Problems with optical train

[Alexp]

Hey guys,

 

it's been quite a long time since anything has happened here because of all this bad weather, but I guess many of us have the same issue in Europe.

Anyway, last time I was able to image an object, I went for IC 5146 and gathered around 150 minutes worth of "usable" light frames at 60 sec each.

In general, I always have eccentricity issues, my stars are always egg shaped around the corners. I have, unfortunately, no experience or clue whatsoever as to what the problem could be, but I believe from looking at the images that there is a pattern to the star shapes across the images but I wouldn't know what the exact issue in the optical train is so I do not know how to solve it. 

My setup is a SW150PDS with an ASI533mc pro attached on a Celestron AVX. I believe that I am reaching the limits of the Celestron AVX weight wise but I doubt guiding was/is the issue. For that particular night of imaging I had a total RMS error of 0.73" according to PHD2 logviewer (I attached the guiding log here). I do have a resolution of 1,03"/pixel with the ASI533mc pro and the SW150PDS and from gathering as much information as I can I am aware that achieving half of the resolution as max RMS guiding error would be better, but I doubt that this is the main issue here?

The optical train itself - beginning with the camera - is my ASI533mc pro -> filter drawer -> baader MPCC -> screw adapter for baader MPCC to the focuser tube of the SW150PDS. At some point I bought the screw adapter for the Baader MPCC because otherwise when using it with the regular focuser tube it would pinch the baader MPCC, hence the solution was to buy the screw adapter for the MPCC.

To find out if the egg shaped stars are consistent over the entire imaging session I compared the stars of the image with high eccentricity (and a low fwhm) with the stars of the image that had a low eccentricity (and a high fwhm) -> the same issue is visible. 

According to ASTAP there is a tilt in both images, I have attached the image analysis of astap, and also, I have attached the unprocessed .fits files as well in case anyone would want to take the time and look at the two images.

I do believe the telescope was collimated well enough when checking with my cheshire. Is there anyone who could be so kind and look through these images and help me find what the issues are? Is the guiding error too high for the resolution? Could it be that the lousy focuser tube has a tilt and I should get a better one or could it be bad collimation? With my current knowledge (and of that I have almost none) I am suspecting something with the focuser tube or could it have something to do with the screw adapter for the coma corrector?

I hope I have added enough information for someone to look at and any help I could get would be highly appreciated!

Have a nice day in the meantime, all of you!!

Cheers,

Alex

 

 

PHD2_GuideLog_2023-09-10_205727.txt L_IC5146_12627_Bin1x1_60s__-10C.fit L_IC5146_12705_Bin1x1_60s__-10C.fit

[alacant]

Hi

Small sensor notwithsatnding, to get stars that consistent under poor seeing using the sw ds focuser and a budget cc is pretty good.

If you want to take it further without upgrading the cc, maybe have a look at the (7 off) rubber rings in the focuser. They distort and perish easily giving rise to wobble, tilt and slip. Increase the guide speed. This may help the PE blip at around 12s although I doubt this can be removed by guiding.

Otherwise, perhaps simply stay as you are and hope for better atmospheric conditions.

Full sized .jpg of the images attached.

Cheers and HTH

 

 

 

Edited November 17, 2023 by alacant

[Alexp]

Thanks for your analysis and input alacant and thank you for having a look at the images.

Oddly enough I usually always had a higher guide speed, between 1 and 2 seconds. But just to see the difference I once tried a lower guide speed and then I saw that overall with my AVX seemed to have performed at a lower RMS error. Sometimes I still increase the guide speed before an imaging run to see if the RMS error lowers itself, but oddly enough the RMS error always seems better at that low guide speed.

I have a few upgrades planned for the scope, a new spider vane and primary mirror mask with a new secondary mirror holder, a new focuser tube (which one is best for my 150PDS I do not know yet, I'd like to budget around €300 for that), and a motor focuser. I still don't believe the mount is the issue so I'll stick to my AVX after having upgraded and regreased it. I do not know why I haven't thought about the comacorrector being the issue, guess I'll put that one on my to-buy-list as well. 

I do not want to go and get all of the upgrades at once so I rather thought I'd prioritise based on what item will have the most effect on reducing the egg shaped stars (and since I saw one of cuiv's recent videos about his upgrades to his new newtonian, he also talks about the low quality of the focuser, which led me to think that might be an issue with mine as well). I don't know why I haven't thought about the coma corrector itself 🤔..

I will still have a look at the inside of the focuser but I guess I will get the upgrades for the newtonion just as I feel like doing it.

Thank you for your kind words and help! In general I feel quite fine with the images, but I reckon I should be able to do better with these upgrades including a new coma corrector.

Cheers and kind regards

[alacant]

8 minutes ago, Alexp said:

higher guide speed, between 1 and 2 seconds.

What you are quoting there is the guide camera exposure. 

It was the guide rate -as a function of sidereal- which I thought may need attention.

11 minutes ago, Alexp said:

quality of the focuser

Before substituting the focuser, (expense!) dismantle it and remove any rubbish along with flexible components . That should help.

Cheers 

[Alexp]

Ahhh thank you, sry, I misunderstood the guide rate as per your advice. Thank you, I will try that out.

And yes, I will check the focuser and see what I'll find inside! 

Thanks again very much!

Cheers