Achieving precise Newtonian collimation for flats - Forums

Topic starter

May 14, 2023

Well this has been bugging me for ages, I have a 12" TS ONTC Newtonian used with a QHY268M and a TSGPU coma corrector. When I first got the camera my images corrected well with flats, no issues. I then cleaned the optics, primary and flat and reassembled and collimated. Then the issues started with bad flat correction. After recollimating several times and gradually getting closer I now get a corrected image as shown below (Luminance is the worst).

Collimation is with a Cheshire, followed by the Catseye XLKP autocollimator and checking with a laser, all agree. Everything looks perfectly collimated including an out of focus star central. I have checked the primary centre spot is central (<1mm) and the laser is collimated - no change when rotated. Also checked with and without filters and get the same result.

I'm frustrated by having to wait for a clear sky to re-test after the tiniest of adjustments, so is there a way to align perfectly day or night to get perfect flat correction as I used to do?

The Ocal collimator seems to get good reviews but I don't want to spend more without knowing this would solve my problem.

Thoughts and comments appreciated!
Rob

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Hasan

May 14, 2023

Hi

Can you post a prior image for comparison?

Have you made any filter changes in your image train?

Best

Hasan

Rob Johnson

Topic starter

May 14, 2023 · Edited: May 14, 2023

Hi Hasan,

Here is an image when everything was almost perfect, I'd be happy if I could get back to this, a little DBE gave a flat field. No filter changes.

Thanks,
Rob

Arun H

Frequent contributor

May 14, 2023

Why do you think this is a collimation issue? Have you checked for light leaks? One issue I personally had was light bouncing in from the gap between the tube and primary cell - an opaque shower cap took care of that. What are you using to generate your luminance flats? If the baffling is poor, which I suspect based on my scope it is, something like a light box or flat panel placed on top of the scope may not sufficiently approximate the light incident from the sky. Your collimation procedure is considered the gold standard - if you are doing it right, which it certainly seems you are, I’d look for other problems before spending money on another piece of hardware which has low probability of meaningfully improving your collimation.

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Rob Johnson

Topic starter

May 14, 2023

Im drawn to that conclusion as very slight adjustments to the collimation give varying corrected images. I have the primary end of the tube covered apart from the fan inlet. I haven't looked for light leaks but that's a good idea to investigate. I use a light box but also used twilight flats with the same results. My tube doesn't have any baffling in place, I was wondering about putting a small shield at the sky end of the focuser to help prevent light reflecting from the outer element of the coma corrector towards the sensor.

Hasan

May 14, 2023 · Edited: May 14, 2023

I do not believe that this is a collimation issue - I also do not believe that this is a FLAT frame/panel issue. The reason is that I still see the bright round center in both images - that is surrounded by a darker segment of the image - looks like a filter that is smaller than the camera sensor - do you think that this might be an issue to consider?

The image appears to be sharp.

Best

Hasan

Rob Johnson

Topic starter

May 14, 2023

Hasan:
I do not believe that this is a collimation issue - I also do not believe that this is a FLAT frame/panel issue. The reason is that I still see the bright round center in both images - that is surrounded by a darker segment of the image - looks like a filter that is smaller than the camera sensor - do you think that this might be an issue to consider?

Best

Hasan

I'd be happy if this is not due to collimation but I don't think it is filters, I get this with or without a filter in place. I use 36mm filters which should be fine with the size of sensor.

Hasan

May 14, 2023

Is light pollution any different?

Have you been having focusing issues - unrelated to collimation?

Best

Hasan

andrea tasselli

Frequent contributor

May 14, 2023 · Edited: May 14, 2023

I strongly suspect this is due to a light leak, the signature is rather apparent in the flattened image. I have assembled and disassembled my primary a number of times and flat correction never actually changed. Similar setup to yours.

Rob Johnson

Topic starter

May 14, 2023 · Edited: May 14, 2023

Hasan:
Is light pollution any different?

Have you been having focusing issues - unrelated to collimation?

Best

Hasan

Light pollution is the same bad, Bortle 7/8 which could be an issue if there are light leaks as Arun has suggested, no focusing issues, I use NINA for focusing and always pretty good.

Rob Johnson

Topic starter

May 14, 2023

andrea tasselli:
I strongly suspect this is due to a light leak, the signature is rather apparent in the flattened image. I have assembled and disassembled my primary a number of times and flat correction never actually changed. Similar setup to yours.

The signature is present in the 'good' image which does point to a cause other than collimation, big and bright M13 masks the signature.
I'll definitely investigate light leaks as you and Arun have suggested, it would please me no end if I can find a leak!

AstroNikko

May 14, 2023

I've seen this issue a few times from different folks, including from folks here in the AstroBin forums. In each case it appears to be a collimation issue. In some cases, the issue was resolved by upgrading to a more robust secondary spider.

Most recently though, this issue was described by Cuiv "The Lazy Geek" in a video he posted a few days ago. He talks about a number of issues, but starts to demonstrate the issue this particular issue around the 11 minute mark: https://youtu.be/xMDKOOwVwvM

In Cuiv's case, it was resolved by adjusting the secondary offset before recollimating the scope.

An OCAL would help, but if you don't have one, you can accomplish the same using a mini guide camera and CS lens with the Multi-Reticule Tool in SharpCap. There's a rather good but lengthy write up here for how to use the Multi-Reticule Tool to collimate a Newtonian: https://forums.sharpcap.co.uk/viewtopic.php?t=5610

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Rob Johnson

Topic starter

May 14, 2023

Thanks Nikkolai, it’s good to know others have similar problems, I’ve had a look at the video and will check on my scope in the daytime. I have a guide camera with a lens so that sounds like an interesting option. I’ll report back when I have looked into this more.

Thanks, Rob

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AstroNikko

May 14, 2023

Sounds good! With the lens and camera pointed down the focuser, you can also check for light leaks.

The red circle in the screenshot outlines the end of the focuser draw tube. The larger circle of light is light leaking through the back of the primary mirror cell, around the primary mirror. The innermost circle of light is light that is leaking from the gap between the draw tube and the body of the Crayford style focuser.

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Brian Puhl

IOTD Staff

May 15, 2023

Collimation looks spot on judging by the circle and vignette. This is light leak. Probably from the back of the scope. I have had similar issues. Just put a shower cap over the back. Also, make sure your bias and flats are at least 0.3s or better, I had a vignette issue that wasn't going away until I took longer calibration exposures. As significant as that is tho, that's light leak.

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Wei-Hao Wang

May 15, 2023

It's highly unlikely a real collimation issue. It's more likely caused by stray light. Could be a leak, or more likely, light coming in from the front opening and reflected multiple times through various paths in your OTA and focuser.

Like in Cuiv's video, making sure the secondary captures all the light from the primary is important. However, his method of adjusting the position of the secondary can only ensure that all on-axis light from the primary is captured. The nature of the eyepiece that he used cannot ensure the same for off-axis light. So there could still be problems. A much better solution would be to use a bigger secondary. Also, make sure that there is no reflecting surfaces in the OTA and focuser, even weak reflection. A hood will also help a lot. It blocks unnecessary off-axis light.

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Rob Johnson

Topic starter

May 19, 2023

Update:
I had a small break in the cloud enough to test again after blocking off all light from the focuser/drawtube and my hopes were very high that this would solve it but as you see I had the same result

Moonlite focuser covered with dense black foam and card and primary covered apart from the fan

I also tried a camera with the lens I had (2.1mm) as @Nikkolai Davenport suggested and although SharpCap latest pro version seems to have the circles feature removed, I measured all of the circles in PI to be concentric to 1-2 pixels and no sign of light leaks, all three primary mirror clips in view. (The highlight to the left is black flocking on the secondary edge)

Something I have noticed is that the vignetting on the flat is not concentric which I have adjuated in the past but the adjustment throws the collimation out. This time I managed to make small adjustments to centre the vignetting and maintain collimation nearly as good as before adjustment. So tonight or the next clear spell I will see if this gives any improvement otherwise I'm out of ideas!

andrea tasselli

Frequent contributor

May 19, 2023

If you had a light leak when imaging then you won't be able to solve your issue other than suppressing said leak and moving forward. Said in another way, the light leak may be appearing during imaging time but not when taking the flats (or vice-versa). What king of flat technique are you using, btw?

AstroNikko

May 19, 2023 · Edited: May 19, 2023

Rob Johnson:
I also tried a camera with the lens I had (2.1mm) as @Nikkolai Davenport suggested and although SharpCap latest pro version seems to have the circles feature removed, I measured all of the circles in PI to be concentric to 1-2 pixels and no sign of light leaks, all three primary mirror clips in view. (The highlight to the left is black flocking on the secondary edge)

You can find the Multi Reticules tool under the Reticule tool dropdown in the main tool bar.

The Multi Reticules tool is a beta feature, though. In this case that means that it's only available in builds less than 60-days old. I just checked, and the version of SharpCap I was running (4.0.9538) was older than 60-day, which prevented me from using the tool.

Checking for updates from the Help menu reported that I was running the latest version. I needed to download and install the latest build (4.0.9556) from the SharpCap downloads page before I could use the Multi Reticules tool again.

The only other thing I could recommend trying is to place the dust cover back on the scope, and with the camera and lens pointed down the focuser, go over the scope with a light to see if you get any leaks.

This also might be a case of the secondary assembly maybe not being as rigid as before reassembly, resulting in a shift in collimation with the tube orientation between light frames and flat frames. Maybe go over the assembly again to be sure all fasteners are tight.

I'll be following this thread to see how things work out.

Good luck!

Nikko

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Rob Johnson

Topic starter

May 19, 2023

andrea tasselli:
If you had a light leak when imaging then you won't be able to solve your issue other than suppressing said leak and moving forward. Said in another way, the light leak may be appearing during imaging time but not when taking the flats (or vice-versa). What king of flat technique are you using, btw?

I did take both the flats and the second image with the focuser/drawtube completely covered. I can shine a light across the drawtube during imaging to be sure my covering method works 100%.
I usually take my flats with a lightbox which I have used for several years now, set to ~25k take 15-20 flats, but to check if that could be the cause I started taking twilight flats which gave the same outcome.

andrea tasselli

Frequent contributor

May 19, 2023

Fair enough. What is your pre-processing workflow then (just to rule out other issues here)?

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andrea tasselli

Frequent contributor

May 19, 2023

Incidentally, you could take a lengthy dark frame with the scope cover on to identify light leaks.

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Rob Johnson

Topic starter

May 19, 2023

It's worth checking, I don't want to rule anything out. I calibrate the flats with a flat dark of 5s, the flats for luminance are taken at 3s and then integrate with these settings to create the master flat. Following that I calibrate the lights with a master bias and dark and the master flat (all taken with the same gain settings)

andrea tasselli

Frequent contributor

May 19, 2023

Again, to rule nothing out I would use only master bias-dark (i.e., darks without removing the bias master frame) to calibrate the lights and see what happens. You definitely have an additive signal there where there shouldn't be any. Incidentally, what is what looks like white felt on the secondary body?

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Rob Johnson

Topic starter

May 19, 2023

Ok I’ll give that a try thanks. The “white felt” is just black flocking which is overexposed in the shot, I’ve been all through everything when I first got the telescope to prevent reflections, the tube was flocked opposite the focuser as supplied.