ring around star

[Joshua Bunn]

Hi all.

Ive noticed a blue ring around bright stars eg sirius. Check the picture bellow. this is sirius at prime focus and the ring around the star would cover about 1/2 the width of the image. (sorry about the crop - its not ideal but i didnt know how else to reduce the image size bellow 200Kb as it was shot in RAW mode).

Im using a lx200 acf 12" and the ring is there whether its through the eyepiece or photographicaly. Does anyone know the source of this.

I was browsing Paul Haese's website, and noticed here (http://paulhaese.net/gsobaffleflocking.html) that he had something simillar. Do you think its caused by the same thing?


thanks
Josh

[Waxing_Gibbous]

Hi josh.
It LOOKS similar and certainly appears to be some sort of internal reflection, but you need to say what sort of scope you are using to be sure! :)

[Joshua Bunn]

thanks Peter - Im using a lx200 12"acf.

could the ring be light getting past the baffle tube thats not from the mirror?
these images show some light leakage when trying to look for it however if i look straight at the secondary mirror i can not see any, BUT, i dont have the diagonal in place with these shots and when it is in place, i cannot see any light leakage as it is shown in these images.

Also could it be bafffle reflections, in image 740 the light from the baffles is just visible if you look carefuly.

I should also point out that just before the bright star is in the FOV, i can see the ring from it in the eyepiece.

thanks
Josh

edit:
I have now been informed that when looking off axis and without a diagonal in place i.e when looking close from the rear of the OTA, i should see what the pictures show. however i shouldnt see it when there is more length off the back of the OTA such as when using a diagonal - which is the case.

[Joshua Bunn]

Hi All

Would someone with a 12" lx200 ACF or R scope mind looking at or photographing the brightest stars in the sky such as sirius, rigel Kent, or canopus. I need to see if other scopes like mine have the same result as the ring that can be seen in the first post.

Ive been asked to check this out by astro optical supplies so they can help judge if its my scope or not.

Thankyou for any help.

Josh

[gb_astro]

Hi Josh.

Sorry I don't have a lx200 so can't help you there.

I did once have a Celestron CPC1100 though.
Like yours I could see light leaking around the outside of the baffle tube as shown in your above photos.
However I never saw any blue rings through the eyepiece
so I am doubting that that would be the cause.

Also I think you would probably see this leakage in most commercial type SCT's.

gb.

[Joshua Bunn]

Hi Gb Astro,

Thanks for your reply, I described what i saw there about the light leaks around the baffle tube to someone with much experience in this field and they said this was normal. I must also say that there were no accesories on the back of the scope when those images were shot, ie no visual back or focuser, and that was taken into consideration when i described what i was seeing re. the light around the baffle tube.

Thanks
Josh

[jenchris]

I got the same sort of thing with my lx90 8"ACF

[Joshua Bunn]

Thanks Jen,

That is interesting. Is your corrector plate clean or good enough?
Can you see where this reflection may be coming from?

Josh

[jenchris]

Someone told me what it was, and for the life of me I can't remember - camera internal reflection or something

[Joshua Bunn]

Thankyou Jen

[Paul Haese]

Joshua, I don't think it is exactly the same problem. That being off axis light getting down past the baffle. It looks more like a standard reflection to me. The blue line is concentric with the star and not distorted.

It looks more like a reflection off you sensor window. What sort of camera are you using? Are you using a reducer? If so which company made the reducer?

A bit more information will make it a little easier to ascertain. Can you reduce the size of the image down to 900 x 900? Try using PS or your stacking program to do that.

[gb_astro]

Paul he says in his OP that:

"the ring is there whether its through the eyepiece or photographically."

gb.

[Joshua Bunn]

Thanks Paul,
It looks similar to what you had with your GSO if im not mistaken:question:.
Ive pretty much rulled out cameras and accesories and i think its the scope. I have not used a reducer or any additional glass in the optical train.

The image in my first post was taken with a canon 550D

The image i have attached to this post is sirius again, a full frame 2 sec. exposure. the optical path is optec tcf-s focuser screwed onto the back of the OTA and an stl 11000 mono camera. the image was taken through a clear filter.

thanks
Josh

[casstony]

Is the 3.25" to sct reducer still in place on the back of the scope? If so check that it is flat black inside rather than shiny anodized (this was a problem on my 9.25 celestron).

[Joshua Bunn]

Ive changed the reducer (not telecompressor) from the standard one that comes with the scope to the one that mounts the optec focuser to the back of the OTA directly and it hasnt changes anything. the one that came with the scope is flat black and i can still see the ring.

Ive been looking down the focuser tube when the scope is pointed at a bright light indoors and can see various reflections.

The dealer who i bought the scope from have contacted meade and meade basicaly said this is normal. But Dr Clay, who ive been corresponding with about this says its not normal after i sent him the images.

I can post the reply Meade gave if anyone wants to see it.

Josh

[Kal]

Is the calculator on this page for working out the distance from the sensor that the reflection is coming from of any use?

http://www.wilmslowastro.com/software/formulae.htm

[Poita]

I'm just waiting for a clear sky to test with my meade, last two nights have been gale force storms, and it is clouding over again now.
The moment I get a piece of clear sky with sirius in it, I'll take a look.

[Paul Haese]

The good news is that it seems to be concentric with Sirius and therefore not a baffle tube problem. So you can rule that out. Good to know that no reducer is in place to cause problems.

Different cameras show the same problem so yes it is likely that the reflection is coming from within the scope.

Off axis reflections will not show up like this. That type of reflection shows up like on the experiments I did.

I tend to think this is caused because Sirius is so bright and some reflection coming back is normal. Could you try imaging Hadar or Acrux or Beetleguese for me? If the reflections are not overly evident in those images or not evident at all it is a minor reflection problem.

[Joshua Bunn]

Thanks everyone for the input.
Kal, I entered some figures into this calculator, 9 micron pixels, f/10 system, and diameter of about 1600 pixels, and it said the replection was 144 mm from the ccd. This would suggest the baffle tube, but i think I already eliminated that by placing a rolled up piece of felt in there. The ring was still there so i removed it. Thankyou all the same.:)

Thanks Peter, It will be interesting to hear the result.

Paul, I will try imaging them when i get my scope out next. Before then, on saturday night i imaged Delta crux, 6 sec exposure (2.75 mag star) and i still saw the ring. Bear in mind, the camera was binned 3x3. see attached image.

Im in contact with the dealer where i bought the scope and they are doing what they can to see if this is normal or not aswell.

thanks
Josh

[Paul Haese]

Yes looking more like an issue rather than just an errant minor problem.

Delta Crux showing is going to mean other stars of the same magnitude will produce the ring. Binning just shows what a 1x1 would look like after several minutes anyway.

Don't bother imaging the stars I requested. Maybe send these images to the dealer and ask them to explain it. If the dealer via Meade cannot explain the reflection then ask to have it replaced.

[Poita]

I can't image tonight, as getting ready to do so, i kind of hurried and I tripped on the cable setting up and snapped the power connector on my mount.

Looking through the eyepiece now at sirius though, I get no ring like your photos. Tried a 24mm and a 12mm and an 8mm, don't get anything like in your photo. Is it that clear through the eyepiece?

[Joshua Bunn]

Don't bother imaging the stars I requested. Maybe send these images to the dealer and ask them to explain it. If the dealer via Meade cannot explain the reflection then ask to have it replaced.[/QUOTE]

Thanks Paul, Ive been in contact with the dealer over there in the eastern states, and they said they would run some tests for me.



Peter, Im sorry to hear about your cable. Thankyou for your efforts. You said you could not see any ring visualy - to clarify, could you see a ring at all? I can see the ring visualy as crisp as they appear in the photo.


To clarify my previous post, I thought 144mm would place me well inside the baffle tube, but that may not be the case. I maybe at the back of the OTA. I will check.

thankyou everyone

[Poita]

I soldered up a new cable, didn't bother polar aligning properly, but took some quick shots with a Pentax DSLR on the back of the 10" ACF, I'm uploading the images now to see what we get.

[Poita]

Here is an image from the 10" ACF of sirius. I didn't align, so the tracking is off.

[Poita]

And a 30 second image. Bothe were taken with the Pentax K-r

[Poita]

Oddly, I did get a similar ring around sirius on my neighbour's C8 very early this morning, but it went away after we cleared the condensation from the corrector plate. I'm not sure if it was identical, but it looked very similar.

[multiweb]

This one is a schmidt reflection from the corrector plate. If the scope is collimated accurately (centering of the secondary) it should fall in the shadow of the secondary and disappear from your picture. You can see in your shot that it doesn't trail with your stars. It's more or less static.

[Joshua Bunn]

Thanks for getting these Peter. If i look carefully in your second image i can just see a blue ring same as mine but fainter - can you see it?



Interesting - thankyou



Here (http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/3373965/Main/3367573) is a link on these reflections aswell. follow the link.

[Joshua Bunn]

Although i can see the ring photographicaly with an exposure long enough to show it up, I can only just see the ring when looking at Rigel Kent - the 3rd brightest in the sky. Stars less luminous than that, I will have to check to see what i can see visually - but my experience is that this is where it pretty much goes away. I will check.

Josh

[Poita]

Yes I can see it if I stretch the image a long way, definitely not visible in the eyepiece though no matter where I look.

I guess a test would be to use a fake star and see if the problem shows up.
It may be something inherent in these scopes, but it shouldn't be as obvious as in your photos.

I'm really interested in tracking down the source.

[EDIT]
I have tried letting the corrector dew up just a little bit, and the ring becomes far more prominent, just like in your images.
It could be a problem with the coatings on your corrector plate, I'd almost put money on it.
Or it could be a thin film of something on your corrector, maybe on the inside?

[Joshua Bunn]

Thanks Peter,
I would agree with you.

I will have to look into the corector plate.

There are some spots on the corrector that only show up when moisture gets on it. Attached are 2 moisture free images of my corrector and 2 moisture laden images showing the CP spots. I have tried cleaning with pure Iso Alcohol but it did not clean sufficiently - the spots are still there. I gues there could be something on the inside, I have never taken the scope appart.

Maybe a trip over east to the dealer or Bintel is in order to see if they can clean it properly - I doubt the coatings have been damaged - I guess the dealer would soon tell me.

Josh

[casstony]

Deposits on the corrector can be water soluble or alcohol soluble; you could try cleaning with purified water to see if the spots are removed.

IME the majority of deposits on the outside of the corrector are water soluble.

[Joshua Bunn]

will do - thanks

[Poita]

I think it just needs cleaning, either be very careful and do it yourself, or find out how much Bintel would charge to do so. I'd reckon there is a fair chance the ring will be minimised then.

[Joshua Bunn]

I think your right Peter, I had a go today and will inspect the CP more thouroughly later and also have a look at the stars to see if the ring is reduced. Warren from astro optical supplies has been good. He had a 8''and 10'' LX200 acf in the shop and he saw the ring increasing in brightness as the appature increased. Next week he has a 12'' coming in and he will look through that one aswell.

josh

[Poita]

I tried again tonight, and as soon as I get the lightest condensation, that ring appears. I never noticed it before as I always use a dew-shield. But literally two minutes after taking the shield off, there it is!

[Joshua Bunn]

Peter, do you think taking the shield off could have caused some stray light leakage? I know you said you let the corrector dew up just a little bit, and the ring becomes far more prominent, but is there a difference between having the dew shield on or off? having said that, i always use a felt linned dew shield.

Josh

[Poita]

I did think of that, and put the shield back on once the ring was evident. It didn't disappear, so in my case the ring is due to refraction of the dew layer on the corrector plate.
No dew, no ring. I guess any layer of gunk on the corrector plate could have the same effect, as could a faulty coating.

[Joshua Bunn]

Thanks Peter

[casstony]

I swapped my C11 for a standard Meade 12" SCT recently and this Meade shows the same ring around bright stars visually. The C11 didn't show it and a Meade 10" ACF that I have doesn't show it.

One difference between the 3 scopes is that the 12" has glossy black paint in the baffle tube which is highly reflective while the other two have flat grey paint in the baffle.

The issue isn't important to me but I thought the report might be interesting to others. I suspect some flat black/grey paint wiped into the baffle from both ends might cure the problem, or perhaps a roll of flocking paper placed in the baffle.

I'd be curious to know if Joshua's old scope has flat or glossy paint in the baffle.

[Poita]

That is really interesting, would be great to see if a rolled up piece of flocking paper removed it.

[PeterM]

I bought Joshuas scope to take out to Leyburn to do my SN search from there and leave my other 12inch in the observatory.
I did 2 weeks of testing once I recieved it, apart from a slight miscollimation I have not seen anything that concerns me optically with this telescope.
I have been using my 12inch LX200R since 2007 and what I see in my 30 second images in that scope are the same as in Joshuas old scope.
I have had a look at a couple of 3rd mag stars visually when collimating and the stars are as I would have expected.
Joshua had cleaned the corrector to what I would say was an excellent job, infact it looks better than my existing scope that gets cleaned once a year.
So I cannot comment on what Joshua saw or imaged other than in my 30 second images I am more than happy.
There is a slight tracking issue that I think I have resolved with a Petersen Ezi Clutch but will know more about that after this weekend at Leyburn. I have added a counterweight set also as well as a Moonlite focuser.
From memory (the scope is in its JMI case) the baffle tube is matt and not gloss, will come back on that to be sure.
Oh yeah I also use an astrozap dew shield that is flocked (nearly spelt that wrong) and a dew zapper built in.

[Joshua Bunn]

Thanks Peter.

I painted the area / circle immediately inside the visual back because i thought it looked shiny, to about 1" deep. I didnt paint the baffle tube. I used what i think was chalk board paint. It made no difference, the ring was still there.

Also, I rolled up some black felt and pushed it to the top of the baffle tube, with a little of the baffle tube left exposed at the visula back end because the felt wasnt long enough. This too made no difference - the ring was still there when looking through the scope.

Thought ide say what had been tried by me.

Josh

[casstony]

Thanks for the info Joshua. I guess the next question is 'are there any 12" Meades that don't show the ring'? Maybe it's inherent to this particular model. Looking into the front of the scope it's clear that more off-axis light gets past the secondary to the rear of the baffle than in the 10".

It's all an exercise of curiosity for me - the scope shows very nice images generally and has given me my best view so far of eta's Homunculus.

[casstony]

Peter, do both scopes show an obvious ring around Sirius or other bright stars, as per the photo's earlier in the thread?

[PeterM]

Hi Tony,
Just waking from 2 days of Flu sorry for not replying sooner. I haven't taken any direct 30 second images of really bright stars, as what I see in my 30 second galaxy/star images looks fine to me. I am working with a Starlight Xpress SXVH9 @F7 so my field is small at 13x11 arc minutes. I will get around to doing that with Joshuas old scope this weekend and mine maybe next week. It would not surprise me if there were a ring around really bright stars given the light collection of the scope/moisture & on the corrector etc and that a few have reported similar, I have seen a similar effect in a 30 second image if a reasonable bright star is nearby . Even so if the galaxy/star images are as good as in my LX200R then I see no need to worry.