Re: How do you fix bloated stars? NEED HELP!
- Hi Ron,
Thanks for your very deetailed reply. You gave me much food for
thought. I have placed your reply on my desktop so as to refer to it
Thanks once again.
--- In email@example.com, "Wodaski - Yahoo" <yahoo@...>
>stars is in
> Let's look at two themes here:
> * Is star bloat really a processing problem?
> * Are your stars really bloated?
> Is Star Bloat a Processing Problem?
> I could make a case that the best place to attach truly bloated
> data acquisition, although there are some things you can do inprocessing to
> make things worse or better (there are some things you can do thatwill
> increase star size, and there are some tricks you can use to reducestar
> size; more shortly).faint
> Among the things that make stars fatter:
> * Dirty optics will scatter light. Careful processing to bring out
> details will also reveal with "dirty toe" around the base of thestar
> profiles.Same issue
> * Poor optics will both scatter light and give you a fatter FWHM.
> as above: careful processing reveals the problem.bloated
> * Poor focus - even just a tiny bit off - creates problems with
> stars as well.blooming
> * Detector characteristics. Many detectors have other methods than
> that allow electrons to flow from bright pixels to dim pixels. Someare
> artifacts of design decisions (e.g., anti-blooming circuitry) orsimply
> substrate characteristics. In addition, many detectors have somelevel of
> significant reflection off of the substrate or circuitry that willspread
> the light out from bright stars.using a
> Here are processing steps that can affect how bloated stars get:
> * When you align images, if you optimize for smoothness (e.g.,
> bicubic method), you are breaking down the existing pixelstructure. This
> spreads each aligned image's star out just a bit. When you combine,you have
> noticeably fatter stars. It is better to get not-quite-perfectalignment,
> even though that will still fatten up your stars a bit due to non-integer
> pixel alignment in the original images.eat away
> * You can use an erosion type of filter in Photoshop to literally
> the outer pixels of the star. This tends to create non-round starshapes,
> and an unnatural star profile results if you get too aggressive.You also
> lose the natural contrast between stars - only the brighter starscan be
> eroded; the little ones will disappear!ones, which
> * You can select just the stars in the image (all but the dimmer
> usually have no detectable bloating due to poor S/N of the "dirtytoe"), and
> then use Curves to modify the star profile smoothly. This requiresa deft
> touch and practice. The technique is explained in complete detailin my Zone
> System book (www.newastro.com/zone).have
> Are Your Stars Really Bloated?
> Your justification for having bloated stars is that your RGB images
> fatter stars than your Ha images. I would propose that using yourHa images
> as a basis for what you expect stars to look like is not apractical idea.
> Why? Well, the Ha filter is blocking nearly all of the starlight.
> "dirty toe" has poor S/N and is much less likely to show up. Thelimited
> amount of light will also limit blooming and other pixel-to-pixelelectron
> shifts that tend to fatten up stars. Additionally, the Ha filter isallowing
> your optical system to focus just a single, narror band of lightyou can
> wavelengths. Especially in refractors, even with APOs, this can
> significantly improve your FWHM.
> In other words, when imaging full or even one-third full spectrum,
> expect significantly higher FWHM than with Ha imaging.just saying
> Now I'm not saying with certainty that your stars are fine; I'm
> that comparing your L or RGB images to your Ha images with respectto star
> size is not a fair comparison; the rules are quite dramaticallydifferent
> and the Ha wins every time on FWHM size. <G>immediately
> I'm a bit of a pragmatist. If you look at pro images, you'll see
> that things like blooms, reflections, and star halos are nottouched up at
> all. (Well, maybe the teams that make the pretty pictures do someof this,
> but usually much less than we amateurs.) They are simply artifactsof the
> imaging process. Since the S/N is always poorer in such areas, I aminclined
> to leave them in my images these days so that the viewer knowswhere the
> good stuff is and isn't!methods are
> If you do want to alter the image to clean up artifacts, the
> there (including the ones I document in various forums andpublications),
> and I have to admit that my opinion on what to do about artifactschanges
> back and forth so I'm not an absolutist on that front. ;-)stars-only
> My own favorite star blot reduction method is using Curves on a
> selection in Photoshop (or, in some cases, such as when there is alot of
> nebulosity or big differences in degree of bloat) and then useCurves to
> carefully alter the star's brightness profile. This takes time, buta
> careful application gives you a very, very high degree of controlso that
> you can balance the various issues in firstname.lastname@example.org] On
> Ron Wodaski
> -----Original Message-----
> From: email@example.com [mailto:ccd-
> Behalf Of Leonard E. Mercer
> Sent: Sunday, August 03, 2008 10:20 AM
> To: firstname.lastname@example.org
> Subject: [ccd-newastro] How do you fix bloated stars? NEED HELP!
> As I improved my processing with time, I have now encountered a
> problem which is really trying me out. That of bloated stars. This
> problem is spoiling good images of mine and it seems I cannot solve
> it no matter what I try. My images are mainly Ha,RGB or Ha,HaR,GB.
> I realised that the problem is imerging as soon as I add the Ha to
> the RGB image.
> I would be very grateful if someone could give me some hints as to
> how I can solve this problem by reducing the bloating.
> I tried Ken Crawfords method of reducing star halos, but got no
> I appreciate any help.
> Thanks & clear skies,