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How to collimate with a not-so-good 2" eyepiece holder?

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#1 bokemon

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Posted 30 November 2022 - 08:49 PM

Hello folks, 

Recently I put a Vixen R200SS into service as a visual instrument on an alt-az mount.  The 2" eyepiece holder on that is not very good, being just a tube with two tapped holes near the top for some thumbscrews.  (Not even a brass compression ring)  But at least it has a 2" thru bore and I can use an Explore Scientific coma corrector with the adjustable top.  The difference in the position of the laser spot between "no thumb screws, push the laser against the focuser" vs using thumbscrews is about 1/4".  And there's a further 1/8" variation depending on how I tighten the thumbscrews.  Also, the coma corrector plus 2" eyepiece is REALLY heavy, so I'm worried that the tilt of that will be different than the laser collimator even if I try to be consistent with the thumbscrews.

 

What should be my procedure when trying to collimate this?  Maybe collimate with the coma corrector already installed?

 

Suggestions on how to "fix" the eyepiece holder?  Maybe put two strips of tape down 120 degrees apart, and then drill and tap a new hole opposed to these two strips, and halfway down the length of the eyepiece holder.  Kind of a poor man's parallelizer.



#2 Jehujones

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Posted 30 November 2022 - 08:58 PM

You shouldn't have to "fix" the focuser as long as you are registering all accessories exactly the same way.

Starting with your collimation tools and repeat for anything put in the focuser. My method for dual thumbscrews has been to snug the first one and tighten the second one always in the same order.



#3 photoracer18

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Posted 30 November 2022 - 09:24 PM

There were at least 3 and maybe 4 Vixen comma correctors for the R200SS. And they mount on the inner part of the drawtube and don't cost you in-focus. I had one of those with Comma Corrector 2. I had a version 1 Paracorr at the time but never considered using it on that scope. I suggest collimating it without the comma corrector.



#4 Alrakis

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Posted 01 December 2022 - 02:30 AM

If this is the original Vixen focuser try this:

 

Baader 2" Deluxe Ultra Low Profile Photo / Visual Back with M60/M68 Threads # T2-32 2458196

 

It does use up some focus distance, but may be worth it. If it is a problem you can look at the solution I implemented.

 

https://www.cloudyni...tor/?p=12223504

 

Chris 



#5 briansalomon1

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Posted 01 December 2022 - 08:49 AM

I always seat collimation tools exactly the way I seat all my eyepieces. That gives my collimation alignments their best chance of resulting in a better image in them.

 

Of course,  you've already read through "How To Collimate Your Newtonian" which is pinned to the top of the Reflectors forum, or the equivalent.

 

On a night with good seeing, I would suggest collimating without the coma corrector and then check the diffraction pattern on a star.

 

http://www.loptics.c.../starshape.html

 

If the star test looks good, I would insert the coma corrector and take a long look at Jupiter at ~300X and compare the sharpness/detail at the edge of the FOV with how it looks in the center.

 

The coma corrector will increase the required collimation precision significantly and will magnify collimation error, mainly towards the edge of the FOV. Assuming the mirrors are cooled off and everything is tight etc... you should be able to get a good idea whether or not your focuser is solid enough for you, the way you use it.

 

Speaking just for myself, I would want quite a bit of proof I really had a problem that was significantly affecting performance before I made any changes.



#6 bokemon

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Posted 02 December 2022 - 02:04 AM

I always seat collimation tools exactly the way I seat all my eyepieces. That gives my collimation alignments their best chance of resulting in a better image in them.

 

This is the reason for me starting this thread.  There is no way for me to know if a laser pointer is going to "seat" the same way as a HEAVY coma corrector plus 2" eyepiece.

The "seating" issue primarily affects secondary alignment.

The star test adjusts primary alignment.



#7 bokemon

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Posted 02 December 2022 - 02:11 AM

If this is the original Vixen focuser try this:

 

Baader 2" Deluxe Ultra Low Profile Photo / Visual Back with M60/M68 Threads # T2-32 2458196

 

It does use up some focus distance, but may be worth it. If it is a problem you can look at the solution I implemented.

 

https://www.cloudyni...tor/?p=12223504

 

Chris 

good idea.

I might also try the APM fastlock with M68 to M60 adapter.



#8 briansalomon1

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Posted 02 December 2022 - 07:35 AM

This is the reason for me starting this thread.  There is no way for me to know if a laser pointer is going to "seat" the same way as a HEAVY coma corrector plus 2" eyepiece.

The "seating" issue primarily affects secondary alignment.

The star test adjusts primary alignment.

When I was testing the collimation stiffness of my telescope, I wanted to know how much flexure there was when I had my  heaviest eyepiece weighs in, which is ~2.25lb.

 

Simply seating the laser collimator and aligning the return beam to the aperture gives you an alignment reference spot at ~2X the focal length of the telescope. If you move the telescope around as you would under normal use you can observe how much the laser spot moves. If you don't have a very narrow laser aperture, or don't want to make one, you can simply steer the beam away form the aperture to some other convenient spot.

 

If you hang a weight on the focuser the approximate weight of the heaviest eyepiece/coma corrector and repeat the test, it will give you a very good idea how your actual alignment is holding under actual use.

 

Here's a picture of my telescope while I was making a measurement. https://www.cloudyni...e/138681-strut/

 

Here's a picture of the laser beam returning to the 1mm aperture. https://www.cloudyni...0920-190412512/ The telescope remains aligned like this no matter what I do.

 

It is possible to calculate the differences in alignment sensitivity of the focuser/secondary/primary. It is simpler and makes more sense to me to just make the entire telescope hold collimation to better than the required alignment.

 

Once you've done that, and have your primary mirror cooled off, you can stop worrying about collimation and enjoy your telescope.


Edited by briansalomon1, 02 December 2022 - 08:10 AM.


#9 AaronH

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Posted 03 December 2022 - 08:39 PM

The 2" eyepiece holder on that is not very good, being just a tube with two tapped holes near the top for some thumbscrews.  (Not even a brass compression ring) 

A compression ring will almost always make registration in a focuser less reliable. I immediately remove them from my focusers and replace them with nylon screws as a non-marring alternative.

 

The compression ring will "float" the eyepiece, coma corrector or collimation tool in the focuser, and it will tilt whichever way gravity pulls it, or whichever unpredictable way the ring forces it.

 

Compression rings are great for gripping accessories hard to stop them falling out. They're excellent at preventing marring. They also help centre the accessory in the focuser drawtube. But this all comes at a cost in terms of repeatable registration.

 

If you have two screws spaced 60-90 degrees apart, with no compression ring, then the screws should push the accessory into the wall of the focuser drawtube quite reliably. If the collimation tool is well-machined with a straight edge, it should reliably sit in the same place flush against the side of the tube. The coma corrector should sit the same.

 

However, with a heavy load (e.g. when imaging), some degree of tilt can still occur as the scope slews around. One trick I use with shorter coma correctors is to add some 2" extension tubes to the end (where they are threaded for a 2" filter). I lengthen it with extension tubes until it almost reaches the end of the drawtube when inserted at its usual position. This reduces the chance of tilt, because any play between the side of the drawtube and the coma corrector is now over a much longer distance.

 

You could possibly try the same thing with the collimation tools sitting in a visual-back extension tube (like the ones that GSO makes). But honestly, if the laser collimator is machined well, it should push just fine against the side with the screws, and register consistently. My Farpoint laser has registered fine in every focuser I've tried it in.



#10 Nemo51

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Posted 07 December 2022 - 06:33 PM

So, the consensus is that collimation is more exact with thumb screws and less exact with compression rings?  Sorry, I’m new to Newts. 



#11 AaronH

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Posted 07 December 2022 - 11:40 PM

So, the consensus is that collimation is more exact with thumb screws and less exact with compression rings?  Sorry, I’m new to Newts. 

There are some previous discussions here:

https://www.cloudyni...mpression-ring/

https://www.cloudyni...pression-rings/

 

The general consensus seems to be that two screws are best, nylon or nylon-tipped screws are the best option for preventing marring, and that the brass split-rings can introduce unnecessary complications.

 

This all applies regardless of scope. It's just that with Newts, the process of collimation brings it to attention.



#12 briansalomon1

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Posted 08 December 2022 - 09:55 AM

I would agree with most of what AaronH wrote, two nylon (or nylon tipped) screws will tend to give slightly more precise registration between the eyepiece or collimation tool and the focuser. It isn't an expensive modification to make.

 

If your collimation tool seats the same way your eyepiece or camera is seats in your focuser, your collimation alignment will match what your eyepiece/camera sees through your telescope. In a newtonian, repeatability between the collimation tool and eyepiece/camera produces best collimation alignment precision using those tools.

 

All of my TeleVue telescopes came stock with a brass compression ring, as does the feathertouch focuser on my dob, and I leave mine stock.

 

http://www.company7....evue/telal.html


Edited by briansalomon1, 08 December 2022 - 09:56 AM.

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#13 Jehujones

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Posted 08 December 2022 - 10:35 AM

This is the reason for me starting this thread.  There is no way for me to know if a laser pointer is going to "seat" the same way as a HEAVY coma corrector plus 2" eyepiece.

The "seating" issue primarily affects secondary alignment.

The star test adjusts primary alignment.

 

 

I would agree with most of what AaronH wrote, two nylon (or nylon tipped) screws will tend to give slightly more precise registration between the eyepiece or collimation tool and the focuser. It isn't an expensive modification to make.

 

If your collimation tool seats the same way your eyepiece or camera is seats in your focuser, your collimation alignment will match what your eyepiece/camera sees through your telescope. In a newtonian, repeatability between the collimation tool and eyepiece/camera produces best collimation alignment precision using those tools.

 

All of my TeleVue telescopes came stock with a brass compression ring, as does the feathertouch focuser on my dob, and I leave mine stock.

 

http://www.company7....evue/telal.html

 

Many of the accessories that I have seen with "compression" rings do not truly compress like the way a compression gland works. I have seen one that did and I'm not familiar with the exact components in the OP's question. The focuser on the AT50 uses a true compression gland and is self centering as it compresses. Many of the ones that I have were simply a brass ring that tightens by being pushed into the groove by a set screw. I suppose that is still a form of compression but it pushes the inserted element against one side of the bore the same way a set screw does. That's why pulling the brass ring and just using the screw(s) doesn't change the registration process. As Brian said, it's all about repeatability and the registration should not change between a lightweight laser and a heavy CC+EP. The change in alignment when switching to heavier components is due to flex. Flex can show up in the focuser draw tube, the focuser mounting plate, the OTA tube wall, the truss connections...etc. If you are experiencing significant flex to affect visual observing, then collimating the scope for the heaviest load will not help you when you switch EPs. I would agree that collimating under load may be beneficial for imaging but even then, if there is significant flex then you will still experience issues as the scope navigates from horizon to zenith. The bottom line is if you're worried about it then you'll have to find the flex.



#14 bokemon

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Posted 10 December 2022 - 09:26 AM

I ended up drilling and tapping an extra set screw hole along the length of the cylinder and used two pieces of tape spread approx 120 deg apart.  It seems to work ok in terms of repeatability

 

Edit: Reminder that this eyepiece holder has no brass compression ring


Edited by bokemon, 10 December 2022 - 09:27 AM.


#15 Tom Stock

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Posted 10 December 2022 - 02:37 PM

The solution is to use a barlow with the laser when collimating primary.  If the laser moves around it wont make a difference.  You just need to have a center spot or circle on the primary and you use it's shadow rather than the laser dot.



#16 Nemo51

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Posted 10 December 2022 - 08:56 PM

The solution is to use a barlow with the laser when collimating primary.  If the laser moves around it wont make a difference.  You just need to have a center spot or circle on the primary and you use it's shadow rather than the laser dot.

Can you point me to an article on this?  Thanks




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