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Old 10-02-2009, 07:39 PM
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gregbradley
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Join Date: Feb 2006
Location: Sydney
Posts: 17,877
You missed my point. I was referring to ugly lines warmer temps on chips can leave behind in the KAI series.

Its no good having optional extras (which are often problematic to use and expensive although the benefits are there for long focal length scopes) if you have an ugly line going through your image due to inadequate cooling. Nice tight stars are wonderful but will be spoilt by the line which is hard/next to impossible to Photoshop out without leaving a trace.

Are you telling me you've never seen these ugly KAI lines in images?

Did you know they nearly disappear with stronger cooling?

Also I never mentioned anything about self guiding. But now you bring it up I think it is overplayed as a marketing point for SBIG.

I get far better autoguiding results using a separate guide scope and an ST402ME than I ever did with self gudiing using the tiny and extremely noisy ST237 chip. It picks up the edge of the FOV of the scope where most optics are weakest, is very noisy and has a very small FOV. Also blue filter often requires longer exposure times which for my setup meant less optimum where 1 second guide exposures have almost uniformly given the best result.
Also with Ha, S11 or O111 self guiding is virtually useless as often unless the object is super bright it requires something like 30 second guide exposures to register a guide star.

Also the way the camera ceases to autoguide when it is downloading an image and in the case of the STL that is something like 26 seconds means tracking errors are building up requiring time for the autoguider to catch up. I see this problem is addressed with the latest STX cameras which do not interrupt the guider when downloading. If you don't program in a delay between exposures you will get eggy stars from the initial errors having built up.

So if you are imaging LRG and no blue or Ha O111 or S11 there is no problem! As well as using a scope with lovely pinpoint stars right to the very outermost edge of the FOV where the pickoff prism collects the guide star. Having said that self guiding is still handy, especially with long exposures, but a guide scope and guide camera are really too easy to use to argue against them.

So self guiding is a bit ho hum. QSI with their offaxis guider built in before the filter would be way superior. If you get flexure you'd be way better off to use an off axis guider than self guiding.

Lets face it, if you weren't selling SBIG cameras and protecting your business we wouldn't be having this discussion which is about protecting SBIG's camera sales. The vested interest factor is too strong.

Greg.




Quote:
Originally Posted by Peter Ward View Post
Sorry Greg, (not surprisingly) I don't agree at all. All other things are rarely equal. Steve's analysis is quite thorough...what aspect of the physics would you care to take issue with or debate?

The few S/N percentage points you gain by extra cooling are easily swamped by having the option to use a device...well, like an AO...which can easily add 20% more flux under a stellar Gaussian curve (translation: brighter & tighter stars)...as does self guiding.... as the pick off chip is looking at the same aplanatic field as the imager. *All* other guiding methods are less terse, and to say otherwise is...as I like to say: "bollocks"

Dismissing these benefits I think is a bit like knocking back Cindy Crawford
because she has a mole....

Last edited by gregbradley; 10-02-2009 at 07:53 PM.
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