Anyone have thoughts about advantages/disadvantages of using a full-aperture filter versus an off-axis filter on a SCT? I'm trying to decide whether to purchase or make a full-aperture filter. If I purchase a sheet of film and make an off-axis filter I'll have enough film left over for one or two pairs of binoculars. I'm always for saving a buck or two - or 25 or 30 (for another piece of film). Kim
I don't know all the reasons for doing the off-axis but that is what I have done with my previous filters. I do know that the Sun is too bright and just like you would mask your telescope down to get a better view of Mars, masking it down to 4" will help with keeping the light from washing out the detail when viewing the Sun. I am planning to buy 1/2 meter of the Baader solar film and cut it into 4.92" squares. I plan to sell them for about $4. That will handle the cost of the film and some poster board to protect the sheets. Thanks, David Dunn -----Original Message----- From: utah-astronomy-bounces@mailman.xmission.com [mailto:utah-astronomy-bounces@mailman.xmission.com] On Behalf Of Kim Sent: Tuesday, March 06, 2012 10:50 AM To: 'Utah Astronomy' Subject: [Utah-astronomy] solar filter Anyone have thoughts about advantages/disadvantages of using a full-aperture filter versus an off-axis filter on a SCT? I'm trying to decide whether to purchase or make a full-aperture filter. If I purchase a sheet of film and make an off-axis filter I'll have enough film left over for one or two pairs of binoculars. I'm always for saving a buck or two - or 25 or 30 (for another piece of film). Kim _______________________________________________ Utah-Astronomy mailing list http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Send messages to the list to Utah-Astronomy@mailman.xmission.com The Utah-Astronomy mailing list is not affiliated with any astronomy club. To unsubscribe go to: http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Then enter your email address in the space provided and click on "Unsubscribe or edit options".
I'm not that familiar with the design of an SCT, but it's my understanding that any telescope that has a spider or similar obstruction will benefit from an off axis mask. It will remove the diffraction spikes from the image. If you have a curved spider than you don't have diffraction spikes any way, so there's no benefit in that case other than reducing the amount of light that enters the telescope. Another thing to keep in mind is that any time you reduce the aperture of a telescope you also reduce the resolving power. However, with solar scopes, 4" is plenty for great detail. On Tue, Mar 6, 2012 at 10:59 AM, Dunn, David <David.Dunn@supervalu.com> wrote:
I don't know all the reasons for doing the off-axis but that is what I have done with my previous filters. I do know that the Sun is too bright and just like you would mask your telescope down to get a better view of Mars, masking it down to 4" will help with keeping the light from washing out the detail when viewing the Sun.
I am planning to buy 1/2 meter of the Baader solar film and cut it into 4.92" squares. I plan to sell them for about $4. That will handle the cost of the film and some poster board to protect the sheets.
Thanks, David Dunn
-----Original Message----- From: utah-astronomy-bounces@mailman.xmission.com [mailto:utah-astronomy-bounces@mailman.xmission.com] On Behalf Of Kim Sent: Tuesday, March 06, 2012 10:50 AM To: 'Utah Astronomy' Subject: [Utah-astronomy] solar filter
Anyone have thoughts about advantages/disadvantages of using a full-aperture filter versus an off-axis filter on a SCT? I'm trying to decide whether to purchase or make a full-aperture filter. If I purchase a sheet of film and make an off-axis filter I'll have enough film left over for one or two pairs of binoculars. I'm always for saving a buck or two - or 25 or 30 (for another piece of film).
Kim
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The central obstruction boogy man is a long held myth. If you take the size of an aperture and subtract the size of the obstruction that gives you the size of an equivalent unobstructed telescope. So a 10 inch SCT with a 3 inch secondary will perform at the level of a 7 inch refractor. I've seen this when Roger Fry's 10 Meade is compared with Sigfried Jachman's AstroPhysics. The best white light view of the sun that I have ever seen was from Ed Erikson SCT with a full aperture Orion solar filter. DT ________________________________ From: Josh <mountaindrifter@gmail.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Tuesday, March 6, 2012 12:05 PM Subject: Re: [Utah-astronomy] solar filter I'm not that familiar with the design of an SCT, but it's my understanding that any telescope that has a spider or similar obstruction will benefit from an off axis mask. It will remove the diffraction spikes from the image. If you have a curved spider than you don't have diffraction spikes any way, so there's no benefit in that case other than reducing the amount of light that enters the telescope. Another thing to keep in mind is that any time you reduce the aperture of a telescope you also reduce the resolving power. However, with solar scopes, 4" is plenty for great detail. On Tue, Mar 6, 2012 at 10:59 AM, Dunn, David <David.Dunn@supervalu.com> wrote:
I don't know all the reasons for doing the off-axis but that is what I have done with my previous filters. I do know that the Sun is too bright and just like you would mask your telescope down to get a better view of Mars, masking it down to 4" will help with keeping the light from washing out the detail when viewing the Sun.
I am planning to buy 1/2 meter of the Baader solar film and cut it into 4.92" squares. I plan to sell them for about $4. That will handle the cost of the film and some poster board to protect the sheets.
Thanks, David Dunn
-----Original Message----- From: utah-astronomy-bounces@mailman.xmission.com [mailto:utah-astronomy-bounces@mailman.xmission.com] On Behalf Of Kim Sent: Tuesday, March 06, 2012 10:50 AM To: 'Utah Astronomy' Subject: [Utah-astronomy] solar filter
Anyone have thoughts about advantages/disadvantages of using a full-aperture filter versus an off-axis filter on a SCT? I'm trying to decide whether to purchase or make a full-aperture filter. If I purchase a sheet of film and make an off-axis filter I'll have enough film left over for one or two pairs of binoculars. I'm always for saving a buck or two - or 25 or 30 (for another piece of film).
Kim
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On Tue, Mar 6, 2012 at 12:22 PM, daniel turner <outwest112@yahoo.com> wrote:
The central obstruction boogy man is a long held myth.
Not strictly true, Daniel, but in the case of solar observing it's usually not an issue so I'm going to agree with the gist of your post and just pick a minor nit. It can become so large as to be noticeable with large exit pupil diameters (very low powers), so much so that you end up with a shadow in the center of the field. But again, this isn't usually a problem with solar observing. Diffraction artifacts such as spikes and rings, being low-contrast, are typically noticed only on night-time bright and point-source objects. The central obstruction's destructive influence isn't noticed otherwise, unless it's huge. Daytime seeing conditions rarely allow diffraction-limited viewing with large apertures, so 4-6" is usually sufficient for even high-powered views. I have found 2" to be a good working minium. Just enough resolution to see smallish details, and bright enough for a pleasing view if your filter extinction isn't too great, or it's a weird color like green or orange or blue. Full-aperture filtration is desireable, for those occasssions when the seeing conditions will support it. Having the option of a hi-res view is nice.
Just out of curiosity, if aperture during the daytime does no good, then why are the professional solar scopes large aperture? Not as large as the night time scopes, but still bigger than anything I have owned. From: Chuck Hards <chuck.hards@gmail.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Tuesday, March 6, 2012 12:33 PM Subject: Re: [Utah-astronomy] solar filter On Tue, Mar 6, 2012 at 12:22 PM, daniel turner <outwest112@yahoo.com> wrote:
The central obstruction boogy man is a long held myth.
Not strictly true, Daniel, but in the case of solar observing it's usually not an issue so I'm going to agree with the gist of your post and just pick a minor nit. It can become so large as to be noticeable with large exit pupil diameters (very low powers), so much so that you end up with a shadow in the center of the field. But again, this isn't usually a problem with solar observing. Diffraction artifacts such as spikes and rings, being low-contrast, are typically noticed only on night-time bright and point-source objects. The central obstruction's destructive influence isn't noticed otherwise, unless it's huge. Daytime seeing conditions rarely allow diffraction-limited viewing with large apertures, so 4-6" is usually sufficient for even high-powered views. I have found 2" to be a good working minium. Just enough resolution to see smallish details, and bright enough for a pleasing view if your filter extinction isn't too great, or it's a weird color like green or orange or blue. Full-aperture filtration is desireable, for those occasssions when the seeing conditions will support it. Having the option of a hi-res view is nice. _______________________________________________ Utah-Astronomy mailing list http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Send messages to the list to Utah-Astronomy@mailman.xmission.com The Utah-Astronomy mailing list is not affiliated with any astronomy club. To unsubscribe go to: http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Then enter your email address in the space provided and click on "Unsubscribe or edit options".
Nobody said large aperture does no good, Brent, I just pointed-out that daytime seeing conditions often limit the potential of larger apertures. I also carefully mentioned that I like the option of full-aperture when conditions allow. I made a full-aperture filter for my 6" Newt. Professional solar scopes are located at higher altitudes, surrounded by water, or other geographic features that maximize observing time. We usually set up in parking lots, or next to huge heat-sinks such as pavement or buildings. I visited Kitt Peak once and got a tour of the solar scope there, a behemoth. We arrived mid-day and the scope was not operating. I was told that the seeing was best in the mornings, on average. The best night seeing comes after the ground has cooled and convection currents have abated. The best day seeing usually comes before the ground has heated up. Large apertures on the sun are best used early in the day. On Tue, Mar 6, 2012 at 2:53 PM, Brent Watson <brentjwatson@yahoo.com> wrote:
Just out of curiosity, if aperture during the daytime does no good, then why are the professional solar scopes large aperture? Not as large as the night time scopes, but still bigger than anything I have owned.
You are, of course, correct, Chuck. I only wanted to bring out that large aperture is not always wasted during the daylight hours. My comment was a bit blunt, and I did not mean to be critical. My time during the day is limited and so my daytime comments here tend to be short and curt. I guess the real story, as you pointed out, is that our daytime resolution is usually limited by our location and conditions rather than the aperture of our scope. From: Chuck Hards <chuck.hards@gmail.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Tuesday, March 6, 2012 3:03 PM Subject: Re: [Utah-astronomy] solar filter Nobody said large aperture does no good, Brent, I just pointed-out that daytime seeing conditions often limit the potential of larger apertures. I also carefully mentioned that I like the option of full-aperture when conditions allow. I made a full-aperture filter for my 6" Newt. Professional solar scopes are located at higher altitudes, surrounded by water, or other geographic features that maximize observing time. We usually set up in parking lots, or next to huge heat-sinks such as pavement or buildings. I visited Kitt Peak once and got a tour of the solar scope there, a behemoth. We arrived mid-day and the scope was not operating. I was told that the seeing was best in the mornings, on average. The best night seeing comes after the ground has cooled and convection currents have abated. The best day seeing usually comes before the ground has heated up. Large apertures on the sun are best used early in the day. On Tue, Mar 6, 2012 at 2:53 PM, Brent Watson <brentjwatson@yahoo.com> wrote:
Just out of curiosity, if aperture during the daytime does no good, then why are the professional solar scopes large aperture? Not as large as the night time scopes, but still bigger than anything I have owned.
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My comments on the obstruction were about the spider, specifically the straight armed cross spiders that dominate dobs. Curved spiders eliminate the diffraction spike effect. Aside from blocking a little light, the secondary has no effect as an obstruction. Having said that, many people aren't bothered one bit by diffraction spikes, and some even like them. So it largely comes down to personal preference. I personally have never had the opportunity to see side by side the difference between a dob with an off axis filter and one with a full aperture filter so perhaps my input is a little unbalanced. :) Josh On Tue, Mar 6, 2012 at 12:22 PM, daniel turner <outwest112@yahoo.com> wrote:
The central obstruction boogy man is a long held myth. If you take the size of an aperture and subtract the size of the obstruction that gives you the size of an equivalent unobstructed telescope. So a 10 inch SCT with a 3 inch secondary will perform at the level of a 7 inch refractor. I've seen this when Roger Fry's 10 Meade is compared with Sigfried Jachman's AstroPhysics. The best white light view of the sun that I have ever seen was from Ed Erikson SCT with a full aperture Orion solar filter.
DT
________________________________ From: Josh <mountaindrifter@gmail.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Tuesday, March 6, 2012 12:05 PM Subject: Re: [Utah-astronomy] solar filter
I'm not that familiar with the design of an SCT, but it's my understanding that any telescope that has a spider or similar obstruction will benefit from an off axis mask. It will remove the diffraction spikes from the image. If you have a curved spider than you don't have diffraction spikes any way, so there's no benefit in that case other than reducing the amount of light that enters the telescope. Another thing to keep in mind is that any time you reduce the aperture of a telescope you also reduce the resolving power. However, with solar scopes, 4" is plenty for great detail.
On Tue, Mar 6, 2012 at 10:59 AM, Dunn, David <David.Dunn@supervalu.com> wrote:
I don't know all the reasons for doing the off-axis but that is what I have done with my previous filters. I do know that the Sun is too bright and just like you would mask your telescope down to get a better view of Mars, masking it down to 4" will help with keeping the light from washing out the detail when viewing the Sun.
I am planning to buy 1/2 meter of the Baader solar film and cut it into 4.92" squares. I plan to sell them for about $4. That will handle the cost of the film and some poster board to protect the sheets.
Thanks, David Dunn
-----Original Message----- From: utah-astronomy-bounces@mailman.xmission.com [mailto:utah-astronomy-bounces@mailman.xmission.com] On Behalf Of Kim Sent: Tuesday, March 06, 2012 10:50 AM To: 'Utah Astronomy' Subject: [Utah-astronomy] solar filter
Anyone have thoughts about advantages/disadvantages of using a full-aperture filter versus an off-axis filter on a SCT? I'm trying to decide whether to purchase or make a full-aperture filter. If I purchase a sheet of film and make an off-axis filter I'll have enough film left over for one or two pairs of binoculars. I'm always for saving a buck or two - or 25 or 30 (for another piece of film).
Kim
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The secondary does decrease effective aperture and therefore resolution. As mentioned before the effect can be calculated by subtracting the secondary diameter from the primary diameter. That yields the effective aperture. Also, don't forget that the larger aperture scopes work at their peak resolution on fewer nights than smaller aperture because it is harder to get a good column of atmosphere in larger sizes. This may be some of the reason for the smaller imported scopes showing a better image than the SPOC scopes. All in all, the larger lines will form behind the larger scopes. The 22" always had the biggest lines. The more interest in the scope, the more interest generated in the public. From: Josh <mountaindrifter@gmail.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Tuesday, March 6, 2012 12:51 PM Subject: Re: [Utah-astronomy] solar filter My comments on the obstruction were about the spider, specifically the straight armed cross spiders that dominate dobs. Curved spiders eliminate the diffraction spike effect. Aside from blocking a little light, the secondary has no effect as an obstruction. Having said that, many people aren't bothered one bit by diffraction spikes, and some even like them. So it largely comes down to personal preference. I personally have never had the opportunity to see side by side the difference between a dob with an off axis filter and one with a full aperture filter so perhaps my input is a little unbalanced. :) Josh On Tue, Mar 6, 2012 at 12:22 PM, daniel turner <outwest112@yahoo.com> wrote:
The central obstruction boogy man is a long held myth. If you take the size of an aperture and subtract the size of the obstruction that gives you the size of an equivalent unobstructed telescope. So a 10 inch SCT with a 3 inch secondary will perform at the level of a 7 inch refractor. I've seen this when Roger Fry's 10 Meade is compared with Sigfried Jachman's AstroPhysics. The best white light view of the sun that I have ever seen was from Ed Erikson SCT with a full aperture Orion solar filter.
DT
________________________________ From: Josh <mountaindrifter@gmail.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Tuesday, March 6, 2012 12:05 PM Subject: Re: [Utah-astronomy] solar filter
I'm not that familiar with the design of an SCT, but it's my understanding that any telescope that has a spider or similar obstruction will benefit from an off axis mask. It will remove the diffraction spikes from the image. If you have a curved spider than you don't have diffraction spikes any way, so there's no benefit in that case other than reducing the amount of light that enters the telescope. Another thing to keep in mind is that any time you reduce the aperture of a telescope you also reduce the resolving power. However, with solar scopes, 4" is plenty for great detail.
On Tue, Mar 6, 2012 at 10:59 AM, Dunn, David <David.Dunn@supervalu.com> wrote:
I don't know all the reasons for doing the off-axis but that is what I have done with my previous filters. I do know that the Sun is too bright and just like you would mask your telescope down to get a better view of Mars, masking it down to 4" will help with keeping the light from washing out the detail when viewing the Sun.
I am planning to buy 1/2 meter of the Baader solar film and cut it into 4.92" squares. I plan to sell them for about $4. That will handle the cost of the film and some poster board to protect the sheets.
Thanks, David Dunn
-----Original Message----- From: utah-astronomy-bounces@mailman.xmission.com [mailto:utah-astronomy-bounces@mailman.xmission.com] On Behalf Of Kim Sent: Tuesday, March 06, 2012 10:50 AM To: 'Utah Astronomy' Subject: [Utah-astronomy] solar filter
Anyone have thoughts about advantages/disadvantages of using a full-aperture filter versus an off-axis filter on a SCT? I'm trying to decide whether to purchase or make a full-aperture filter. If I purchase a sheet of film and make an off-axis filter I'll have enough film left over for one or two pairs of binoculars. I'm always for saving a buck or two - or 25 or 30 (for another piece of film).
Kim
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Resolution is more a function of aperture (diameter) than throughput, but it can be argued that increased diffraction from a larger secondary reduces resolution due to poorer energy concentration in the central spot. An 8" mirror with a 100% relfective coating has the same theoretical resolution as one with a 90% relfective coating, however the 100% reflective mirror has a dimmer limiting magnitude. On Tue, Mar 6, 2012 at 3:02 PM, Brent Watson <brentjwatson@yahoo.com> wrote:
The secondary does decrease effective aperture and therefore resolution. As mentioned before the effect can be calculated by subtracting the secondary diameter from the primary diameter. That yields the effective aperture.
I understand the simple arithmetic of calculating the effective aperture when there is an obstruction, but is there an objective measurement of the degradation due to the diffraction of the obstruction itself? Or is the effect simply one of reducing contrast? And finally, is this why SCT's are generally large focal ratio instruments, i.e. to increase contrast and compensate for the large obstruction? Kim -----Original Message----- From: utah-astronomy-bounces@mailman.xmission.com [mailto:utah-astronomy-bounces@mailman.xmission.com] On Behalf Of Brent Watson Sent: Tuesday, March 06, 2012 3:02 PM To: Utah Astronomy Subject: Re: [Utah-astronomy] solar filter The secondary does decrease effective aperture and therefore resolution. As mentioned before the effect can be calculated by subtracting the secondary diameter from the primary diameter. That yields the effective aperture.
Remember that Meade used to sell dedicated f/6.3 SCT's (I don't think they do anymore). Those secondaries were significantly larger than the secondaries on dedicated f/10 SCT's. A wider FOV, but at the expense of lower throughput and increased diffraction. The market has spoken, and focal reducers are what most imagers now use to accomplish the same thing. They have the flexibility of going with either f/6.3 or f/10, with the same instrument. (I even own one for my own f/10 Celestron) The larger focal ratio is just the way Cassegrains work. The secondary is in effect a reflective Barlow. Smaller secondaries mean higher magnification values required to put the final focal plane behind the primary. Kim, have you read Dick Suiter's book "Star Testing Astronomical Telescopes"? He goes into great detail on the effects of diffraction due to secondaries. On Tue, Mar 6, 2012 at 3:10 PM, Kim <kimharch@cut.net> wrote:
I understand the simple arithmetic of calculating the effective aperture when there is an obstruction, but is there an objective measurement of the degradation due to the diffraction of the obstruction itself? Or is the effect simply one of reducing contrast? And finally, is this why SCT's are generally large focal ratio instruments, i.e. to increase contrast and compensate for the large obstruction?
Kim
-----Original Message----- From: utah-astronomy-bounces@mailman.xmission.com [mailto:utah-astronomy-bounces@mailman.xmission.com] On Behalf Of Brent Watson Sent: Tuesday, March 06, 2012 3:02 PM To: Utah Astronomy Subject: Re: [Utah-astronomy] solar filter
The secondary does decrease effective aperture and therefore resolution. As mentioned before the effect can be calculated by subtracting the secondary diameter from the primary diameter. That yields the effective aperture.
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I haven't read Suiter. I intuitively (I think) understand the effect(s) - maybe I'll hunt online for more information but if anyone thinks of a helpful article I'm game to read. Thanks all. Kim -----Original Message----- From: utah-astronomy-bounces@mailman.xmission.com [mailto:utah-astronomy-bounces@mailman.xmission.com] On Behalf Of Chuck Hards Sent: Tuesday, March 06, 2012 3:23 PM To: Utah Astronomy Subject: Re: [Utah-astronomy] solar filter Remember that Meade used to sell dedicated f/6.3 SCT's (I don't think they do anymore). Those secondaries were significantly larger than the secondaries on dedicated f/10 SCT's. A wider FOV, but at the expense of lower throughput and increased diffraction. The market has spoken, and focal reducers are what most imagers now use to accomplish the same thing. They have the flexibility of going with either f/6.3 or f/10, with the same instrument. (I even own one for my own f/10 Celestron) The larger focal ratio is just the way Cassegrains work. The secondary is in effect a reflective Barlow. Smaller secondaries mean higher magnification values required to put the final focal plane behind the primary. Kim, have you read Dick Suiter's book "Star Testing Astronomical Telescopes"? He goes into great detail on the effects of diffraction due to secondaries.
I failed to mention that the diffraction caused by a spider is not eliminated by a curved vane. It is only minimized visually. The curved vane does affect resolution, especially if it is thick. Visually you don't see it as much because the angle of the vane changes constantly as a function of its position in the aperture. This makes the spike seem to dissappear, but its effects are still there. For those who have not seen my 10 inch scope, I purposefully constructed it with the idea of minimizing the effects of the secondary and spider. It has no spider, and he secondary is only 7/8 inch minor axis. It was purpose built to see how close I could get to a refractor like image. Effective aperture of tis scope is 9 1/8 inches. Comparing this scope to Siegfried's Clark refractor, I would say I almost succeeded. Under excellent conditions at the Wedge, the Clark still had slightly higher resolution images (only slightly), but my 10" had no secondary color. Richard Suiter's book is an excellent reference for these questions. From: Brent Watson <brentjwatson@yahoo.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Tuesday, March 6, 2012 3:02 PM Subject: Re: [Utah-astronomy] solar filter The secondary does decrease effective aperture and therefore resolution. As mentioned before the effect can be calculated by subtracting the secondary diameter from the primary diameter. That yields the effective aperture. Also, don't forget that the larger aperture scopes work at their peak resolution on fewer nights than smaller aperture because it is harder to get a good column of atmosphere in larger sizes. This may be some of the reason for the smaller imported scopes showing a better image than the SPOC scopes. All in all, the larger lines will form behind the larger scopes. The 22" always had the biggest lines. The more interest in the scope, the more interest generated in the public. From: Josh <mountaindrifter@gmail.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Tuesday, March 6, 2012 12:51 PM Subject: Re: [Utah-astronomy] solar filter My comments on the obstruction were about the spider, specifically the straight armed cross spiders that dominate dobs. Curved spiders eliminate the diffraction spike effect. Aside from blocking a little light, the secondary has no effect as an obstruction. Having said that, many people aren't bothered one bit by diffraction spikes, and some even like them. So it largely comes down to personal preference. I personally have never had the opportunity to see side by side the difference between a dob with an off axis filter and one with a full aperture filter so perhaps my input is a little unbalanced. :) Josh On Tue, Mar 6, 2012 at 12:22 PM, daniel turner <outwest112@yahoo.com> wrote:
The central obstruction boogy man is a long held myth. If you take the size of an aperture and subtract the size of the obstruction that gives you the size of an equivalent unobstructed telescope. So a 10 inch SCT with a 3 inch secondary will perform at the level of a 7 inch refractor. I've seen this when Roger Fry's 10 Meade is compared with Sigfried Jachman's AstroPhysics. The best white light view of the sun that I have ever seen was from Ed Erikson SCT with a full aperture Orion solar filter.
DT
________________________________ From: Josh <mountaindrifter@gmail.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Tuesday, March 6, 2012 12:05 PM Subject: Re: [Utah-astronomy] solar filter
I'm not that familiar with the design of an SCT, but it's my understanding that any telescope that has a spider or similar obstruction will benefit from an off axis mask. It will remove the diffraction spikes from the image. If you have a curved spider than you don't have diffraction spikes any way, so there's no benefit in that case other than reducing the amount of light that enters the telescope. Another thing to keep in mind is that any time you reduce the aperture of a telescope you also reduce the resolving power. However, with solar scopes, 4" is plenty for great detail.
On Tue, Mar 6, 2012 at 10:59 AM, Dunn, David <David.Dunn@supervalu.com> wrote:
I don't know all the reasons for doing the off-axis but that is what I have done with my previous filters. I do know that the Sun is too bright and just like you would mask your telescope down to get a better view of Mars, masking it down to 4" will help with keeping the light from washing out the detail when viewing the Sun.
I am planning to buy 1/2 meter of the Baader solar film and cut it into 4.92" squares. I plan to sell them for about $4. That will handle the cost of the film and some poster board to protect the sheets.
Thanks, David Dunn
-----Original Message----- From: utah-astronomy-bounces@mailman.xmission.com [mailto:utah-astronomy-bounces@mailman.xmission.com] On Behalf Of Kim Sent: Tuesday, March 06, 2012 10:50 AM To: 'Utah Astronomy' Subject: [Utah-astronomy] solar filter
Anyone have thoughts about advantages/disadvantages of using a full-aperture filter versus an off-axis filter on a SCT? I'm trying to decide whether to purchase or make a full-aperture filter. If I purchase a sheet of film and make an off-axis filter I'll have enough film left over for one or two pairs of binoculars. I'm always for saving a buck or two - or 25 or 30 (for another piece of film).
Kim
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_______________________________________________ Utah-Astronomy mailing list http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Send messages to the list to Utah-Astronomy@mailman.xmission.com The Utah-Astronomy mailing list is not affiliated with any astronomy club. To unsubscribe go to: http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Then enter your email address in the space provided and click on "Unsubscribe or edit options". _______________________________________________ Utah-Astronomy mailing list http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Send messages to the list to Utah-Astronomy@mailman.xmission.com The Utah-Astronomy mailing list is not affiliated with any astronomy club. To unsubscribe go to: http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Then enter your email address in the space provided and click on "Unsubscribe or edit options".
Excellent points, Brent, and well worth repeating. In fact a curved vane spider actually INCREASES overall diffraction due to the longer vane in the light path. As you note, the overall scattered light is distrubuted over a larger area- this means decreased contrast, but the absence of an obvious visible spike fools our brain into thinking we have better contrast and a better view. More aesthetically pleasing- to some- but not intrinsically better. Diffraction spikes can be used as an observing tool, escpecially if you like close, unequal double stars. By rotating the OTA, you can place the dim companion between the spikes and increase it's visibility. That's because the spikes are drawing energy away from the main diffraction rings around the brighter companion, and increasing contrast closer to it. Result: The companion is more visible. Even the best-made refractors have diffraction rings, it's a property of light and we can't do anything about it. Being a lifetime Newtonain user primarily, I've learned to live with spikes, and use them to my advantage when needed. On Tue, Mar 6, 2012 at 4:38 PM, Brent Watson <brentjwatson@yahoo.com> wrote:
I failed to mention that the diffraction caused by a spider is not eliminated by a curved vane. It is only minimized visually. The curved vane does affect resolution, especially if it is thick. Visually you don't see it as much because the angle of the vane changes constantly as a function of its position in the aperture. This makes the spike seem to dissappear, but its effects are still there.
I can accept being wrong, but only if I learn something. :D Josh On Tue, Mar 6, 2012 at 7:54 PM, Chuck Hards <chuck.hards@gmail.com> wrote:
Excellent points, Brent, and well worth repeating. In fact a curved vane spider actually INCREASES overall diffraction due to the longer vane in the light path. As you note, the overall scattered light is distrubuted over a larger area- this means decreased contrast, but the absence of an obvious visible spike fools our brain into thinking we have better contrast and a better view. More aesthetically pleasing- to some- but not intrinsically better.
Diffraction spikes can be used as an observing tool, escpecially if you like close, unequal double stars. By rotating the OTA, you can place the dim companion between the spikes and increase it's visibility. That's because the spikes are drawing energy away from the main diffraction rings around the brighter companion, and increasing contrast closer to it. Result: The companion is more visible.
Even the best-made refractors have diffraction rings, it's a property of light and we can't do anything about it. Being a lifetime Newtonain user primarily, I've learned to live with spikes, and use them to my advantage when needed.
On Tue, Mar 6, 2012 at 4:38 PM, Brent Watson <brentjwatson@yahoo.com> wrote:
I failed to mention that the diffraction caused by a spider is not eliminated by a curved vane. It is only minimized visually. The curved vane does affect resolution, especially if it is thick. Visually you don't see it as much because the angle of the vane changes constantly as a function of its position in the aperture. This makes the spike seem to dissappear, but its effects are still there.
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There are a lot of common misconceptions about the cause and effects of diffraction, Josh, It took me decades to sort it all out, myself, and I'm still learning. Thanks for bringing it up, actually. People spend a lot of time and energy trying to optimize an optical system that in the end only produces either marginal or purely subjective gains. On Wed, Mar 7, 2012 at 11:14 PM, Josh <mountaindrifter@gmail.com> wrote:
I can accept being wrong, but only if I learn something. :D
As has been stated, diffraction effects, or lack thereof in a Newtonian reflector, is mostly aesthetic. I have never been particularly bothered by the spikes coming off bright objects, but while building my daughters 8" f4 ball scope, we thought that it would be cool to do away with the spikes. We made a curved vane spider which consisted of one ~170 degree vane to which the secondary hub was mounted. The views were really great without the spikes and I really liked it. However, that vane wiggled like Santa Claus's belly any time the scope was touched. I worked, and worked to improve the design and did make some strides, but it just didn't perform well enough for me. Having to wait for a couple of seconds for the image to stop jumping around after touching the scope was just to much for my daughter, so we eventually swapped the limp spider for a tried and true, 4-straight-vane, solid as a rock spider. I have not seriously considered monkeying with a curved one since. I encourage people to do what is pleasing to them and if a curved vane spider is in your future, I can help you avoid some pitfalls. Mat -----Original Message----- From: utah-astronomy-bounces@mailman.xmission.com [mailto:utah-astronomy-bounces@mailman.xmission.com] On Behalf Of Chuck Hards Sent: Thursday, March 08, 2012 9:17 AM To: Utah Astronomy Subject: Re: [Utah-astronomy] solar filter There are a lot of common misconceptions about the cause and effects of diffraction, Josh, It took me decades to sort it all out, myself, and I'm still learning. Thanks for bringing it up, actually. People spend a lot of time and energy trying to optimize an optical system that in the end only produces either marginal or purely subjective gains. On Wed, Mar 7, 2012 at 11:14 PM, Josh <mountaindrifter@gmail.com> wrote:
I can accept being wrong, but only if I learn something. :D
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sounds great mat! I've got lots to learn and do. Looking forward to every bit of it. Josh On Thu, Mar 8, 2012 at 8:53 AM, Hutchings, Mat (H USA) <mat.hutchings@siemens.com> wrote:
As has been stated, diffraction effects, or lack thereof in a Newtonian reflector, is mostly aesthetic. I have never been particularly bothered by the spikes coming off bright objects, but while building my daughters 8" f4 ball scope, we thought that it would be cool to do away with the spikes.
We made a curved vane spider which consisted of one ~170 degree vane to which the secondary hub was mounted. The views were really great without the spikes and I really liked it. However, that vane wiggled like Santa Claus's belly any time the scope was touched. I worked, and worked to improve the design and did make some strides, but it just didn't perform well enough for me.
Having to wait for a couple of seconds for the image to stop jumping around after touching the scope was just to much for my daughter, so we eventually swapped the limp spider for a tried and true, 4-straight-vane, solid as a rock spider. I have not seriously considered monkeying with a curved one since. I encourage people to do what is pleasing to them and if a curved vane spider is in your future, I can help you avoid some pitfalls.
Mat
-----Original Message----- From: utah-astronomy-bounces@mailman.xmission.com [mailto:utah-astronomy-bounces@mailman.xmission.com] On Behalf Of Chuck Hards Sent: Thursday, March 08, 2012 9:17 AM To: Utah Astronomy Subject: Re: [Utah-astronomy] solar filter
There are a lot of common misconceptions about the cause and effects of diffraction, Josh, It took me decades to sort it all out, myself, and I'm still learning. Thanks for bringing it up, actually. People spend a lot of time and energy trying to optimize an optical system that in the end only produces either marginal or purely subjective gains.
On Wed, Mar 7, 2012 at 11:14 PM, Josh <mountaindrifter@gmail.com> wrote:
I can accept being wrong, but only if I learn something. :D
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My Holmes-built (think Dan) Baader film solar for my 8" nexstar SCT uses an off axis opening of 2" or maybe 3" at most. Haven't looked at it for some time. It works just fine, and so saves you film for binos, or whatever. I certainly see no need for to build one to cover the whole front of the 'scope. BTW, before each use, I hold it to the sun & check very carefully for any pinholes. etc. 73, lh On 3/6/2012 10:49 AM, Kim wrote:
Anyone have thoughts about advantages/disadvantages of using a full-aperture filter versus an off-axis filter on a SCT? I'm trying to decide whether to purchase or make a full-aperture filter. If I purchase a sheet of film and make an off-axis filter I'll have enough film left over for one or two pairs of binoculars. I'm always for saving a buck or two - or 25 or 30 (for another piece of film).
Kim
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participants (8)
-
Brent Watson -
Chuck Hards -
daniel turner -
Dunn, David -
Hutchings, Mat (H USA) -
Josh -
Kim -
Larry Holmes