Optimising an SCT for APS-C DSO Imaging - Bright Star Reflections - Forums

Topic starter

Feb 12, 2023

I would very much like to optimise my trusty old C8 SCT for Deep Sky Imaging. I appreciate that I am never going to match the perfromance of an APO Triplet but that's no reason not to aim for it. It's a great all rounder and I have the benefit of attaching a Hyperstar. I love that I can shoot close up planetary, planetary nebula, galaxies and wider nebula all with one set up.

I hope to tune my scope in every way possible and am thinking of starting a thread on each topic. There will be those out there that think I am wasting my time and I politely ask those not to leave any comments. I am hoping that those with specific knowledge of the issues to overcome are able to come forward with practical advice to help those of us who wish to pursue a path to self destruction!

I have recently moved to an APS-C size colour chip (Risingcam IMX571) which has highlighted some of the shortfallings of my scope. Firstly, this has led me to fit a Starizona SCT Corrector IV - 0.63X Reducer / Coma Corrector Starizona. The Celestron reducer just didn't cut it.

Since fitting the Starizon reducer, when shooting broadband RGB I am often seeing these strange 'diffraction loops' eminating from bright stars on the edge of the FOV. It doesn't always appear in my initial subs and so simply re-framing is not a bullet-proof way of preventing it from occuring. It is sometimes not evident until the images are stacked and processed.

Is this something that others have seen with this reducer and an APS-C chip?
Is there anything I can do to eliminate it?

Helpful Engaging

Steve Spring

Topic starter

Feb 13, 2023

Here's a 15 sec sub of The Pleiades. The 'loops' are relly extreme.

Any pointers would be greatly appreciated

Ethan Pearson

Feb 13, 2023

Hey Steve!
So I have the C6 running an ASI071MC and it has the same rings. My own research into it is I found that the baffle down the middle is reflecting the light from those bright stars just right to cause the loops. A couple of things ive tried was painting some sand paper and sliding it to the inside of the tube to help break up that reflection. This works the best but its not an ideal fit, if the paper isnt perfectly round then the stars will match that shape. I ended up getting some Musou black paint that absorbs 99.4% of light. There is still some glare but its a lot less noticeable.

Hope this helps!
Ethan

Helpful Engaging Supportive

Steve Spring

Topic starter

Feb 13, 2023

Hi Ethan,

Thank you. I really appreciate you taking the time to reply.
That would make sense, I've tried alot of different things and it just wasn't present in any of my images of the last two years using an ASI533mc. The step up in chip size has certainly brought out any issues with my scope.

My tube has been flocked, but I never thought about the baffle. I will be taking close look at it now though!
I've tried moving the optical train with reference to the back of the scope, an aperture mask on the outside of the corrector plate and moving the filter position but none of which have made any difference.

Taking the baffle apart is not something I'll rush into but an opportunity may arise. I'm thinking of starting another thread about mirror flop.
I would like to tune in my back focus and tilt but the focus mech on the C8 is so inconsistent.
I would purchase a crayford focuser but have no way of locking the mirror so everything changes after a meridian flip anyway

Helpful Respectful Engaging

Steve Spring

Topic starter

Feb 14, 2023

I've taken a look at the baffle of my scope. At first glances I assumed that it had been flocked because it had a very dull coating. However, following some reading up I think it is the stock grey coating.

There cetainly seem to be alot of reflections.

Does anyone else out there have any experience of flocking the baffle and what they found best please?

Frank "Voloire"

Feb 14, 2023

This is my C6, same boat

https://www.astrobin.com/z6qgey/

Didn't find any solution yet.

CS

Steve Spring

Topic starter

Feb 24, 2023

Well, I m categoriclly not advising anyone else to follow my own pursuit but I am happy with the preliminary results and wanted to feedback. Thanks Ethan for point+ing me in the right direction.

I have stripped down my scope (terrifying, do not undertake his unless you are well prepared, you are more likely to cause problems than fix them, i am a compulsive tinkerer but i do plan, prep and research and accept the risks).

I have painted the inside and outside of my baffle tube, firstly with =searchliteSennelier 759 mars black abstract matt acrylic paint and then a top coat of Stuart Semple Bacl 3.0. In, hindsight, I wish I had looked into using traditional flocking materials. The paint is excellent straight on but is more reflective at low incident angle. I prefer the flocking material but fitting that into the baffle cleanly is not for my skills. I can't imagine how that would be possible?

This thread was very informative https://www.cloudynights.com/topic/716767-an-effective-black-paint-for-telescopes/page-4?hl=+sennelier

Whilst i had the scope apart I have cleaned the mirror and corrector and lined the inside of the tube with flocking material.

I now have a before and after shot of Pleaides and the difference is night and day.

i don't believe this will hve entirly solved the issue of internal reflections but should help in all but the most extreme of targets. I did experience a reflection pattern with Alnitak just out of shot but the pattern is more diffuse and was lost when reframed.

This will need futher testing but happy witht the results.

Helpful

Ethan Pearson

Feb 24, 2023

Steve Spring:
Well, I m categoriclly not advising anyone else to follow my own pursuit but I am happy with the preliminary results and wanted to feedback. Thanks Ethan for point+ing me in the right direction.

I have stripped down my scope (terrifying, do not undertake his unless you are well prepared, you are more likely to cause problems than fix them, i am a compulsive tinkerer but i do plan, prep and research and accept the risks).

I have painted the inside and outside of my baffle tube, firstly with =searchliteSennelier 759 mars black abstract matt acrylic paint and then a top coat of Stuart Semple Bacl 3.0. In, hindsight, I wish I had looked into using traditional flocking materials. The paint is excellent straight on but is more reflective at low incident angle. I prefer the flocking material but fitting that into the baffle cleanly is not for my skills. I can't imagine how that would be possible?

This thread was very informative https://www.cloudynights.com/topic/716767-an-effective-black-paint-for-telescopes/page-4?hl=+sennelier

Whilst i had the scope apart I have cleaned the mirror and corrector and lined the inside of the tube with flocking material.

I now have a before and after shot of Pleaides and the difference is night and day.

i don't believe this will hve entirly solved the issue of internal reflections but should help in all but the most extreme of targets. I did experience a reflection pattern with Alnitak just out of shot but the pattern is more diffuse and was lost when reframed.

This will need futher testing but happy witht the results.

oh yeah that is much better! And if those bright objects have been reduced that much you should be able to image just about anything now!

chefjedidiah

Feb 25, 2023

Hi Steve,

I too must be crazy then because I attempt the same thing with my setup. LX200GPS 10" Wedge + QHY268C

I've always just suffered with the internal reflections as part of SCT life.

There's just something about imaging at that long focal length that is so elusive, but when you get a good result it's so rewarding.

Clear skies

Jed

Supportive

Steve Spring

Topic starter

Feb 25, 2023

Hi Jed,

Definately, I can't help wanting to focus in on the detail in many of the Nebula and of course the SCT is great on galaxies.
It comes with it's challenges but I'd really like to optimise it.
I'm hoping to pick up an external auto focuser, not sure it will make alot of difference but only one way to find out

I've been working on collimation and started dabbling with Metaguide which has potential.
I'd really like to obtain a flatter field. I have the Starizona reducer but still have some coma and tilt to work out.
I'm hoping the focuser will help with consistency.
My mount has been tuned and I am able to get 0.4rms on a good night but normally is at 0.6rms, there is little more room for improvement here and it is pretty much matching my seeing now anyway.

I've managed to get imaging this evening and will hopefully be gatting data on M106. I have some data from before the baffle mod which has internal reflection so I'll be keen to see the difference.

Clear Skies

Steve

Helpful

Related discussions

Starizona SCT Corrector IV - 0.63X Reducer / Coma Corrector (Backspace requierments)?

SO I have an older C8 and I was encouraged to invest in the Starizonia .63 reducer. Info I found was to establish a back space of 90.3 mm "but at 90.3", the telescope does not focaus clearly. The ASIair can't establish the scopes focal ...

Starizona reducer/coma corrector directly relevant for C8 SCT optimization and deep sky imaging.

Dec 22, 2024

Optolong L-PRO and SC8

Hello, I am looking to purchase a broadband filter with my SC8 to improve further my acquisition data. I was wondering if anyone is using it with an SC8 shooting at F10 or if you use it with a focal reducer (celestron or starizona). If you have any e...

Optolong L-PRO filter relevant for improving C8 deep sky imaging acquisition data quality.

Aug 14, 2024