Help for imaging stars

[CraigT82]

I captured some subs of the double cluster last night and just getting round to looking at them.

I think I’ve blown out the stars but as a bit of a newbie to DSO imaging I’m not totally sure how to tell.

Opening up a single raw frame in gimp and using the colour picker tool to hover over the brighter stars in the image the values are maxed out (255,255,255 - 65535 pixel value) so I’m guessing I’ve overcooked the exposures and nothing can save these stars. 

For info I’m it was shot at 102mm aperture f5.6, 120 second exposures at 100 gain. ASI533mc and no filter at all. 
 

Really want to take nice images of stars - GCs , OCs, asterisms etc…. Any help gratefully received! 

[ONIKKINEN]

Some bright stars will always saturate and there isn't really much you can do about it. But so long as the core isn't completely blown white all the way to its halo, then you can still get good colours out of it. Just by looking at your screenshot here i think you'll still get good star colour so probably not much to worry about.

If you just want to focus on the stars in an image then you would probably want to do shorter exposures. Your 533MC is very sensitive and low readout noise, so 60s will almost certainly be more than good enough. Probably even 30s, but now you have to decide how much data is too much data since you'll be taking more of those short exposures. I'd expose for 60s and call it a day.

[geeklee]

13 minutes ago, CraigT82 said:

ASI533mc and no filter at all. 

You'll need a UV/IR Cut filter if you're using the 533MC Pro.  

I'm not familiar with GIMP but it's OK for a small number or brighter star core pixel values to be maxed out in the linear state.  I'd try 60s and even 30s.  Personally I like 60s on the 533MC at Gain 100 with a typical speed scope.

EDIT: Have you debayered that image yet?  Check how it looks stretched after this.

Edited September 20 by geeklee

[CraigT82]

Thanks both. I did calibrate and integrate in PI (only 15 subs though so 30 mins total) and then did colour calibration and gradient removal, then took it into gimp for stretching, and the stars didn’t look *too* bad  (for a first attempt) but there was barely any colour in most of them. Think I might try 60 or 45 seconds next time. 
 

I do have an Astronomik L2 filter to go in but just waiting for the right adaptor to arrive so I can screw it into the camera. Just had to try it all out last night filterless as the camera, EAF and ASIAir all arrived in the morning and it was clear by nightfall so just went for it. 

[geeklee]

2 minutes ago, CraigT82 said:

I do have an Astronomik L2 filter to go in but just waiting for the right adaptor to arrive so I can screw it into the camera.

If it's 1.25" there's usually one in the 533MC Pro box that it screws into and then the whole things screws into the 11mm M42 F-F adapter on the camera.  Apologies if you're aware already and have something different!  

[CraigT82]

8 minutes ago, geeklee said:

If it's 1.25" there's usually one in the 533MC Pro box that it screws into and then the whole things screws into the 11mm M42 F-F adapter on the camera.  Apologies if you're aware already and have something different!  

Ahhh THAT’S what that little ring is! I picked that out the box and wondered what it was for, didn’t click at all. Thanks for telling me! Oh well at least I have another one coming that I can use on the guide camera then and get rid of the 1.25” nosepiece on that. 

[ONIKKINEN]

51 minutes ago, CraigT82 said:

and the stars didn’t look *too* bad  (for a first attempt) but there was barely any colour in most of them.

Try a little cheat code for colour: Arcsinh stretch for when you start stretching the image after all the basic preparations (gradients, colour calibration etc.)

You can try the full 1000 and see how it turns out, if there are no strong colours present in the image other than stars it usually works (with nebulosity probably not). If its too much try something like 300-500. You can also run it again at a very low power after the first go, like a factor of 5 or less and get some extra stretch to the colours. Then once you finish stretching with something else like Histogram transformation you'll notice the stars already have a lot more life to them and they need less saturating to get them where you want them to be.

[CraigT82]

Thanks I’ll give that a go tomorrow and see how it goes 👍🏼

[michael8554]

Is there a histogram in the imaging software ?

Michael

[Elp]

For star colour you normally have to boost saturation a lot in post, raising contrast will dilute/clip the colour as well as star bloat.

[CraigT82]

Coming back to the question of getting nice stars, for imaging star fields, OCs and GCs would it be better to use a lower gain to get a larger full well with longer exposures, in order to avoid saturating the brighter stars?