Posts Made By: Dmitri Kulginov

I've bought a replacement visual back with compression ring for my C8. I notice that it is longer than the original back. Does it play any role? (In principle it should, since the corrector plate is supposed to correct best at one particular position of the main mirror, but the effect is probably very small.) Does a longer visual back interfere with use of reducer/corrector?

Dmitri

Inge,

Don't we have profiles for that?

Dmitri

We all know halo that surrounds bright stars and planets in the telescope. While I know where it comes from - chromatic aberration, scattering on lens/mirror surfaces, dirt and scratches, scattering in the optical path of the telescope, scattering by the atmosphere, scattering in the eye - I have very vague idea about the relative importance of those factors. Experience with many (types of) telescopes whould help here, and that's where I lack.

Any ideas? Refractors vs reflectors? Achromats vs apochromats? Newtonians vs catadioptrics? Eyepiece contribution?

Dmitri

I realized yesterday that rotating the diagonal in my C8 when observing East/West has much larger effect on the collimation than a possible mirror displacement. The effect is not large and is only noticable on focused stars at hight power.

How do rotating focusers fare in this respect, in particluar the WO SCT Crayford focuser?

Dmitri

Last Thursday I finally had my chance to observe from Curacao. The first half of my stay there the Moon was out, and the second half it was mostly cloudy. I was using my ST80, the sky was rather dark at S, not as good at SE, and lousy at E.

First of all, I looked at M104 that was too low in Sweden. It was rather bright and elongated; the bright edge looked "sharp", but I couldn't see the sombrero. Then I looked at two last Messier objects that I hadn't seen: M68 and M83. I logged M68 as "dim" (there was not much more to say), but M83 was magnificent. I could see that it was large and irregular, much more so than M31 and more than M33 (obviously that was a trace of the spiral arms), and had a bright core. I wish I could look at it with bigger aperture... That concluded my Messier survey!

After that I looked at open clusters NGC3766 (that was unremarkable) and the Jewel Box, NGC4755 that I missed during my earlier travel South. This arrowhead is beautiful! Of course I had to look at Alpha Crucis which is surely the most beautiful multiple star in heaven, and Alpha Centauri that was somewhat low above the horizon.

The highlight of the night was NGC5128, bright galaxy with a dust lane. I could see the dust lane with deeply averted vision; I checked the orientation later and it was dead on. The galaxy was followed by Omega Centauri that was huge, looked grainy at 32x and started to resolve at 100x.

By that time Scorpius moved to a darker part of the sky, and I looked at the globulars M80 and M4. The latter was much easier to resolve than Omega Cen - already at 32x. Finally I checked M6 and M7 that I had never seen so high, and headed to the hotel. I also logged a young hare that was very intrigued by my headlights.

Dmitri

I'm a little surprised that nobody seems to offer an 1.25" replacement visual back with compression ring for SCT. Am I missing something?

Dmitri

Before the observing season has started here in Sweden, I'm trying to find a battery operated hair dryer, ideally a compact "toy" dryer on AA batteries. Can anyone point me in the right direction?

Dmitri

Televue recommeds on their web site three eyepieces for owners of 8" SCT:

24mm Panoptic for low power
13mm Nagler T6 for medium power
9mm Nagler T6 for high power

(There are some other recommendations that have to do with long eye relief - not important to me.)

While this looks very attractive, what is your experience with these eyepieces in an 8" SCT? Are there cheaper alternatives to 24 Pan that work just as good? (As good in 80% of FOV is not good enough.) Do you feel there are any magnification gaps here? Finally, how do the three work with reducer/corrector?

[ ... and no, I'm not interested in 2" eyepieces. ]

Dmitri

Where can I buy a 4-5lb counterweight for CG-5? (Bore diameter 0.810" I think.) Scopestuff used to have them, but they are listed as no longer available there.

Dmitri

A question to the optical experts:

When I tried to collimate my C8 with R/C on (bad idea!), I immediately noticed that a slightest shift of the (artificial) star from the field centre yields asymmetric diffraction pattern out of focus. I'd say it looked as out-of-focus coma; yet there was no noticeable coma in focus and the stars were sharp over the FOV - that's what R/C are for.

So how does it work that this asymmetry disappears in focus? (Or doesn't it?)

Dmitri