Posts Made By: Roland Christen

Your full size image is very sharp. Great results from such a difficult location.

Rolando

Realistically what do you expect this scope to do for you compared with all the others that you have tried?

If you evaluate it in terms of having a perfect figure with the large c.o. of this RC design, you will get the light grasp of approximately a 6" to 6.5" clear unobstructed aperture. You can expect the resolution under normal atmospheric (i.e. good seeing but not super great) to be the equivalent of a 4" clear aperture.

Although the 8" aperture has a theoretical resolution of 0.57 arc sec, I would expect the star FWHM sizes to fall between 2.5 and 3.5 arc sec at best. It will be interesting to see what your results are.

I see RCs as being of value in the 12" and larger sizes. Reason is that the large central obstructions needed for flat fields and ease of construction significantly reduce their ability to produce tight stars due to atmospheric blurring of the first diffraction ring. Above 12" the size of the Airy disc with first diffraction ring approaches the minimum achievable on earth in our atmosphere for deep sky exposures (around 1 arc sec). So for these sizes, it matters little how large the obstruction becomes, and some fast scopes have over 60% obstruction by diameter. For smaller aperture you are not as limited by the atmosphere, but you are always fighting sharpness. This is where an unobstructed scope, or one with very small obstruction will produce the best results.

Here is an example of 10" RC scope with 50% obstruction. Resolution 3.7 arc sec: http://astrosurf.com/papilain/posts/M%2027%20au%20RC%20,%20juin%202009.3.jpg

Did you measure the FWHM size of the stars? If so, what are you getting? Do you have a screen shot?

Forget the 12" Questar. There's one up here in Wisconsin, and the images are terrible. It never cools down. It had no proper thermal design. The secondary obstruction is around 40%. All in all a very poor scope optically with an impressive looking tube assembly.

Here is a crop of the nebula.

Amazing what a consumer camera can do. The ultimate point and shoot outfit.

Rolando

Interesting results. I've never used CCD Inspector so I don't know how that correlates with Maxim DL. With Maxim on a good night an 8" refractor will get around 1.2 arc sec FWHM and a 6" around 1.5 to 1.6 arc sec. But again I don't know whether that is measured the same way as CCD Inspector. I'll have to dig out some of my Meade 10" shots from 2 years ago and see what that scope was able to accomplish. I know it wasn't as good as the 6" refractor, not even close.

Attached image is resolution of an 8" EDF refractor at 1 arc sec per pixel (1.13 arc sec resolution on that night)

Rolando

Moisture has gotten under the coatings at the edge of the mirror and oxidized the aluminum from underneath. The mirrors were probably allowed to get real wet with dew aluminum during use. Dew has dissolved carbon dioxide in it. This is a weak acid that will eat into the coatings from the edges and through any small pinholes in the surface of the coating. Mirror coatings usually have a protective overcoat that resists oxidation, but the edge is vulnerable. Eventually the entire mirror will get a haze over it, but this can proceed very slowly and may take years to develop.

Rolando

>>The diffraction pattern I observed looked like a textbook picture of the effects of tube currents. But a refractor with such a small temperature differential would show this?>>

It does not take a lot of temperature difference to produce tube currents. If in doubt, and you want to rule out any objective problem, and you don't have time to wait for the scope to settle down, look at a star straight up. Tube currents will be at a minimum at the zenith.

>>But the artificial star spikes got to go>>

It's the Star of Bethlehem effect. It reminds me of Christmas and wintertime by the fire while the snow blows around the house outside.

Rolando

I wonder how well it would work on my Roush supercharger? Would it open up the air intake even more and really soup up the charger?
I've got some buckets of polishing powder that would put a mirror finish on the intake.

Rolando