Elliptical circles around bright stars in long exposure

[graemlourens]

Hi everyone.

First post! Started Astronomy / Astrophotography few months ago, learnt my first hard lessons for example with 'dew', but need some help from the masters to understand a few things. Hope you're willing to help me.

My setup is a Nexstar Evolution 6", Nikon D610, Prime focus.

Photos are generally very good, absolutely no problems with quality.

But 2 days ago i was making photos for stacking, and prime focused on Capella. (Just for sharpening my skills, setting up alignment correctly, handling lightpollution and so on)

With 1600 ISO and 30sec exposure, with or without stacking, i have elliptical circles around (the very bright) capella. 

See image here: 

(this image is composed from 10x 30sec exposures, deepskystacker, just basic adjustments)

My research shows me that this is caused by reflections in my equipment, but as with prime-focus i would really not know where this is happening (no lense)

Can you give me some idea why/when this happens?

Kind regards & thx for any help.

Graem

[VigdisVZ]

Dew?

[graemlourens]

Hey VigdisVZ

No not possible, it was shot from my window inside the apartment shooting out of the open window, and checked the corrector lense after seeing the first image.

But i'm not meaning the very faint gradient circles (thats normal) really just the elliptical unter Capella (in the middle)

Regards, Graem

[graemlourens]

Have to add:

I made slight underpressure in the apartment, so cold air was coming in, and the telescope was acclimating for around 1h.

And to my knowledge air disturbance would not cause that kind of 'nice' rings.

Regards, Graem

[VigdisVZ]

Maybe it's just the vignetting of the optical train that gets stronger with a bright lightsourse. 

Have you tried taking flats to see if this is the case?

[graemlourens]

i haven't.

I will try this at the next clear sky, maybe thats it, sadly currently weather is horrible, but will try asap.

Obviously making such long exposures of a bright star as capella does not make sense, but would still like to find out whats causing this, so i can counter-act on it in future in other situations.

Regards, Graem

[graemlourens]

another question about flats:

as with darks, they should be taken at the same time and the same conditions as the lights.

Do you think it makes sense to partially reconstruct the conditions, make flats and then integrate them to stacking with the original lights or it does not make sense at all?

(Flats do not need nice weather )

Kind regards

[sharkmelley]

You are using a Schmidt Cassegrain.  So there are a variety of ways that stray light can reach the sensor via internal reflections.

To narrow down what is going on, take images with the star in different positions on the sensor and see if and how the pattern of stray light changes.   Apply the same amount of histogram stretch (if any) to each image.

Mark

[graemlourens]

Hello Mark.

Good idea, it is also possible that this is coming from reflexions around me (obviously shooting out of window is not good! but was just for practise)

Will try that trick and see further

Thx for the advice! Regards, Graem

[VigdisVZ]

another question about flats:

as with darks, they should be taken at the same time and the same conditions as the lights.

Do you think it makes sense to partially reconstruct the conditions, make flats and then integrate them to stacking with the original lights or it does not make sense at all?

(Flats do not need nice weather )

Kind regards

Flats is taken when the camera is mounted in the exact same position as when taking an image, meaning, they're best done right before or after, since only then can you make sure everything is the same in the setup. Also, the idea is to catch any dust on the lens/mirror, so it would be useless to take them later as moving the setup would cause spots to fall off or new ones to stick.

Darks can be taken with the camera removed or with lens/telescope cap on, just the ambient temperature is the same. People can build a darks library with a temperature controlled fridge, although I've heard some people discarding older darks and taking new ones.

I think what you need to do here is to start experimenting, like Mark says, and remember that the 6" SCT isn't exatly known for "perfect" flat field And a properly taken flat will tell you a lot about your optical train.

[davew]

I think Sharkmelly has it.

Long fix is to spend a month or two trying out various pointing or flocking regimes. The short answer is stop imaging Capella

Dave.

[graemlourens]

Will do!

And will inform you guys what it was when i've found it.

Thx!, Graem

[sharkmelley]

Coincidentally there is another thread currently discussing a problem which may have the same cause as yours.  Take a look at the links posted in comment #6 on this thread:  http://stargazerslounge.com/topic/228664-semi-circular-artefacts

Mark

[graemlourens]

interestingly, i added the celestron f6.3 focal reducer, and its changed totally (largely reduced, but still there)

Its an unstacked raw that i quickly shot 2 days ago.

But i think i just should ignore it, it only happens on very bright stars, and capella obviously isn't my target for astroimaging

So i guess i'll let this rest and concentrate on taking my time to go to a dark location! You can see in the image how much light pollution i have

[zakkhogan]

could it be collimation ?

[graemlourens]

Hey guys.

This glare has bothered me more and more, as its happening as soon as something gets bright (or with long exposures is forced bright)

Its not collimation, i checked and its dead on, furthermore the scope is brand new.

So i wrote celestron, and they told me that they have never heared of this problem in the nexstar evolution 6 series and it could be faulty and would have to be replaced.

Attached is a video that i made. It even happens in daylight with very short exposures. This video is done f6.3 with i think 1/50th second exposures.

I guess i should send the scope in for replacement/checking?

Or is this a 'common' phenomena with telescopes?

You can also see in the video, that in the center i have a larger circle that is much brighter than the rest, this is also indicating me that smth is wrong?

Love you hear your opinions before i send poor scope on a journey....

Regards, Graem

DSC_2955.mp4.zip

[graemlourens]

My local dealer by chance had another Celestron Evolution 6 (brand new) and we tested it, its the same effect.

But i guess Mark is right, that other Thread http://stargazerslounge.com/topic/228664-semi-circular-artefacts/

is talking about exactly the same issue.

After researching more and more, it seems this is a common problem with celestron's SCT's ? I am surprised the support tells me they've never heared of this problem :/

So i guess flocking will be the only solution or getting a different scope. 

Will keep you all up to date.

Regards, Graem

[Owmuchonomy]

My local dealer by chance had another Celestron Evolution 6 (brand new) and we tested it, its the same effect.

But i guess Mark is right, that other Thread http://stargazerslounge.com/topic/228664-semi-circular-artefacts/

is talking about exactly the same issue.

After researching more and more, it seems this is a common problem with celestron's SCT's ? I am surprised the support tells me they've never heared of this problem :/

So i guess flocking will be the only solution or getting a different scope. .

Will keep you all up to date.

Regards, Graem

They have heard of the problem because I wrote to them with the same issue. I get it on bright objects through my SCT 9.25 especially with the .63 reducer when it can be quite horrific? The only thing that works is as mentioned above, stop imaging bright objects or reduce exposure times. It can be done, see my Flickr link.

[graemlourens]

GOOD NEWS AND BAD NEWS!

Good: its purely due to light entering the rear baffle tube directly (bypassing the primary mirror) and is totally removed with flocking the baffle tube for example with this: http://www.firstlightoptics.com/misc/black-velour-telescope-flocking-material.html

I tried it out, and all elliptical glare has vanished, as well as the allover contrast has improved a lot.

Bad: without disassembling the OTA its very difficult to make this perfect and i havn't been able to do it perfect yet.

This velour is self-sticky so i stuck it to thin wrapping plastic, rolled it up and shov it up the rear letting it unroll inside.

But especially close to the secondary its far from nice and round, and my vignetting has worsened a lot...

I'm running out of ideas how to ge this perfect, but will continue trying as seeing the difference i am very surprised celestron does not do this in the factory (or some other technique)!

Anyone having any good ideas?

[graemlourens]

Ok i got it Calculated the inside tubes diameter as perfectly as possible (btw for the Evolution 6" its exactly 80mm in my case) 

but already connected both ends together to form a perfectly round 'tube' with stickytape. It fits perfectly and snug in the baffle tube.

Now the vignetting is still a little worse, but very acceptable when using the focal reducer, without you do not see much difference.

I am so happy to have finally got rid of these horrible reflections! If anybody needs help just PM me, glad to help.

And if somebody needs a little of the Velour, i really only needed a small piece, so if anybody needs i can easily put some in an envelope and sent it to you.

I will attach a few before/after pictures!

[graemlourens]

Glare totally disappeared

and also a comparison of vignetting before/after

[Cath]

Yeah, Celestron DO know about the design problem they have with their SCT's, but they have NO interest in fixing it. You'll never get an honest reply about it from them.

It's the same reasoning a lot of companies have these days - sod the problems so long as we're making the money we want.

[graemlourens]

I suspect as the 4-6" 'normal' SCT's from celestron are not designed for imaging its not a priority for them as only with like 40mm eyepieces you can see the glare, but if you attach a DSLR its obviously catastrophic for imaging.

Well i'm happy its now solved, and i can continue my AP-testing without worrying about that anymore it was bugging me a lot... like having a broken scope it felt!

Regards, Graem

[Cath]

oh right. I've not seen them mention anywhere that their 6" and smaller SCT are not meant to be used for imaging.

[graemlourens]

Well not officially.

I just suspect that they don't advertise it as imaging equipment and therefore (to our disadvantage) don't take too much care in those kind of problems :/

But i agree with you, none the less, when so many people have the same problem they should react.

The supports reaction was 'just flock it, its not difficult'. I was a little surprised they use their customers to fix their problems

Regards, Graem