You are certainly welcome at any ATM session. However, don't expect to see much being done on the 21" project currently. I need to sell the 16" before much more happens on the 21". One scope finances the next... Mat -----Original Message----- From: utah-astronomy-bounces@mailman.xmission.com [mailto:utah-astronomy-bounces@mailman.xmission.com] On Behalf Of Erik Hansen Sent: Wednesday, October 30, 2013 11:42 AM To: Utah Astronomy Subject: Re: [Utah-astronomy] Journey to the 1.8 meter
Well there you go, at f4 you may be right. The only thing that would have made mine lower would have been using thinner plywood. A certain height is always needed for the mirror box to swing easily. I think the low profile designs are good because of larger diameter of the altitude bearings, it makes it easier to adjust friction. I should probably check it out at one of your ATM sessions.
I'm 6'4", and plan on a very low profile rocker/mirror box. This should
get the eyepiece down low enough for me, but again, I will probably go slightly faster than f4 just to make sure. I will need to bring a ladder or step of some kind for most others to view through the scope.
Yes, someone would do well to buy the 16"!
Mat
-----Original Message----- From: utah-astronomy-bounces@mailman.xmission.com [mailto:utah-astronomy-bounces@mailman.xmission.com] On Behalf Of Erik Hansen Sent: Tuesday, October 29, 2013 7:24 PM To: Utah Astronomy Subject: Re: [Utah-astronomy] Journey to the 1.8 meter
With a 20 inch f4 at the zenith you needed about one step. Had a ladder that we added a half step. I think you will need some sort of ladder, unless you are very tall. We selected a secondary size some where in the middle of min and max, as I recall 3.5 inches. Collimating was not difficult, of course you can always make it more difficult than is needed if you get obsessed, a Cheshire usually worked fine.
Sounds like someone would be fortunate to get your 16.
A 6" refractor is a very nice scope.
My 16" is f5.6 and its secondary mirror is 2.6" M.A. The secondary is very nice and was handmade by Louis Wilkinson (an early optician for Cave Optics). I have a hand written note from him stating the quality of the mirror; it is very good.
The 21" will be an f/4 or slightly faster. The secondary will be in the 4" to 4.5" range. The main goal is for me to be able to stand flat-footed at the scope when it is pointed at the zenith. Ladders don't bother me, but would rather not have to carry one around. With the current generation of parabola correctors, one does not need to shy away from fast focal lengths (at least for coma issues). Figuring a fast, larger optic is another beast. Then there is collimation, which needs to be right on the nut with a fast, fat optic But what the hey, I can man up and do this.
Mat
-----Original Message----- From: utah-astronomy-bounces@mailman.xmission.com [mailto:utah-astronomy-bounces@mailman.xmission.com] On Behalf Of Erik Hansen Sent: Tuesday, October 29, 2013 5:43 PM To: Utah Astronomy Subject: Re: [Utah-astronomy] Journey to the 1.8 meter
Good luck with 21 inch btw, I did a 20 f4 and enjoyed the views a great deal. What is your f ratio and secondary size? A question for your 16 inch as well.
Personally I have given up on building and did get cured from aperture fever, very happy with a 6 inch refractor.
The popcorn is done. But my, was it an informative discussion. Thanks to
all the contributors. I was too "yella" (yellow for you yankees) to jump in....
Mat
-----Original Message----- From: utah-astronomy-bounces@mailman.xmission.com [mailto:utah-astronomy-bounces@mailman.xmission.com] On Behalf Of Erik Hansen Sent: Tuesday, October 29, 2013 4:53 PM To: Utah Astronomy Subject: Re: [Utah-astronomy] Journey to the 1.8 meter
It will for certain be a different view than the Grim. I would still like to hear people's impression anyway. Let's hope Mike is willing to set it up for a 3 or 4 hour SPOC star party.
My apologies for the error in light gathering vs resolution, you can put the popcorn away Matt.
The 41 inch aperture comes from the rule of thumb that resolution is approximated by the diameter of the primary minus the diameter of the secondary. That is 41 inches, and is an approximation of the resolution. The light gathering power, however is equal to the square of the primary minus the square of the secondary. That is NOT 41 inches, but is very close to a 64 inch scope.
These are not new rules, and I am certain that Mike has looked at them multiple times. He is not in the dark - well, all astronomers are up in the night I guess. I am sure that Steve knows all about the tradeoffs employed in this scope as well. This has got to be a fantastic telescope, and will do really well against any other amateur scope available including the Grim.
Just out of curiosity, what is the diameter of the secondary in the Grim scope?
From: Chuck Hards <chuck.hards@gmail.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Tuesday, October 29, 2013 2:17 PM Subject: Re: [Utah-astronomy] Journey to the 1.8 meter
I don't understand your "true aperture" claim. It makes no sense.
The aperture is 70 inches with the resolution of a 70 inch scope.
The clear surface area of the 70 inch with a 29-inch secondary is 3,188 square inches. The clear surface area of a 40 inch with a 10-inch secondary is 1,178 square inches.
Yes, it does have a very small true field even at a 7mm exit pupil. Mike knows this, knew it well ahead of time.
He plans to add motor drives for tracking.
The usable magnification range is a low of about 280X up to whatever the seeing will allow, but probably less than 400X.
Again, Mike knew this in advance.
On Tue, Oct 29, 2013 at 1:29 PM, Erik Hansen <erikhansen@thebluezone.net>wrote:
I was not suggesting a smaller secondary, I was suggesting a smaller primary with a more efficient design.> The true aperture is 41 inches, so you are getting the resolution of a 41 inch scope, my point is a 45 inch primary with a more conventional secondary would give same light gathering with probably same ladder height as a 70 inch with a 29 inch secondary. I doubt it puts the Grim to shame, it would be interesting to compare the 2 at SPOC, and hear what the comments are.
The other issue, is what is the largest field of view with the scope? With some eyepieces any object centered will not be there long. The time it takes someone to climb down the ladder and another one up, the object will be out of the field of view....will they have more than a minute or less than a minute.
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