Hello all, I am just getting ready to cut my trusses for the dob I am making. The question I have is where to make the eyepieces I have come into focus. I have read that you use a 2" eyepiece racked all the way in and cut your trusses when it just reaches focus. Then I ready just the opposite. You should cut your trusses so everything is in focus with your eyepiece racked all the way out. What are your feelings on the matter. I feel I should make eyepieces come into focus in the middle of the travel of the focus-er. ?????? Thanks for your help in this matter. Mark Shelton
I would go with near middle of travel. This would seem to make it more likely that any eyepiece you get in the future will work. My focus travel is 4 inches and I have eyepieces that require it be racked all the way in and all the way out. Extension tubes can be used if you need more out travel.
Hello all,
I am just getting ready to cut my trusses for the dob I am making. The question I have is where to make the eyepieces I have come into focus. I have read that you use a 2" eyepiece racked all the way in and cut your trusses when it just reaches focus. Then I ready just the opposite. You should cut your trusses so everything is in focus with your eyepiece racked all the way out.
What are your feelings on the matter. I feel I should make eyepieces come into focus in the middle of the travel of the focus-er. ??????
Thanks for your help in this matter.
Mark Shelton _______________________________________________ Utah-Astronomy mailing list Utah-Astronomy@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Visit the Photo Gallery: http://www.slas.us/gallery2/main.php
Mike: Having been there before, and gotten it wrong, my preference is to build a scale drawing that accurately shows you what's going on in detail. I built a beatiful hexagonal plywood tube and then drilled the hole for the eyepiece holder in the wrong place. I was able to plug the hole and redrill it in the right place and the patch just barely showed and no one ever noticed it but me to whom it was a persistant reminder of the misadventure. You can get the exact focal length of your mirror from a Foucalt tester. They are easy to make if you have some bench space and some modest tool skills. Or you might ask Steve Dodds to do it for you. The tester will also show you the quality of your mirror's figure and you can even record a picture of the figure on a DSLR. The position of the focal plane within the barrel of an eyepiece can be determined from the manufacturer in some cases and by direct inspection in other cases. With Plossls, just turn it over and look inside for the position of the field stop. For eyepieces with a field lense in front of the field stop you need to get specs from the manufacturer. Televue has this on their webpage. In general on modern eyepieces the field stop is within a half inch of where the barrel meets the top extension of the eyepiece. Lastly the the scale drawing will you how well the secondary is doing its' job. A common mistake is to buy a secondary that is so small that it blocks light from the edge of primay from reaching the focal plane. This is often done is the mistaken belief that it will improve the image by minimizing the obstruction of the primary. What it actually does is turns your 13 inch telescope into a 10 inch telescope. The unscrupulous will use this to mask a turned down edge on a poorly figured mirror. DT
Hi Mark; With all the scopes I've made, I just made sure that the eyepiece which needs the most "in" travel, comes to a focus near the racked in position. Don't forget to also test your eyepieces with your barlow if you have one! It can get costly to not get this right if you cut the tubes too short; so take your time. But also consider that you can adjust with shims if you end up needing the tubes to be a bit longer (ask me how I know...) Mat -----Original Message----- From: utah-astronomy-bounces@mailman.xmission.com [mailto:utah-astronomy-bounces@mailman.xmission.com] On Behalf Of Mark Shelton Sent: Thursday, April 28, 2011 6:31 PM To: Utah Astronomy Subject: [Utah-astronomy] Dob Help? Hello all, I am just getting ready to cut my trusses for the dob I am making. The question I have is where to make the eyepieces I have come into focus. I have read that you use a 2" eyepiece racked all the way in and cut your trusses when it just reaches focus. Then I ready just the opposite. You should cut your trusses so everything is in focus with your eyepiece racked all the way out. What are your feelings on the matter. I feel I should make eyepieces come into focus in the middle of the travel of the focus-er. ?????? Thanks for your help in this matter. Mark Shelton _______________________________________________ Utah-Astronomy mailing list Utah-Astronomy@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Visit the Photo Gallery: http://www.slas.us/gallery2/main.php ---------------------------------------------------------------------------- This message and any included attachments are from Siemens Medical Solutions and are intended only for the addressee(s). The information contained herein may include trade secrets or privileged or otherwise confidential information. Unauthorized review, forwarding, printing, copying, distributing, or using such information is strictly prohibited and may be unlawful. If you received this message in error, or have reason to believe you are not authorized to receive it, please promptly delete this message and notify the sender by e-mail with a copy to Central.SecurityOffice@siemens.com Thank you
participants (4)
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daniel turner -
erikhansen＠thebluezone.net -
Hutchings, Mat (H USA) -
Mark Shelton