Hollow Star Issue

[Wayne76]

Hi Everyone,

 

Recenlty I am start to useing QHY10 and HyperStar to take photo of deep sky objects,

Currenlty I am using C11, Hyperstar, QHY10,

Pls see below pic, one is taking with 20s single frame, other one is with 25s X 30 lights, 25s X 30 dark, 25s X30 bias,

then stack it in DSS, process in Pixinsight

In the single frame pic, my star is sold round, but after I process them in the DSS, the small star become hollow,

 

1) Could it due to focus issue ?

    but in my single frame the all my star is solid,

 

2) Could it be something wrong with what I did in the DSS,

   but before when I process with DSLR images, it didn't had those issue before,

 

Did anyone had this problem before,

 

Thanks

Wayne

19-Dec-2017

 

[Handy_Andy]

I did this the other day, you need to change your stacking settings from kappa-sigma to average and that will get rid of your hollow stars

[Wayne76]

2 minutes ago, Handy_Andy said:

I did this the other day, you need to change your stacking settings from kappa-sigma to average and that will get rid of your hollow stars

Hi Andy,

 

Should I change setting to average on the light, dark and bias as well,

 

Thanks

 

Wayne

 

[Handy_Andy]

I'm no expert, having only recently got into stacking properly myself but from my limited playing with dss, average on the lot seems to be best

[happy-kat]

Are you sure the hollow stars are evident straight after DSS stack or are they there only after process in PI?

[Wayne76]

2 minutes ago, happy-kat said:

Are you sure the hollow stars are evident straight after DSS stack or are they there only after process in PI?

Hi Happy,

 

Yes, as you can see from the photo I post above,

One is with single frame, as you can see the star is at least all solid,

but after I stack them up then it become hollow,

 

Wayne

[happy-kat]

I would check each light frame make sure there is no bad poor image in there. I always use kappa clipping and don't have hollow stars 

After registering any poor light frame score above 4 I choose to remove.

Edited December 19, 2017 by happy-kat

[Wayne76]

24 minutes ago, Handy_Andy said:

I'm no expert, having only recently got into stacking properly myself but from my limited playing with dss, average on the lot seems to be best

I change to average, but it still do the same thing,

[Wayne76]

3 minutes ago, happy-kat said:

I would check each light frame make sure there is no bad poor image in there. I always use kappa clipping and don't have how stars 

After registering any poor light frame score above 4 I choose to remove.

Ok, I will give a try tonight, by going through each light frame !!

 

[Wayne76]

13 minutes ago, happy-kat said:

I would check each light frame make sure there is no bad poor image in there. I always use kappa clipping and don't have hollow stars 

After registering any poor light frame score above 4 I choose to remove.

Forgot to ask you, which program did you use use to check 

[happy-kat]

I don't know what file format your images are in

 

[carastro]

If you are using DSS to stack, check what settings you have on the cosmetic tab.  If it's ticked, then untick it, it is supposed to remove hot pixels, but seems to eat the centre of stars as well.  

Carole 

Edited December 19, 2017 by carastro

[MarsG76]

To me it looks like peak level clipping... definitely post processing issue.

 

[carastro]

I think it's definitely the cosmetic tab, I had a similar conversation with some-one on another forum only yesterday, who had holes in his stars, and after me advising him to re-stack unticking the cosmetic tab, they were perfectly fine and the stars looked pretty much the same as the ones you are showing.  

Carole 

Edited December 19, 2017 by carastro

[Wayne76]

Thank you, White Dwarf, donut effect is gone now !!

Some of the star is not round, it is due to the tracking issue, since my mount is still unguided,

Should have brought the CGEM not the CPC mount in the first place....

I will shorten the exposure time to 15s next time I try,

 

 

Edited December 19, 2017 by Wayne76
Wrong photo

[carastro]

Glad it worked.  BTW my name is Carastro, White Dwarf is my classification on the SGL.

Shame about the mount, is it on a wedge? 

Carole 

 

Edited December 20, 2017 by carastro

[Wayne76]

2 hours ago, carastro said:

Glad it worked.  BTW my name is Carastro, White Dwarf is my classification on the SGL.

Shame about the mount, is it on a wedge? 

Carole 

 

I am so sorry, Carastro, 

My CPC 1100 is on its own, no guide cam or wedge

I did brought Celestron Nexguider, but still haven’t got it to work...

what is the max exposure time do you suggest on just normal CPC mount, 

thank you so much for all your advices

[carastro]

Imaging is very difficult on a mount that only moves in Alt/Az mode.  The hyperstar is fast, but you're going to be cheating yourself of good data by taking such short exposures.  

I have no idea on length of exposure as never imaged on the sort of set up you have.  

Is there a wedge available for this mount?  You would stand a better chance with that, and if you say you have bought a guider for it, it is obviously capable of guiding, you just need to make it work, but it will struggle if not in wedge mode.

It looks like a nice piece of kit you have but just not really suitable for long exposure imaging with the current mount.  

Might be worth starting a new thread on this topic and hopefully people who have used the same or similar kit can give you better advice. 

Carole

Edited December 20, 2017 by carastro

[Wayne76]

Yes wedge is available on the CPC, but it really required lots of time just to do the set up,  that is why I brought the HyperStar system....?

 

i will start another topic in the equipment section 

 

Thank you Carastro !!

[The Admiral]

Wayne, welcome to the world of Alt-Az imaging! EQ mounts seem to be king when it comes to imaging, a wedge on your mount would go some way there, but imaging on an Alt-Az mount is by no means without reward. The answer to this is to stack many, and I mean many, short exposure subs. The maximum length of the sub is limited by field rotation, and this will depend on where you are looking in the sky; in some cases it will be seconds, and at the most probably 90s. The other issue you will encounter is the accuracy of the tracking that the mount can achieve during the exposure time, and it is not unusual to find a percentage of subs which show star streaking. You should reject these and not stack them. This would be a severe problem with the C11 because of its long FL, but with Hyperstar you have a very fast, short FL set-up, which greatly helps. The light gathering power of your C11 compared to my 102mm refractor is huge!

Might I suggest that you have a look at the long thread "The No EQ Challenge" (https://stargazerslounge.com/topic/228101-the-no-eq-dso-challenge/) where you will find a lot of information about doing Alt-Az imaging, and you can see what others (mainly with short FL refractors) achieve.

Good luck, and keep us informed of progress.

Ian

PS. Visited Cape Town some years ago. Fabulous country.

Edited December 20, 2017 by The Admiral

[Wayne76]

40 minutes ago, The Admiral said:

Wayne, welcome to the world of Alt-Az imaging! EQ mounts seem to be king when it comes to imaging, a wedge on your mount would go some way there, but imaging on an Alt-Az mount is by no means without reward. The answer to this is to stack many, and I mean many, short exposure subs. The maximum length of the sub is limited by field rotation, and this will depend on where you are looking in the sky; in some cases it will be seconds, and at the most probably 90s. The other issue you will encounter is the accuracy of the tracking that the mount can achieve during the exposure time, and it is not unusual to find a percentage of subs which show star streaking. You should reject these and not stack them. This would be a severe problem with the C11 because of its long FL, but with Hyperstar you have a very fast, short FL set-up, which greatly helps. The light gathering power of your C11 compared to my 102mm refractor is huge!

Might I suggest that you have a look at the long thread "The No EQ Challenge" (https://stargazerslounge.com/topic/228101-the-no-eq-dso-challenge/) where you will find a lot of information about doing Alt-Az imaging, and you can see what others (mainly with short FL refractors) achieve.

Good luck, and keep us informed of progress.

Ian

PS. Visited Cape Town some years ago. Fabulous country.

Hi Ian,

How are you,

The main reason I brought the CPC unit is due to it mobility, but still it is heavy..........

It is still a mission to move in and out,

 

And I brought the HyperStar is due to it short exposure advantage,

For example the photo below it taken with 60 X 25s light frams, but only manage to keep 45 of them, 30 darks and 30 Bias, no flate,

According to the Starizona, on the C11 with HyperStar, it reduce the focal length of f10 to f2,  1s = 30s exposure on standard equipment,

Which means 30s X 25s X 45 = 33750 / 60 / 60 = 9 hour 37.5 minutues exposure, ( I hope my calculation is correct...)

 

Issue I have now,

1 - Due to the mount issue, need to short the exposure time to 20s -15s to avoid flying stars.....

     Need to find out from other forum the recommend time for exposure

2 - Can only 10 -15 pic a time, then I need to move the object back to center of the screen

3 - Can't do to long exposure, other wise the whole pic will go over exposure, could also be my QHY10,  

 

Thank you for liking Cape Town !!

 

Wayne

20-Dec-2017

 

 

[The Admiral]

The link I gave you contains some information on field rotation, but I've used this site: http://calgary.rasc.ca/field_rotation.htm. There are other sources if you Google. This excludes any issue resulting from poor tracking, which it seems you may have as you say you have to re-centre often. I would expect the object to remain more or less centred over a couple hours, not perfectly mind, but drift needs to be slow otherwise you'll get star trailing. I can achieve this using my lowly Nexstar SE. It all comes down to initial alignment. I level my tripod accurately using a proper level, not a button level, and do a 2-star alignment.

I guess bright stars will readily saturate your camera, given your aperture. M42 is a difficult target anyway, in that there are some bright regions, and so it is common to take short exposures and blend with longer exposures in order to cope with the dynamic range. There is some star trailing visible - aim for better alignment.

Edit. Well, I say star trailing but I see it isn't over the whole field. I wonder if you have camera alignment issues as well. At f/2 I know that camera alignment is critical, but I have no experience to advise on that.

Ian

Edited December 20, 2017 by The Admiral

[Wayne76]

Thank you Ian

i will take long and short exposure, and shorten to exposure time ?

[happy-kat]

There is no given answer for how long to expose unless you want to build a spreadsheet for your exact latitude and your exact equipment. In brief point East or west upto 60 degrees is most favourable in northern hemisphere. The higher you go and the more North or south the quicker star trails will show.

[Wayne76]

On 20/12/2017 at 15:54, happy-kat said:

There is no given answer for how long to expose unless you want to build a spreadsheet for your exact latitude and your exact equipment. In brief point East or west upto 60 degrees is most favourable in northern hemisphere. The higher you go and the more North or south the quicker star trails will show.

 

After checking, CPC without a wedge the you should keep exposures to maximum of 20 seconds.

 Otherwise  field rotation will occur,

f/2 (HyperStar) is 25X faster than imaging at f/10 (at cassegrain focus).