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Old 19-04-2008, 02:31 PM
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Kirkus (Kirk)
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Back focus problem

I learned a little something tonight on IIS (who needs Google?). After attaching my DSLR to my 6" Meade (Schmidt-Newtonian) I discovered that I can't rack the focuser in tight enough at prime focus to attain focus. Thanks to the search feature here, I learned that this is a "back focus" issue. I also learned that my options are to either move the primary up the OT (bleh!) or try a low-profile focuser.

I also have my 10" Celestron reflector that I'm able to use on my mount (I really wanted to use the 6" for my DSLR because it's so much easier to set up and is less strain on the mount). I took a rough measurement and the focuser on the 10" is about 50% the distance from it's base to the eyepiece hole than the focuser is on the 6". But there's also a focal length difference of about 500mm between the two scopes. I wonder if that will be enough for me to rack the DSLR in tight enough?

I'm going to try it on the 10" tomorrow night.

(I rambled on about all that just to get to this question) Are there any other options I might have other than moving the primary up the OT or trying a low profile focuser?
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Old 19-04-2008, 02:41 PM
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skwinty (Steve)
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Hi Kirkus
I chose to cut my ota and move the mirror rather than opt for a low profile focusser. The reason for this decision is the low profile focusser is not as sturdy as the existing high profile and I believe that there will be a fair amount of "slop" when the camera plus any projection pieces etc is mounted.
All in all I am very happy with my decision to shorten the ota
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Old 19-04-2008, 02:52 PM
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peter_4059 (Peter)
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Kirkus - another option is to fit longer colimation screws and springs to the primary mirror cell so you can move the mirror forward without cutting the tube. I managed to move my Newt primary 10mm forward this way. You need to ensure the springs are stiff enough so the mirror is held firmly but the advantage is you can always put the original screws and springs back in later.

Another option is to use a Barlow although this has the disadvantage of narrowing the field of view.

Peter
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Old 19-04-2008, 08:25 PM
Ian Robinson
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Quote:
Originally Posted by peter_4059 View Post
Kirkus - another option is to fit longer colimation screws and springs to the primary mirror cell so you can move the mirror forward without cutting the tube. I managed to move my Newt primary 10mm forward this way. You need to ensure the springs are stiff enough so the mirror is held firmly but the advantage is you can always put the original screws and springs back in later.

Another option is to use a Barlow although this has the disadvantage of narrowing the field of view.

Peter
I designed my OTA to accomodate the required backfocus for my kit and SLR.

If need be I can do same as Peter suggest's and recollimate. My Orion Optical (UK) cell has quite long collimating screws too.

Working this with a SNT .... pass .... never owned one.
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Old 19-04-2008, 08:45 PM
Alchemy (Clive)
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Originally Posted by Kirkus View Post
(I rambled on about all that just to get to this question) Are there any other options I might have other than moving the primary up the OT or trying a low profile focuser?
the Baader MPCC will give you a little more in focus it may help both in improving your images and the focus problem
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Old 20-04-2008, 03:24 AM
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EzyStyles (Eric)
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agree with the baader MPCC. you'll get slightly more infocus . Also, adding filters helps with infocus too.

Depends on what type of low profile focuser you are talking about. If it is the GSO one, yes you'll get abit of slop with that. Quality low profile focusers such as the moonlight focuser, JMI etc you won't get any play/slop at all.
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Old 20-04-2008, 05:40 AM
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Thank you all for the ideas. I think I'm going to try Peter's idea of longer collimation screws and springs first. Depending on what I can find at the local hardware store.

10mm isn't a very long distance, but if 10mm (or even 20mm) more at the focuser would've done the job, that same distance at the primary would do the same thing, right?

This is actually kind of fun. Discovering a problem, learning ways of fixing it, then doing so.
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Old 20-04-2008, 08:11 AM
Alchemy (Clive)
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yes longer screws will give the same result, i would suggest you remove the focuser and then focus at the hole this will give you an indication of difference required to change...... mine was about 50mm
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Old 20-04-2008, 07:52 PM
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I was able to find and replace the collimation screws and springs with longer ones. Unfortunately I wasn't able to find longer screws to replace the locking screws. However, the springs I bought are stiff enough to keep the primary in place after collimating (thanks for that suggestion, Peter! ). I was able to move it up about another 20mm.

That, unfortunately wasn't enough.

But like an old dog with a bone I wouldn't let it go. After doing some reading online and poking around with the focuser I discover that Meade took this into consideration when they built the thing. There's an extension tube on the focuser that's removable (doh!). This blasted thing is like 60mm long!! Apparently there's a low profile T-Adapter that comes with this telescope but because I bought it used I didn't get the T-Adapter and wasn't aware that one was made for it. I bought a universal 1.25" T-adapter that slides into the eyepiece and is nowhere near low profile.

Anyway, between losing the extension tube and moving the primary up I was just able to attain focus! I'm going to buy the Meade T-adapter and if it's profile is low enough, I'll put the original hardware back on the primary so I can continue to use the locking screws.

I posted 3 photos from my first prime focus night out on the town.

http://www.iceinspace.com.au/forum/s...d.php?p=317655

Thank you all so much. For this, and all the help this beginner has received since joining.
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Old 23-04-2008, 04:36 AM
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Just so I understand (I love learning about this stuff!)...

If I discovered that my DSLR is about 30mm short of reaching focus, could I use that measurement to know how far up the OT to move the primary?

Would those 2 measurements be equal?
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Old 23-04-2008, 03:46 PM
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There's an article on this forum that explains how to make the measurements - have a look in the projects and articles section - it's probably under astrophotography.
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Old 24-04-2008, 02:25 AM
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There's an article on this forum that explains how to make the measurements - have a look in the projects and articles section - it's probably under astrophotography.
Thanks, Peter.
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