#1  
Old 03-07-2014, 08:51 PM
E_ri_k (Erik)
Registered User

E_ri_k is offline
 
Join Date: Jun 2011
Location: Lakes Entrance
Posts: 817
Bloated stars

A basic question I guess, but are there many other causes of bloated stars in images, other than focus and seeing?

Erik
Reply With Quote
  #2  
Old 03-07-2014, 09:59 PM
Andy01's Avatar
Andy01 (Andy)
My God it's full of stars

Andy01 is offline
 
Join Date: Mar 2012
Location: Melbourne
Posts: 2,318
Interesting question..
How about overexposure? Especially with small pixel chips perhaps.
Reply With Quote
  #3  
Old 04-07-2014, 12:45 AM
MrB's Avatar
MrB (Simon)
Old Man Yells at Cloud

MrB is offline
 
Join Date: Jan 2007
Location: Rockingham WA
Posts: 3,408
Collimation, dew, vibrations, thin high cloud, quality of optics (chromatic aberration, wavefront error, scatter etc), guiding errors, tube currents, warm mirror, just to name a few.

But seeing(depending on fl) and focus (and maybe collimation for reflectors) would have to be the most common.

Dew is easy to overlook, heavy dew is easily noticed on lenses and in images, but light dew can be barely seen on a lens/mirror but can easily bloat brighter stars. Same goes for very thin cloud at a dark site, almost invisible by eye but will bloat stars.

Last edited by MrB; 04-07-2014 at 01:07 AM.
Reply With Quote
  #4  
Old 04-07-2014, 07:42 AM
gregbradley's Avatar
gregbradley
Registered User

gregbradley is offline
 
Join Date: Feb 2006
Location: Sydney
Posts: 15,477
I find sometimes when there are bright stars in the field when using a chip with small wells - less than 40,000 electrons, you can, with some scopes with largish aperture, see bloated stars. I see this with my KAF8300 and ICX694 cameras sometimes.

Otherwise bloated stars are poor focus, bad seeing (hard to notice unless you are imaging at around 2 metres or more focal length), thin cloud.

If you use a clear luminance filter you will get a bit more light but slightly bloated stars from the IR component which is normally filtered.

Greg.
Reply With Quote
  #5  
Old 04-07-2014, 08:49 AM
E_ri_k (Erik)
Registered User

E_ri_k is offline
 
Join Date: Jun 2011
Location: Lakes Entrance
Posts: 817
Interesting, and quite a lot of variables to consider. I was imaging last night, the seeing was very good, first two subs looked great, and very sharp. However the next few subs deteriorated, and the stars grew larger and larger.

The focuser reported that the temperature was stable, but I put the mask back on anyway and checked focus, which was also good.

Ended up going to bed!
Reply With Quote
  #6  
Old 04-07-2014, 08:53 AM
Shiraz's Avatar
Shiraz (Ray)
Registered User

Shiraz is offline
 
Join Date: Apr 2010
Location: ardrossan south australia
Posts: 4,787
I don't think that blooming is exacerbated by small wells - the idea behind that concept seems to be that when the wells fill up, the excess charge spreads across the chip to nearby pixels. But this cannot happen with ABG chips, where the excess charge is drained to earth before it gets above the potential walls of the pixels - it cannot ever get out of the pixels to spread elsewhere - so small wells/blooming is not a reason for fat stars. What happens to excess charge is illustrated here: http://www.olympusmicro.com/primer/j...ing/index.html
non-ABG chips allow excess charge to overflow the pixels, but this charge is constrained to flow down the readout channels producing (vertical) blooming lines - it does not just spread out in all directions to produce fat stars.

Erik, I think your last post hits it on the head - fat stars are mostly caused by bad seeing. It has been terrible down here for the last two nights - huge 4 arcsecond (FWHM) blobby stars of no use to anyone. Happens to the big guys as well - the two images in the attached link are of the same bit of sky, but under vastly different seeing conditions - look familiar? http://ftp.aao.gov.au/images/captions/aat050.html
It is also interesting that the SN has produced a huge blob - this is photo film and the blobbiness is clearly nothing to do with CCD charge overload - it is just the shape of the wings of the point spread function of the scope.

Another possible source of bloating is the chromatic aberration of refractors. Even the best APOs produce significant defocus at the extremes of the spectrum - have a look at the images from FSQ or TV NP scopes - the images can be excellent, but they show bright stars with beautiful big and smooth halos around them that I assume is from the CA affecting the luminance channel.

Last edited by Shiraz; 04-07-2014 at 06:28 PM.
Reply With Quote
  #7  
Old 05-07-2014, 01:12 PM
LewisM
Novichok test rabbit

LewisM is offline
 
Join Date: Aug 2012
Location: Somewhere in the cosmos...
Posts: 9,326
Ray,

Isn't there a function of glass type used with bloat and halo? I seem to recall there is, and have noted that images produced through true fluorite crystal elements suffer far less from the blue halo than do images with FPL53/ED element refractors.

I definitely can notice a difference between the FSQ106N (Fl) and FSQ106ED (ED) images - much more pronounced halo with the ED (and another reason why owners hang onto the FSQ106N till their dieing days). I also note a significant lack of any halo - apart from diffraction spikes caused by the element spacers - with my Vixen FL102S with the rear element being true Fl crystal.

Could be all conjecture, but it is noticeable in images comparing the 2 types of "glass".

I also have HEARD there is a difference between oil spaced and air spaced because of the reflection/refraction surface standpoint, but that one is a VERY large can of worms.
Reply With Quote
  #8  
Old 05-07-2014, 02:23 PM
Shiraz's Avatar
Shiraz (Ray)
Registered User

Shiraz is offline
 
Join Date: Apr 2010
Location: ardrossan south australia
Posts: 4,787
Hi Lewis. Interesting observation on the difference between the FSQs.

But sorry, discussion of which types of refractors produce the least CA is out of my comfort zone (ie I have no idea ). My comment was based on the observation that stars from even the most highly respected APOs have more halo than those from reflectors - looking at figure 149 in the reference seemed to show that CA could be the culprit. That is not a criticism though, just an observation - APOs can produce very fine images. Regards Ray
ref: http://www.telescope-optics.net/semi...o_examples.htm

Last edited by Shiraz; 05-07-2014 at 02:34 PM.
Reply With Quote
  #9  
Old 06-07-2014, 09:12 AM
RickS's Avatar
RickS (Rick)
PI cult recruiter

RickS is offline
 
Join Date: Apr 2010
Location: Brisbane
Posts: 10,584
I've seen some strange artifacts around bright stars with my FSQ-106ED and KAF-8300 camera. My guess is that these are caused by the microlenses on the sensor. Maybe not bloat, but another cause of less than perfect stars.

Cheers,
Rick.
Reply With Quote
  #10  
Old 06-07-2014, 11:15 AM
gregbradley's Avatar
gregbradley
Registered User

gregbradley is offline
 
Join Date: Feb 2006
Location: Sydney
Posts: 15,477
FSQ106N versus FSQ106ED. I have had both. FSQ106N is better in many ways, FSQ106ED has a larger more perfect correction, more backfocus, a reducer is available, is smaller, does not vignette bright perimeter stars, is very sharp.

FSQ106N has minor vignetting expressed as a dark bar through bright stars along the perimeters of the image. You occassionally see this in other scopes.

The main difference between Fluorite and FPL53 is not chromatic aberration but light scatter. Also Fluorite per Yuri from TEC allows the designer to build a faster lens without chromatic aberration compared to FPL53.

FSQ106ED coatings are another matter. The FSQ106N coatings are a lovely grass green and colour from FSQ106N was always rich and saturated.

FSQ016ED I immediately noticed in intial images the colours were very muted, biased towards the mustard green of the coatings and to my eye used to FSQ106N a little bit unpleasant. There are thousands of excellent FSQ106ED images around so part of that can be corrected with processing. But I did bring this up on another site and one guy did some testing. Sure enough there is a green bias. His testing though seemed to show it came from the black tube paint rather than the lens coatings and that flocking the internal tube corrected this.

Having switched from FSQ106ED to TEC110 fluorite oil spaced triplet I was hoping to regain the rich colours of the FSQ106N again. I was happy that was indeed what I found. So perhaps the Fluorite play a bit part in this - hard to say. I find my AP140 which is an FPL53 oil spaced triplet a very superior imaging scope and matches my TEC180 fluorite oil spaced F7 triplet.

FSQ106N has a much sturdier focuser (endless threads about flex in the focuser of the FSQ106ED and the various versions that came out to address this), the focus lock on the FSQ106ED was faulty it would shift focus when you locked it. The other main advantage of the FSQ106ED was the extra backfocus (FSQ106N was very limited) plus the availability of a reducer. Its also shorter. The captains wheel was a mistake and was removed on some versions of the FSQ106ED.

Both FSQs are very sensitive to temperature change and its common practice to refocus with every 1C change in temp. Not an issue for those who use automated focus routines. You can get temp compensation electronic focusers (Robofocus if you go to the trouble to set it all up).

With regards to CCD small wells, I think the stars bloat because with a deep well pixel it can contain the data without filling and the whole PSF of the star is recorded with its diminishing values as you go away from the core of the star. With a small well the core is filled, saturated, the next pixel out is also filled, the next the next and so on and it goes out a fair way before the wells contain the info without bloating.

A proof of this is that one solution to this overly bloated star is to reduce exposure times so the wells can better contain the data further out from the star core without filling. I doubt its an APO issue as in my case for example the same scope does not show this behaviour with other cameras with deep wells (I have used quite a few cameras over the years).

Generally small wells can mean limited exposure time on faster or larger aperture scopes and are better suited to F7 or above and smaller apertures so there is less bright light overwhelming the wells and neighbouring wells.

Also keep in mind when you say anti blooming cannot spread - the specs rate the effectiveness of the antiblooming usually expressed like 100X or some other number so if it were bright enough the antiblooming system would still be overwhelmed although I doubt that is what is happening here.

Another theory I have is perhaps some chips are more sensitive to IR than others and that is showing up in bloated stars. One solution would be to try a UV/IR or simply an IR filter in front of the CCD window to see if that helps. It may not but its worth a test on a known bloating target.

Greg.
Reply With Quote
  #11  
Old 06-07-2014, 01:16 PM
Shiraz's Avatar
Shiraz (Ray)
Registered User

Shiraz is offline
 
Join Date: Apr 2010
Location: ardrossan south australia
Posts: 4,787
Quote:
Originally Posted by gregbradley View Post

A proof of this is that one solution to this overly bloated star is to reduce exposure times so the wells can better contain the data further out from the star core without filling. I doubt its an APO issue as in my case for example the same scope does not show this behaviour with other cameras with deep wells (I have used quite a few cameras over the years).


Greg.
good point - saturated star cores can result from overexposed subs - another one for the list Erik. The very simple answer, as you point out, is to reduce the sub exposures. There seems to be a residual belief that long subs are inherently better than short subs. This made sense in the days of CCDs with high read noise, where you needed lots of signal to get above the read noise - there are still some CCDs on the market that need long subs to work properly. Modern CCDs with small pixels have much lower read noise, so can be used with much shorter subs for the same SNR - if you use shorter subs, the wells do not saturate.

For example, your 694 needs subs about 1/3 as long as your 16803. A recent image by JJJ taken with 40 second subs shows just how far modern cameras have come.
Reply With Quote
  #12  
Old 06-07-2014, 01:20 PM
gregbradley's Avatar
gregbradley
Registered User

gregbradley is offline
 
Join Date: Feb 2006
Location: Sydney
Posts: 15,477
It seems that way Ray.

I was getting burnt out cores of M104 with 10 minute subs and the CDK17/reducer/694 camera. 5 minutes was fine. I would have had to do separate core exposures which I could do I suppose and perhaps more of that dim dust lane would expose in 10minutes.

Greg.
Reply With Quote
Reply

Bookmarks

Thread Tools
Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT +10. The time is now 08:19 AM.

Powered by vBulletin Version 3.8.7 | Copyright ©2000 - 2019, Jelsoft Enterprises Ltd.
Advertisement
Bintel
Advertisement
OzScopes Authorised Dealer
Advertisement
Meade Australia
Advertisement
SkyWatcher Australia
Advertisement
NexDome Observatories
Advertisement
Lunatico Astronomical
Advertisement
Celestron Australia
Advertisement
Astronomy and Electronics Centre
Advertisement