Telescope Reviews: SCT Optical Coatings

One of the primary changes to the later XLT scopes was actually not so much the coatings. The Starbright Coatings were quite good.

The biggest difference I think was in the chance of the GLASS in the corrector.. They went to a "Water White" glass which has much better transmission properties.

As for the difference? Some people report seeing a difference, some don't.

I have a C14 with XLT coatings. The tranparency of the corrector is freaky... At night when you look into the front of the scope, you can't even SEE the corrector. It is TOTALLY invisible.

My own belief is that it does help.

Not only do the coatings and the new glass improve transmission, but one of the main Contrast issues with the SCT design is that off axis light entering the scope can reflect against the BACK of the corrector. Anything that improves the corrector's ability to let that light pass back out improves contrast by keeping it from getting to the focal plane or eyepiece.

My guess though is that the difference is very subtle.

But I have every confidence that people that say that they can see a difference do indeed see a difference.

As an example, for decades, we have been told that we shouldn't be concerned with off-axis illumination falloff in many scope designs. Over the years, I have become far more sensitive to it. And when someone tells me that they can see a difference in one coating or another, I am not at all inclined to doubt it.

But again, whatever difference thats going to be there is going to be VERY VERY SUBTLE.

If I were buying used and had to choose between an SCT with XLT, and one where the seller was standing behind the Non-XLT scope with a solid appraisal guarentee that is scope was an excellent sample, I would rather have that scope than the one with unknown optical quality even if it had XLT coatings.

Finally, I would be less interested in an early 80s scope. I don't know when they started coating the rear of the corrector (see above) but I know that early SCTs did not have this.

Also, it is very hard to know how long the coatings from back then will last. Newer coatings (Starbright) have overcoating that will probably make the coatings last longer. But this is ONE topic that I wish we could get more substancial guidance on. It would be great if one of the Magizines or some hobbiest group could do some formal testing. But anyway, that is what might keep me from getting an older scope even if the seller said the optics were excellent.

If buying new, get the XLT simply because it will help resale. Lots of people think that just because a scope doesn't have XLT it is going to be inferrior in some way to an XLT scope.. They simply don't want them. GREAT way to find a superfine C8 though. The last C8 I owned had Starbright coatings but the optics were superb for a SCT. I got it for a freaking STEAL on it. I think I paid $375.

Good luck.

--------------------
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.