CN FORUMS ARCHIVE Telescope Reviews: C130

That's correct. For imaging, the Omni refractor has several things against it...the achromatic lense, the long focal length (and physical length) and the mount.

In an achromat, the light that is bent coming thru the lense is not all bent equally across the light spectrum. So some colors are not coming to focus at the same point. Your eye can ignore these false colors, but the camera picks them all up. The second thing about the Omni that's a disadvantage is it's long focal length (f/10). For imaging you really want to get a scope that has an f/ratio down as low as possible. With a scope with fast f/ratio you're capturing more light in less time. That equates to better mount tracking and more image captures in a nights imaging session. With digital imaging, you might want 60 or more images to stack to help eliminate camera noise...60 x 2 minutes exposure is 2 hrs of imaging. 60 x 5 minute exposures is 5 hours of imaging and the amount of light you have would be the same. So, faster f/ratio is better for imaging. As an aside, unguided mount tracking will be anywhere from 30 secs to a couple of minutes depending on the mount. That means a 5 minute exposure would not be possible without star trails in most mounts...30 seconds with an Omni would be good. You're just not going to get much light in 30 seconds with a slow f/ratio scope.

The other part of the equation is focal length. The longer the focal length of the scope, the higher the magnification will be. The higher the magnification, the better the mount needs to track and the more stable it needs to be.

My intro level CG5-GT mount can go about 1 minute before it starts to show star trails with my 320 mm focal length imaging refractor. That's not very long. To get longer exposures I have to autoguide. To autoguide means you'll have to have a computer or an expensive mount (talking $$$ here). To see my setup, go to My Astronomy Pages in my sig line below.

As I said earlier, imaging and visual observing have very different requirements. When you try to combine both in one scope, you'll end up with a fast f/ratio, large aperture scope with a short focal length and a short physical length sitting on a sturdy EQ mount. All those things are compromises of course and your desired portability starts to get compromised as well.

The setup you see in my sig line was my attempt to combine visual and imaging functionality in a portable setup on a budget. I put the setup together over the course of a year. For visual use only, I can take the C6S-GT out by itself. The 6" of aperture shows the brighter deepsky objects nicely. I added a 2" visual back, 2" diagonal, and a 2" eyepiece and can get 1.5° true fields of view (double the C130). The mount itself is very stable with the small C6 sitting on it (and it has goto which is a plus). I can carry that combo out to the deck in one piece quite easily.

As an imaging rig, the C6 works nicely for planetary and lunar imaging. With an f/6.3 focal reducer, I can image deepsky objects at a focal length of 960mm (that focal length frames the moon nicely). My second scope is the one I use the most for imaging. It has a fast f/4.8 focal ratio with a 0.8 field flattener/focal reducer and a focal length of 320mm.

I image with a DSLR, not a CCD camera because I like the wider fields of view possible with the DSLR's large sensor, plus I use the camera as my daytime camera. I use a Meade DSI Color CCD camera as my autoguider camera and it can double as a planetary imaging camera as well.

As I said earlier, I put together this setup over the course of a year or so, but I started with the C6S-GT. Astrophotography can be an expensive hobby, but it can be pursued a little bit at a time. Just start out with the right foundation or you'll be spinning your wheels and frustrated in the end.

Patrick

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CPC1100;AT66ED;Denk S1;
EdgeHD C8; C6 SCT
6"f/4.8MN ES Comet Hunter
10"f/6 Newt
Vixen GP2;CGEM Mount
Canon 60D;Lodestar Guider

The Lord sits enthroned above the circle of the earth...He stretched out the heavens like a canopy.

Edited by Patrick (12/27/07 08:10 AM)