Star testing a Celestron Omni XLT 102.... WOW!!

I guess the "hand figured" optics claim by Celestron isn't a joke!

Star tested my 102 tonight on Sirius.

OTA was initially out of collimation, but a few tweaks on the mirror cell corrected that.

Waited about an hour for the tube to acclimate to the surrounding temperature, then did several inside and outside focus tests. Unfortunately, I have no way of recording what I saw visually, but will do my best to describe what I saw. Diagonal was a 1.25" Orion dielectric, EP was a "generic" 6mm plossl. A Baader fringe killer was used as well. Mount is an LXD75.

Inside focus: nice, concentric, well-defined discs
Outside focus: also very nice, well-defined discs

No obvious spherical aberration or over/undercorrection. No sign of astigmatism, or turned down edge. In fact, inside and outside star test patterns are essentially identical. If I could make an estimated guess, optics show at least 1/7 (maybe even 1/8) wavefront pte.

Confirming my visual test with images on Mark Ludes' website, the following pictures are the closest in representing what I see through the EP.


http://aberrator.astronomy.net/scopetest/html/refractor100_8.html


http://aberrator.astronomy.net/scopetest/html/refractor_100_5.html

Suffice it to say, I am VERY pleased with the optics in this refractor; with star test results(and I'm bound to irk some here with this) rivaling those refractors of much higher cost. Of course, there are some caveats; this is the second Omni OTA I've gotten from Celestron. The first was a 120mm with metal flakes between the doublet pair, and uneven coating. As I said before as well, collimation was way off out of the box. The fact that this is an achro also shows CA on bright objects, but a reasonable trade off considering the price paid for the OTA and mount together. All in all, I would highly recommend the Omni XLT refractors.

Regards,
James Sarte

>>I guess the "hand figured" optics claim by Celestron isn't a joke!>>

It is not surprising that an achromat is well figured. Achromats by their very nature have the most forgiving of all optical designs and the easiest to make. It is a fact that the easiest optic to make will inevitably be the most precise, because one needs to really screw up in polishing to get anything less than perfection. These lenses are not "hand figured" but simply polished accurately on modern polishing spindles. The 4 curves of an achromat are very shallow and do not cause any alignment problems. With the standard crown and flint elements and simple spherical surfaces the result is essentially a perfect spherical null at one wavelength in the middle of the visual spectrum. This is really quite easy to do with modern equipment, and quite hard to screw up (but can be screwed up if one pushes the envelope).

All other optical systems from the simple Newtonian to the Cassegrain, SCT, Apo refractor, RC, Dall Kirkham, etc., have much more complicated elements that must be manufactured to a very high standard in order for the entire package to work effectively.

Rolando