gold price: Re: [SBIG] ST-7 disappointing

Minggu, 24 Februari 2008

Re: [SBIG] ST-7 disappointing

Hi, Bill,

The ST-7XME chip is relatively small at 6.9 x 4.6 mm, (the Canon chip is
22.5 x 15 mm) so the field of view at your f6.3 focal length (1760 mm)
is 13.5 x 9 arc-minutes. M42 is around 60 arc-minutes square. Which is
why all you can see is the Trapezium.

For large targets like this you either need a large chip (and then a
telescope with a wide flat field) or a shorter focal length instrument -
you will find that for M42 people are either using an STL11000 on a big
telescope, or a smaller camera on a small refractor.

Regarding the overexposures, you could try a red or better still H-alpha
filter on the Moon. Or stop the telescope down with an off-axis mask to
reduce the amount of light on the chip.

The minimum shutter speed is actually 0.12 seconds (I know you can
choose a lower setting in the software, but it's a mechanical speed
limitation), so you will always be limited on these very bright objects,
particularly with a large aperture telescope. These cameras are
designed for use on faint objects. You would need one with a different
chip structure such as the ST2000 with electronic shuttering (and anti
blooming) to get down to 1/1000 second.

But give it a try on something fainter and of a suitable size - M51 will
fit nicely with the chip vertical, there are a couple of 11th mag stars
to the East which will fall onto your guide chip, and you'll be
pleasantly surprised by the result.

Best of luck,

Peter.

http://www.madpc.co.uk/~peterv/index.html
Approx. 55ºN, 2ºW (Northumberland, UK)

Bill Logan wrote:
> Hi group,
>
> I have spent the last two nights trying to image with my new ST-
> 7XME. I am very disappointed in its highly touted performance. I
> am using an NexStar11GPS with a f/6.3 focal reducer. I am using
> three different camera control software programs; CCDOps, CCDSoft
> and Nebulosity. All with the same results. I have LRGB and V
> filters loaded in the filter wheel. As with all new cameras and
> telescopes, I always try the easy objects first. After carefully
> focusing the camera on a dim star with my FeatherTouch microfocuser,
> I cannot get an image of the moon. I've tried the lowest possible
> shutter speed of .001 second and all I get is black and white
> streaks. Ok, so it's too bright and the camera is too sensitive so
> I moved on to another object. I tried Saturn and all I can get is a
> white blob with ears and that was at the zenith. Seems I can't get
> the exposure down far enough. Is there a gain adjustment somewhere
> in the software?
>
> Tried to image M42; an easy object. Looking at the image, you would
> think I was at f/10, but I have a .63 focal reducer on the rear
> cell After the focal reducer comes a short T-adapter and then the
> CWF9 and camera. I cannot physically get the camera any closure to
> the C11 without removing the focal reducer. All I can get is the
> trapezium. If I lengthen the exposure to 30 seconds, I begin seeing
> the some of the nebulous filaments, but now the stars start blooming.
>
> How do you folks take those pretty pictures that I see everyday? One
> final note. I bought the camera primarily for differential
> photometry, but for some reason, the FOV is less than 15'. Any
> suggestions and advice would be appreciated. The ST-7 cost too much
> money to give up, but I am about ready to go back to my Canon Rebel
> XT.
>
> Clear skies,
> Bill