Jump to content

Banner.jpg.692da9ed9af2eace53e1dc94cfc0e912.jpg

Why do my stars look like rings?!


Recommended Posts

Hi all,

Just starting out in DSO imaging, so I am most definitely still in the steep learning curve phase. I've managed to take a couple of OK images previously, but I've come across something really strange on the images I took on Saturday night. Except for a couple of very bright stars, all the stars look like they're dark in the centre, and I can't think what could have caused this.

Now I will say that I am imaging unguided at the moment (should hopefully have the funds for the guide equipment next month - yey!), and the tracking wasn't fantastic either that night - I'd originally planned a visual only evening. I didn't notice any particular problem with dew, although the target, M13, was fairly close to the zenith.

I'm attaching a cropped copy of an unprocessed image to show the effect (no dark frame subtraction/stacking, etc has yet been applied - and please excuse the shocking light pollution - an LP filter is also on the shopping list...). This is actually a separate jpg taken at the same time as the RAW/CR2 images, but the same effect is visible on those as well.

To give some info on my setup - 925 XLT on NEQ6, Canon 600d + focal reducer. Focused using a bahtinov mask as well, then 1 min subs taken at ISO 800.

So, does anyone know what the problem was? Did I do something wrong or was this just the conditions on the night?

Thanks in advance, am completely stumped !

Matsey :)

post-14710-0-16117400-1369732081_thumb.j

Link to comment
Share on other sites

Lol really?! That's a tad embarrassing :shocked: .

I'm wondering how I managed that, as I'm positive it was properly focused with the bahtinov mask. Could it be my method - what I did was slew to Arcturus to sort out the correct focus, then went back to M13 to take the shot without re-focusing - would that have done it?

Thanks guys, really appreciate the quick responses.

Matsey :)

Link to comment
Share on other sites

Lol really?! That's a tad embarrassing :shocked: .

I'm wondering how I managed that, as I'm positive it was properly focused with the bahtinov mask. Could it be my method - what I did was slew to Arcturus to sort out the correct focus, then went back to M13 to take the shot without re-focusing - would that have done it?

Thanks guys, really appreciate the quick responses.

Matsey :)

That method 'should' be fine, provided the focuser isn't slipping during the transition, which it often can when carrying a load. Make sure it's all nice and firmly (but not overly) tightened up :)

Link to comment
Share on other sites

Yes, the dark centre is the shadow of the secondary mirror from being out of focus. Focusing with a Bahtinov is easy and was probably right when you did it. Focusing SCTs by using the flawed moving mirror system is not the best idea. An aftermarked conventional focuser on the back is a good idea or the Feathertouch upgrade for the standard one. In any event one way to reduce the likelihood of this kind of thing is always to focus by pushing the mirror up the tube, never by pulling it down. If you pull it down it may not come all the way at once but may slowly drift on down after focusing. Just check which way the Celestron knob works. I don't know.

You can focus on a different bright star but it should really be at the same altitude as your target since the refractive index of the atmosphere varies with elevation from the horizon.

Olly

http://ollypenrice.smugmug.com/Other/Best-of-Les-Granges/22435624_WLMPTM#!i=2277139556&k=FGgG233

  • Like 2
Link to comment
Share on other sites

Focus with the mask , lock the focuser , then recheck the diffraction pattern before moving to the target.

The focus can , and usually does , change slightly when you lock the tube.

Rather than just using the Liveview screen I take a short , 4-5 second exposure to make sure that I can see the fine spikes , not just the central clump.

Link to comment
Share on other sites

Sorry , missed the part about the OTA , you can't lock yours.

As Olly suggests , an extra focuser will make life a lot easier , I've done just that to my Mak180 with a Revelation and it makes all the difference.

Link to comment
Share on other sites

You can focus on a different bright star but it should really be at the same altitude as your target since the refractive index of the atmosphere varies with elevation from the horizon.

Olly

http://ollypenrice.s...39556&k=FGgG233

Interesting. I'd never thought of that. I suppose that derives from layering of the atmosphere, as if the atmosphere was all of a single composition, the refractive index would be the same all round?

Link to comment
Share on other sites

Thanks all, some really really helpful suggestions here.

Steve - thanks for the correction, I was busy googling how to lock focus, and yes can see that it's not an option on my OTA. I have been using the LiveView to do the focusing, zooming in on the star, it does seem to work well as a method, at least until I sort out proper connection to my laptop, which is the ultimate goal when I'm a bit more comfortable with my setup.

Olly - didn't think about with the mirror going down the tube, I'll definitely make sure I do that next time. And also keeping the altitude the same, I was wondering if focussing would shift depending on where I was pointing, so I will keep this in mind as well.

Regarding the focuser - that is an excellent suggestion and one I have been given serious thought to. Why I've been undecided is that I am planning to get something like an ED80 as my "real" imaging scope - the intention was always to use the 925 for mainly visual, I'm just using it for imaging now until I get the other scope. So it makes me think I'd be better saving the money I'd spend on an upgraded focuser to put towards the imaging scope. No doubt I'll come back and upgrade the focuser at some point anyway, but there are other purchases higher up the shopping list I'd like to get first.

Thanks again, really appreciate the responses, and very relieved it did turn out to be something pretty simple!

Matsey :)

Link to comment
Share on other sites

Interesting. I'd never thought of that. I suppose that derives from layering of the atmosphere, as if the atmosphere was all of a single composition, the refractive index would be the same all round?

Even if the atmosphere were homogeneous I think this effect would apply. The angle of attack of the incident rays to the atmosphere changes and so does the thickness of the atmosphere in the light path.

Olly

Link to comment
Share on other sites

Hi all,

Just starting out in DSO imaging, so I am most definitely still in the steep learning curve phase. I've managed to take a couple of OK images previously, but I've come across something really strange on the images I took on Saturday night. Except for a couple of very bright stars, all the stars look like they're dark in the centre, and I can't think what could have caused this.

Now I will say that I am imaging unguided at the moment (should hopefully have the funds for the guide equipment next month - yey!), and the tracking wasn't fantastic either that night - I'd originally planned a visual only evening. I didn't notice any particular problem with dew, although the target, M13, was fairly close to the zenith.

I'm attaching a cropped copy of an unprocessed image to show the effect (no dark frame subtraction/stacking, etc has yet been applied - and please excuse the shocking light pollution - an LP filter is also on the shopping list...). This is actually a separate jpg taken at the same time as the RAW/CR2 images, but the same effect is visible on those as well.

To give some info on my setup - 925 XLT on NEQ6, Canon 600d + focal reducer. Focused using a bahtinov mask as well, then 1 min subs taken at ISO 800.

So, does anyone know what the problem was? Did I do something wrong or was this just the conditions on the night?

Thanks in advance, am completely stumped !

Matsey :)

Hi,

I regret to inform you that IMHO the solution is not as a simple as it has been made out to be. I have had exactly the same problem as yours and I only use nearly brand new ( budget class ) ED Fracs for imaging not Newts or Maks, the focus is with FWHM asist and visually on the laptop screen I can tell accurately if the star is in critical focus or not, the focuser may slip afterwards but there is not much that I could do with that.

I sorted out the problem once by reducing the detection threshold of the DSS in the Cosmetics module. I feel that since I shoot with DSLR and the captures are inevitably noisy, this may have had something to do with it , the DSS not being able to align the stars correctly and leave a black hole in the middle. The other night the problem came back even though I had over 2 hours of data to work with , ( now I believe that I need a minimum of 4 hours of data from the 1000d to do anything useful as anything else is just covered with noise), so a weak S/N maybe part of the problem as well as a none critical focus. Perhaps the more experienced good guys in the forum could shine a light on this common problem.

A.G

Link to comment
Share on other sites

The problem you have is a focus issue first and foremost .

I can't believe that DSS would misalign subs to give holes in the stars ... !

Believe me you don't need 4 hours of data to get a good result with a DSLR , although more is certainly better.

This was with 1 x 300s and 1 x 600s frames stacked together from a Standard Canon 1000D + ED80Pro.

M313006001200P.png

Yes the final version was better , but as a 15 minute test it kinda proves my point.

Link to comment
Share on other sites

Hi AG, Thanks for your comments, definitely open to any suggestions :)

I will say though that the problem manifested in each single raw image, ie without any processing or attempt at stacking, so at least for me it won't have been misalignment in the stacking process that caused the problem. For now at least I'm inclined to believe the issue was user error (ie, not quite focused), as I have had a few previous attempts with the DSLR none of which had this problem. Will have to wait for the next clear night to check again, but I will report back when I do.

Thanks :)

Link to comment
Share on other sites

I'm going to argue the opposite. Plenty of data is good! This SGL collaboration had 47.5 hours in four telescopes. Nobody ever had too much data!

Olly

http://ollypenrice.s...TEC CORE-X3.jpg

That is an absolutely stunning image Olly, if I ever get to produce something a fraction as good as that, I'd be very happy indeed ! Baby steps though, I'm slowly getting there :)

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.