Towards a right spacing for coma corrector

[v4169sgr]

All,

Will be very grateful for any insights.

Equipment: 20 cm f 4.5 Newt on an NEQ6; Canon 450D fitted with Baader MPCC (probably type II).

The first image below was taken with the assembly as-is. The second with the addition of two very thin spacers on the camera side of the MCC (the max. I could fit in and still have some thread left!).

Is there any improvement in the second image - accounting for the orientation and slight processing differences? I'm inclined to think not, and even to believe the situation has been made worse.

What do you think?

[Nigella Bryant]

Are these cropped images? Looks to me like tilt rather than coma. Coma would be more uniform around the outer sides of the image whereas yours seem to be more one sided. I  no expert on these things so others with more experience hopefully will give their knowledge on the subject. 

[woldsman]

Not an expert but use the same coma corrector. The standard test is to take an image, rotate the camera/optical train, take another image and compare. If the issue is the same in the same place, you have tilt. 

[michael8554]

1. There are two MPCC spacings, depending on whether you are using M42 or M48 fittings:

2. As I always say in these circumstances, you shouldn't compare stacks of long exposures, which may contain stacking and guiding artefacts.

Look at one fast exposure, stretched if necessary.

3. The second exposure looks worse, could be Tilt or guiding.

Michael

[alacant]

On 13/02/2024 at 22:02, v4169sgr said:

Is there any improvement in the second image

Hi
 

Excellent shots, but if you feel you want to take it further...
Both images are tilted; make sure that the primary mirror is adjusted correctly and ensure that the camera-cc assembly be inserted correctly into the focuser.
Baader cc? Lose the m42 adapter and go with 58mm from the m48 shoulder. If your focuser has a compression ring, avoid clamping across the shoulder of the undercut of the cc. Or just remove the compression ring. Simply dismantling and re-seating usually helps.

But hey, 450d on a 200 f5; if you're satisfied with the images perhaps stay as you are.
Cheers and HTH

Edited February 14 by alacant

[v4169sgr]

Thank you for all the responses! Food for thought.

Happy to acknowledge the limitations of my equipment, however given my motivating interest on photometry, I'd like to see if I can achieve nice circular stellar images throughout as much of the frame as possible. Added to which for me it is as much about the process as the result.

In efforts to eliminate tilt (clearly not successful!) I use a Hotech Laser Collimator to ensure all the optical elements of the OTA are lined up - and crucially use the same focuser configuration and clamping mechanism as I use to hold the camera, adapter and coma corrector. I use a Baader Clicklock Clamp attached to my BDS-NT focuser to ensure circularisation so that I am not playing with unevenly tightened grubscrews. I attach the coma corrector to the camera body using a WO high precision "copper" M48 T-mount, so I am not using the M42 threads.

In the first photograph below I show the MPCC itself without any covers (and in the edge of the picture a stack of two of the thin spacers I am adding). The second photograph shows the MPCC (capped) mounted to the camera body using the M48 T adapter. The third photograph shows the full assembly: I push home the camera + CC assembly as far as it will go into the clamp, then secure it with the locking mechanism.

Which software do you use to determine the tile, by the way? Would it run on Linux? I run Linux Mint (Ubuntu based).

My principal concern though is in determining the correct spacing. I don't think I am particularly close at the moment, and welcome insights. This is the reason I am introducing the 2 mm of spacing, to try to achieve the correct 57.5 mm separation between MPCC and detector, since I am using the M48 thread. I welcome any thoughts.

[alacant]

11 hours ago, v4169sgr said:

software

Hi
If you're able to reach focus with the clicklock, then it looks just about as good as you're gonna get. If not, use a low profile eos-m48 adapter and a 12mm extension tube.
The tilt was measured using Siril under Ubuntu 22.04.3. The same version will run under Mint but if you're interested in the journey as much as the destination, you may wish to build the latest version from source. Testing and reporting bugs helps the developers toward the next release.
HTH

Edited February 15 by alacant

[v4169sgr]

Thanks again. Focus is no issue ... I use a Bhatinov mask to achieve critical focus - and the Pleiades are bright enough that I don't then need to slew to target.

I've taken some measurements ... within the precision of the tools I have available, I estimate 56-57 mm between the image plane and the M48 thread. Might not be spot on with this one and could easily be a mm or so more as I found it difficult to determine with any greater precision where the M48 thread is in the assembly. However this should not be any great surprise since the T adapter and MPCC are obviously designed to work together with a DSLR. So the spacing should be good enough.

I use Siril to preprocess my images already, so quite a revelation that I could simply type in 'tilt' in the command line and see for myself. In answer to a comment above, I verified that the stacked image is in line with (produces the same result as) one of the subs. I've seen stacking issues before - they are not pretty - but that doesn't seem to be an issue here.

If the weather holds up I should be able to carry out some experiments in the next couple of nights with single frames under various conditions.

An obvious question that arises is what sort of results I should *expect* from the equipment I have? Is it really legitimate that the MPCC II doesn't remove all the coma towards the edges of the frames? How precise should I expect the match between the image plane and the detector plane in my DSLR to be?

[alacant]

4 minutes ago, v4169sgr said:

I estimate 56-57 mm between the image plane and the M48 thread.

Therefore, and as your images show, apart from tilt, you are too close. Can't recall exactly but we needed at least 58mm from the shoulder of the m48 thread. The m48 t-ring alone will give you 55mm. With your 2mm spacers that's probably around 57mm, but you're now at the safe thread limit. Baader give 57.5mm. Use 58mm as a starting point. The low profile t-adapter with a set of extension rings makes this easy.

9 minutes ago, v4169sgr said:

is it really legitimate that the MPCC II doesn't remove all the coma

It removes the coma, but introduces astigmatism. If you want out-of-the-box-corner-to-corner over aps-c, perhaps consider the GPU.

Cheers and HTH