Telescope Reviews: Celestron C6-(XLT) SGT,how do u change to 2"

The C6 starts vignetting at the field stop of about 5mm (2.5mm off axis).

The falloff is smooth and progressive up to the field stop of about a 27mm eyepiece, where the illumination plunges. Just because you can't see it doesn't mean it isn't there. I can see it easilly in comparison to similar field in a refractor.

Once you go past an effective field stop of 27mm (.37x smaller if you are using a focal reducer), the edge of field illumination drops from about 75% to 50% by going just a few millimeters larger. It continues to decline until the point where you see visible vignetting.

Does this mean that you can't enjoy a field that is only 30% as bright at the edge as it is in the center? Well, I guess not.. Clearly, a lot of people use combinations like this and seem to enjoy it.

But considering that the vast majority of the field is dimmer than in a 4" refractor used at 1.4 degree true field, (remember, more of the area of the field is in the outer diameter), by comparison, the field will look quite distorted visually. The center of the field will be far brigter than the outside of the field.

The effect is that when you look at very large targets like M44, the scope with a fully illuminated field will show a FAR FAR more interesting field.. In the SCT, it will appear that the cluster is somewhat condensed with the brighter stars all near the center.

IN a 4" refractor with the same size field, you will see that there are bright stars everywhwere in the field, and many many many faint outliers that won't even be visible in the 6".

SO sure, you can do it. And hey, a lot of people do, and it works for them.

But if you want to do wide field viewing and see the feild properly rendered with a target framed in a field of background stars, get a refractor or a Newtonian.

I used to do this years ago, and after making many many comparisons on a large number of targets, I determined that the damage to most targets was far more severe than I would have thought. In my 6" refractor, targets I had seen many many times in small SCTs and MCTs became dramatic swarms of stars. Not because of the "Sharpness" of the refractor,but rather because the field was fully illuminated. It wasn't until I started doing those comparisons that I started to realize how much I had been missing.

So do it, but do it with the understanding that the field will look better in a 4" refractor. And the 4" refractor can of course go MUCH wider than even 1.4%.

Small SCTs and MCTs have pretty extreme off axis illumiation dropoff. Going bigger than 27mm in my own opinion damages the view to the point that it is not worth it.

Regards.

--------------------
Celestron C14, CGE (Big Al)
Astro-Physics 6" f/8 (Buffy)
Televue 101 (No name, but I call it my Widescreen HD Space TV)

The night sky is my mistress. She seduces me away from all other lovers.

Edited by Eddgie (08/11/08 10:46 AM)