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Adjusting collimation on a cheap mirror diagonal?

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#1 drneilmb

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Posted 24 January 2023 - 09:15 PM

After a pause of a couple years from reading Cloudy Nights, I am back and thinking more critically about my astro equipment. My grab-n-go is a Meade Infinity 102 f5.9. I got a 2" mirror diagonal because I like to use a 32mm SWA as a finder and space walk eyepiece. But I got a really cheap mirror diagonal. So cheap that our esteemed Jon Isaacs threw one away (?) but it was what I could afford.

 

Compress_20230124_195954_4619.jpg

 

In starting up my CN reading again, I am learning about adjusting the focuser and collimating a refractor. I put my Orion Lasermate in the focuser, checked its collimation rotating through 360 degrees and got it as centered as I could on my handmade target.

 

Compress_20230124_200429_9164.jpg

 

Then I put the diagonal in the focuser and the laser in the diagonal and as you all expected, the laser spot moved, just over a centimeter.

 

Compress_20230124_200429_9289.jpg

 

Is it possible to fix this? Are there  adjustments somewhere to aim the mirror differently? If not, are there ways to shim the mirror to make the aim better? Or should I just save up and buy a different cheap mirror diagonal like an $80 GSO?

 

If it isn't fixed, what is the impact of continuing to use it like it is, just like I have been doing for the past few years? What am I giving up with a tilt like this?

 

Thanks in advance for any advice you can offer.

 

-Neil


Edited by drneilmb, 24 January 2023 - 09:17 PM.


#2 luxo II

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Posted 24 January 2023 - 09:43 PM

Many diagonals don't have adjustments.

 

Whether it matters is for you to decide. if you can't see any ill effects I'd suggest fuggedaboutit.


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#3 TOMDEY

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Posted 24 January 2023 - 09:52 PM

Star diagonals suffer both angular and lateral misalignments. I measured those on a bunch of them years ago. The problem is that it takes four degrees of freedom (i.e. adjustments) to bring it to perfection, but a flat mirror only allows three, at most. So, even in theory --- there is no guaranteed fiddling with that mirror or prism inside to bring it to book. The only way to enjoy perfect alignment is --- if the blasted thing is made right in the first place! That usually means --- a premium diagonal... no free lunch... unless you just get lucky.

 

Your paper target set up only measures two degrees of freedom - so even satisfying it does not mean the thing is providing identical alignment.

 

Here's my test report summary. >>> Some of those were brand new and some were used, so condemning any one doesn't necessarily mean that all ~identicals~ would be no good. But the trend is pretty obvious. Premium tends good, bargain generally does not.    Tom

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  • 88 diagonals table 1200.jpg


#4 luxo II

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Posted 24 January 2023 - 09:59 PM

Its a budget scope with a budget eyepiece, Tom; in current $ many of those cost would more than his entire rig.


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#5 TOMDEY

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Posted 24 January 2023 - 10:05 PM

Its a budget scope with a budget eyepiece, Tom; in current $ many of those cost would more than his entire rig.

Yep... I know... it's just a reality thing. In that case, I would recommend getting out the tools and taking it apart to see what's inside. More prudently --- just enjoy it as-is.    Tom



#6 sevenofnine

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Posted 24 January 2023 - 10:46 PM

Astro-Tech has a 2" diagonal that's reasonably priced and worth saving for. It will work well on your future scope borg.gif

 

https://www.astronom...refractors.html.



#7 Redbetter

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Posted 25 January 2023 - 12:58 AM

That 2" Meade DS series diagonal isn't likely worth any trouble or what you paid for it.  Comments I have seen are that the mirrors aren't good, and the housing is cheaply/poorly made.  I have sworn off diagonals with poorly made housings as they have been far more trouble than they were worth, actually costing more in the end for a diagonal that didn't allow good images and had to be replaced anyway.  Commonly the barrel/nosepiece/body were misaligned to one another which is not really fixable, and in some cases the mirror was badly misaligned within the body, and/or astigmatic. I keep some around as reminders not to do that again.

 

My problematic experiences were with cheap 1.25" diagonals.  The 2" mirror diagonal housings I own have been well made and properly aligned.  A good run-of-the-mill 2" mirror diagonal is not very expensive. 

 

Unfortunately, as painful as it is, the reality is that I don't think you could afford the one you have, but something like an AT 2" diagonal would have been affordable and saved you money in the long run.  I guess the silver lining is that you learn something about checking the various aspects of alignment--I know that is what I learned from going through it while trying to adjust a refractor's objective and focuser.



#8 norvegicus

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Posted 04 February 2023 - 09:12 AM

Neil, a quick experiment is to look at a critical target, like a star at high enough magnification to see the Airy disc, or if you want to use your wide field eyepiece choose a daytime target that is far away and has some detail that you can barely make out, with and without the diagonal, i.e. put the eyepiece directly into the focuser and view straight through.  If the view is significantly degraded with the diagonal in place then you have your answer.

 

You might run out of focus travel when going straight through; the solution can be to use a spacer tube (anything the correct diameter will work for this quick experiment), or you might be able to just back the eyepiece out enough.




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