HELP! Coma Corrector Issue

[Anthony RS]

Hello!

I bought a GSO 2 inch Coma Corrector for my 8inch Newtonian. I'm still having terrible coma on one of the corners which is actually worse than without the coma. The rest of the corners look fine with proper guiding.

Here are some details and what I've done to try and solve the issue:

1- Spacing is 75 mm just as recommended.

2- Used a Cheshire, laser collimator, and a webcam to check collimation.

3- Squared the focuser so that it's orthogonal to the optical axis.

4- Tried another DSLR

Note that once I collimate and double check that I'm collimated with all the tools, I always try to do a star test and collimation appears off (the dark spot isn't in the middle). Last time I tried to collimate the primary using a star but it's really tough to fine tune since due to the focuser's sag, I can't be sure that the star is in the middle of the fov.

My take on the problem is that when I'm using the DSLR with the coma corrector, the weight is moving the focuser away from the center of the optical axis but I'm not sure this if this is the case. I've tried everything and I'm out of ideas.

Below is a test image I took for Lagoon Nebula. Notice the coma on the left top side mostly, while the right side usually appear to have no coma (not the case in the attached image though for some reason, but usually the right side is fine). One more thing I've noticed, the stars on the left corner appear to be out of focus while the rest of the image appears well in focus (used a Bahtinov mask with APT bahitnov focus tool)

There's also another image I took for a distant light to check collimation (since clouds covered the sky as soon as I decided to do a star test). It appears to be fine and I was able to center it better but didn't capture the image.

Please let me know if you have any idea what's causing this. My number one suspect is the focuser's sag preventing me from doing accurate collimation although everything appears to be normal while collimating. I'm giving this another 2 weeks of my time, if it doesn't work I might quit astrophotography till I'm able to afford  an APO.

One more thing that needs to be added, when using the GSO CC I had to move the primary mirror up the scope by around 2 cm to be able to achieve focus.

 

Clear Skies!

[david_taurus83]

What scope exactly are you using and what focuser has it got?

[Anthony RS]

Just now, david_taurus83 said:

What scope exactly are you using and what focuser has it got?

It's a Celestron AVX 8 inch. 8 inch newtonian, f5. The focuser is the one that came with the scope. It's a Celestron AVX 8 inch.

 

[alacant]

1 hour ago, Anthony RS said:

the weight is moving the focuser

Not sure; post a photo of it.

Is it the old fat 2 1/2" single speed rack and pinion focuser? If so, there are two grub screws (you don't normally noice them) on the opposite side of the focuser to the focus knobs that can be used to take up the slack. Loosen the pinion (4 screws which hold the knobs) and tighten the grub screws until there's a firm grip and you can't wobble the focus tube. Tighten the pinion back to just take up the backlash. Re-collimate and you should be good to go.

I've some photo somewhere if you get stuck.

HTH and clear skies

 

[david_taurus83]

Is it the standard Crayford focuser? If so, tbh, they ain't the best and their certainly not designed to take the weight of a DSLR. I upgraded mins to the low profile one to achieve focus but even then I struggled with tilt in the system. I bought a Baader MPCC and it certainly helped but I still had coma on one side of the image but maybe not as bad as yours. Your coma still points to the centre whereas mine all pointed in the same direction. I'd take the 75mm spacing with a pinch of salt and experiment with more and less. I have a refractor now for imaging and the flattener and camera all screw together so more or less eliminate tilt issues but the quoted spacing I needed to use was no where near right so I had to experiment. You may find a sweet spot but at the same time you may have to accept that it might not completely eliminate it right to the edges. DSLR chips are quite big when you compare them to 'proper' astro CCD's. It's a frustrating hobby indeed! But the challenge is also what makes it so interesting!

[Anthony RS]

Just now, david_taurus83 said:

Is it the standard Crayford focuser? If so, tbh, they ain't the best and their certainly not designed to take the weight of a DSLR. I upgraded mins to the low profile one to achieve focus but even then I struggled with tilt in the system. I bought a Baader MPCC and it certainly helped but I still had coma on one side of the image but maybe not as bad as yours. Your coma still points to the centre whereas mine all pointed in the same direction. I'd take the 75mm spacing with a pinch of salt and experiment with more and less. I have a refractor now for imaging and the flattener and camera all screw together so more or less eliminate tilt issues but the quoted spacing I needed to use was no where near right so I had to experiment. You may find a sweet spot but at the same time you may have to accept that it might not completely eliminate it right to the edges. DSLR chips are quite big when you compare them to 'proper' astro CCD's. It's a frustrating hobby indeed! But the challenge is also what makes it so interesting!

I'm sure it's not from the spacing since I'm having the same issue even without the CC. i know it won't be perfect and I'd expect some Coma to remain near the edges but this is far from acceptable. The left corner actually looks worse than not using the CC.

[Anthony RS]

3 minutes ago, alacant said:

Not sure; post a photo of it.

Is it the old fat 2 1/2" single speed rack and pinion focuser? If so, there are two grub screws (you don't normally noice them) on the opposite side of the focuser to the focus knobs that can be used to take up the slack. Loosen the pinion (4 screws which hold the knobs) and tighten the grub screws until there's a firm grip and you can't wobble the focus tube. Tighten the pinion back to just take up the backlash. Re-collimate and you should be good to go.

I've some photo somewhere if you get stuck.

HTH and clear skies

 

Yep that's the focuser. I already tried to tighten the grub screws but without loosening the pinion (4 screws on the bottom). I'll try that. Thanks!

[david_taurus83]

3 minutes ago, Anthony RS said:

I'm sure it's not from the spacing since I'm having the same issue even without the CC. i know it won't be perfect and I'd expect some Coma to remain near the edges but this is far from acceptable. The left corner actually looks worse than not using the CC.

Have you bought the GSO new? Can you send it back and swap for the Baader MPCC or the Skywatcher 0.9? These 2 seem to be popular? Failing that it may need an upgraded focuser. Some have the ability to adjust the tilt out of them.

[Anthony RS]

Just now, david_taurus83 said:

Have you bought the GSO new? Can you send it back and swap for the Baader MPCC or the Skywatcher 0.9? These 2 seem to be popular? Failing that it may need an upgraded focuser. Some have the ability to adjust the tilt out of them.

Ye I bought it new and that was a while ago so I don't think I can send it back. The thing is that if I get 100% sure that the problem is the focuser, I'll invest in a good focuser but I can't afford spending more money now without actually fixing the issue ?

 

[alacant]

1 hour ago, Anthony RS said:

without loosening the pinion

Ah, ok. That's probably it then as if not, the pinion prevents the focus tube moving and mating against the two Teflon strips. It is also fully adjustable for tilt but collimating should take care of that anyway; simply make sure the three sets of push-pull tilt screws are adjusted fully home against the baseplate.

 I have the same focuser on my sw250 and -foolishly- swapped it for the dual speed Crayford having been persuaded that for ap I needed fine focus. What a waste of time and money that was. 

The other item you need (essential) is the compression collar and in so doing, the ability to move your primary mirror back to where it should be. Attached is my rebuild...

HTH

[Waldemar]

The image shows some elongation of he stars pointing to the center which tells your spacing is too short.
Baader's delrin rings (https://www.baader-planetarium.com/en/baader-t-2-adjustmentspacer-rings.html) provide a possibillity to go longer  bit by bit, by adding more thin rings between the CC and the camera.

Often this is just a matter of tenths of a millimeter. To do the spacing right means you will have to experiment. What the producers say is just an average.
The best way to get it exactly right is with an adjustable spacer like this: 
https://www.baader-planetarium.com/en/baader-varilock-29-lockable-t-2-extensiontube-20-29mm-with-spanner-tool-(t-2-part-25y).html
But there is also an issue with sagging, for the elongation is not the same in all corners. On the right side it is definetly less, but still visible.

 

[Anthony RS]

59 minutes ago, Waldemar said:

The image shows some elongation of he stars pointing to the center which tells your spacing is too short.
Baader's delrin rings (https://www.baader-planetarium.com/en/baader-t-2-adjustmentspacer-rings.html) provide a possibillity to go longer  bit by bit, by adding more thin rings between the CC and the camera.

Often this is just a matter of tenths of a millimeter. To do the spacing right means you will have to experiment. What the producers say is just an average.
The best way to get it exactly right is with an adjustable spacer like this: 
https://www.baader-planetarium.com/en/baader-varilock-29-lockable-t-2-extensiontube-20-29mm-with-spanner-tool-(t-2-part-25y).html
But there is also an issue with sagging, for the elongation is not the same in all corners. On the right side it is definetly less, but still visible.

 

I'm having the same issue without using the CC so I don't think spacing is the problem however I will try to experiment more with the spacing maybe that works.

[Anthony RS]

4 hours ago, alacant said:

Ah, ok. That's probably it then as if not, the pinion prevents the focus tube moving and mating against the two Teflon strips. It is also fully adjustable for tilt but collimating should take care of that anyway; simply make sure the three sets of push-pull tilt screws are adjusted fully home against the baseplate.

 I have the same focuser on my sw250 and -foolishly- swapped it for the dual speed Crayford having been persuaded that for ap I needed fine focus. What a waste of time and money that was. 

The other item you need (essential) is the compression collar and in so doing, the ability to move your primary mirror back to where it should be. Attached is my rebuild...

HTH

The compression ring is a great idea actually! I will be able to get the primary back where it was since I'd have enough inward focus then, and it should center the Collimation tools and the DSLR perfectly as opposed to what I currently have. I guess I'll try that although it's going to be hard to find it where I live so I'm going to have to order it and wait for the shipping ? Hopefully that would solve my problem.

[Anthony RS]

18 minutes ago, Anthony RS said:

The compression ring is a great idea actually! I will be able to get the primary back where it was since I'd have enough inward focus then, and it should center the Collimation tools and the DSLR perfectly as opposed to what I currently have. I guess I'll try that although it's going to be hard to find it where I live so I'm going to have to order it and wait for the shipping ? Hopefully that would solve my problem.

one question, would the TS Optics 2" Adapter with compression Ring work with this focuser and which brand did you use?

[alacant]

Scopestuff have it: M60x1 male to 2". They've a few left but have taken it from their listings. Send them a photo of the focuser to nail it with them. I'm sure TS ** have it too, at a price;) HTH.

** This one I think

[Waldemar]

10 hours ago, Anthony RS said:

I'm having the same issue without using the CC so I don't think spacing is the problem however I will try to experiment more with the spacing maybe that works.

That is why you use the CC... if the CC does not work as it should, the spacing is wrong

[Anthony RS]

6 hours ago, Waldemar said:

That is why you use the CC... if the CC does not work as it should, the spacing is wrong

I tried different spacings. The best one was the one I'm already using ? The CC is working fine on the right side of the image but not the left side which is why I'm suspecting that's it's something wrong with the optical axis. 

[Waldemar]

You are right about the optical axis. probably some sagging in your focusser, or a bad spacer or adapter. Could also be play in your camera T2 adapter.
You say you tried different spacings, but the difference between good and bad or not so good is just tenths of millimeters, not millimeters.

Normal spacers go in steps of 5 mm or so and that are far too big steps, that is why I advised the Baader Varilock.
To get things really right in AP costs a lot of effort and money...

[Davey-T]

Try  rotating the camera and also shooting straight up to see if the effect alters.

Dave

[Anthony RS]

Just now, Davey-T said:

Try  rotating the camera and also shooting straight up to see if the effect alters.

Dave

I've tried rotating the camera with no effect. I also tried another DSLR without any effect. I'll try to shoot straight up though and check if anything changes. 

[Anthony RS]

31 minutes ago, Waldemar said:

You are right about the optical axis. probably some sagging in your focusser, or a bad spacer or adapter. Could also be play in your camera T2 adapter.
You say you tried different spacings, but the difference between good and bad or not so good is just tenths of millimeters, not millimeters.

Normal spacers go in steps of 5 mm or so and that are far too big steps, that is why I advised the Baader Varilock.
To get things really right in AP costs a lot of effort and money...

The image attached is why I'm quite sure it has something to do with the optical axis. I've traced the direction of the coma from all corners which should lead me to the center of the axis if I'm not mistaken; and the result is that it's way off the center of the image which should imply that my collimation is way off, but according to all my collimation tools, collimation is spot on.  This is frustrating ?

[Anthony RS]

Just now, Anthony RS said:

The image attached is why I'm quite sure it has something to do with the optical axis. I've traced the direction of the coma from all corners which should lead me to the center of the axis if I'm not mistaken; and the result is that it's way off the center of the image which should imply that my collimation is way off, but according to all my collimation tools, collimation is spot on.  This is frustrating ?

Plus the excess vignetting on the left side kinda confirms it's the optical axis that's the issue. Am I making sense? ?

 

[Anthony RS]

On 20/08/2018 at 02:13, alacant said:

Scopestuff have it: M60x1 male to 2". They've a few left but have taken it from their listings. Send them a photo of the focuser to nail it with them. I'm sure TS ** have it too, at a price;) HTH.

** This one I think

I could only find this compression ring https://www.amazon.co.uk/TS-Optics-compression-Skywatcher-TSM54-2/dp/B00EE2CJ86 on amazon. All the other sellers don't ship to my country. Will this one fit my Celestron drawtube is it specifically made for a Skywatcher? Are they different?

[david_taurus83]

16 hours ago, Anthony RS said:

Plus the excess vignetting on the left side kinda confirms it's the optical axis that's the issue. Am I making sense? ?

 

I think you've pretty much figured out the issue. A small tilt shouldn't put you that far off centre. I know you've said you've centred and collimated the optical path but something is clearly not right. I think you need to remove the focuser and start again from scratch. I followed the below tutorial and it got me pretty close, as much as my patience would allow!

 

 

[Anthony RS]

30 minutes ago, david_taurus83 said:

I think you've pretty much figured out the issue. A small tilt shouldn't put you that far off centre. I know you've said you've centred and collimated the optical path but something is clearly not right. I think you need to remove the focuser and start again from scratch. I followed the below tutorial and it got me pretty close, as much as my patience would allow!

 

 

I've tried it more than 5 times. I followed the same tutorial.