Opticstar DS-335C any opinions on this?

Hi David,

I'm sure Dave will note if I mentioned anything incorrectly below.

It's to do with the image scale of the scope & camera in combination with seeing conditions.

It is generally thought that the optimum resolution is 2 arcsec/pixel (although some people have said to aim for 3 in Ireland due to the gulf stream!). Basically if you have an image scale of, say, .5 arcsec/pixel and seeing limits you to 2 arcsecs/pixel many of the pixels are being wasted as you're setup far exceeds the resolution you can obtain. In that case you would get as good an image with far less but larger pixels.

The smaller the pixel size, the shorter your scope needs to be deal with seeing conditions (i.e. to get near to that 2arcsec/pixel value). You can bin 2x2 to effectively double your pixel size (quadrupling area) but that would only give you a small image and you would also end up with a mono image.

Another thing to note is that the chips with smaller pixels are often not as sensitive as the larger ones. I think this camera (& the Sac10) have a QE of around 50%. The more expensive cams often have around 70-80% I believe.

By the way, Ron Wodaski has an excellent & free calculator program for download on his site. It will do all of the resolution and field of view calculations for you.

www.newastro.com/newastro/book_new/camera_app.asp

Hope that helps. Nearly ended up confusing myself typing this!

Trev

What is the drawback of small pixcels v large pixcels and when is one better then the other.

I would have thought that smaller pixels mean greater resolution on the dots-per-inch scale of things. But, I think also that smaller pixels heat up faster, so you will have a greater amount of noise in a picture taken with smaller pixels. Of course, I assume there is trade-off i.e. there comes a point where the pixels are the 'right' size for resolution AND heat resistance.

Forgot to mention this as well but smaller pixels usually means smaller chip, which in turn means smaller FOV. e.g. I need to use the WO 66 to get M45 in the field of this chip.

Thanks for this Trevor. So basically I would need to reduce the amount of pixcels by half to even get the optimon result with an f3.3 reducer.

It is generally thought that the optimum resolution is 2 arcsec/pixel

The current trend is to go for a pixel scale of 0.5 * your typical seeing in FWHM.

So, my typical FWHM in Celbridge is about 3 arcsec per pixel. So I chose my CCD of 24 um pixels based on this giving me a scal of 1.28 arcsec per pixel.

Ron Wodaski's calculator will give you all the data you ned to make the choice.

HTH

Dave

Hi David,

After seeing Dave's mail I think you'd probably get reasonable results with the f3.3 reducer. I had one but it is so sensitive to collimation that I ended up just using the refractors for imaging.

I definitely wouldn't go for an uncooled version though.

As it is the SAC suits my setup, but if I were using an SCT I think I would spend the extra for a more suitable image though. On the other hand, if you were got the f3.3 working properly you would get around one of that chip's limitations; the low sensitivity. It just didn't work out in my case.

Trev