Posts Made By: Roland Christen

>>The other night I observed Saturn at low power with my 4" APO. I was using a 40mm Pentax XL and the 2" dielectric coated diagonal that came with the scope.>>

Rule of thumb: At low powers the eye defects are totally dominant, overshadowing everything else in the optical train. At high powers the optical system is totally dominant, overshadowing every defect in the eye's lens (except for floaters which are not in the eye lens).

There is absolutely no reason to look at planets with low powers. It is also quite common for a beginner to attribute optical defects seen at low powers to the telescope optics. Low power defects point to defects at the eye end (and this will include the eyepiece). High power defects point to defects in the front end (lens, mirror etc).

Rolando

Nice! I have the convertible version.

Rolando

Having high performance optics is not a requirement for splitting most doubles. Aperture is the main criteria. A 1/4 wave optic will split equal mag doubles as well as a perfect optic. When splitting unequal doubles, aperture plus high correction is a requirement.

Rolando

The heat from the mirror gets tranferred into the central baffle tube, and then this heat flows up the tube and out the front of the tube. The resultant plume will not disappear until the mirror is at the same temperature as the inside air. Unfortunately the air inside the SCT tube continues to fall because the aluminum tube (with its black paint) is very efficient at radiating into the clear dark night sky. One thing you might try is to wrap the tube in some insulating material, which will slow down the cooling of the internal air and allow the mirror to stabilize.

Trying to get the warm air out via fans and coolers won't really work. The moment that you remove the cooling device, the mirror will begin warming the air inside the baffle tube and the tube currents will resume.

The cooling problem is at its worst when the sky is pristine clear. That's when maximum radiative cooling takes place. On still nights when the air is damp and the sky is a bit hazy, the scope will perform well without those nasty plumes. That is also the time you will see the most detail on the planets.

Rolando

Your work is super. All your images are very well processed with excellent resolution.

Rolando

Most lenses are designed to have zero coma. They will have off-axis astigmatism and field curvature. The field curvature will cause off-axis stars to be out of focus, and the astigmatism will show the stars as ovals on a flat field and small cross shapes at best focus off-axis.

Field curvature of a lens, where all elements are up front, will be approximately 0.35 times the focal length. It will be inward curving. A short lens will have a steeper curvature than a long lens. Double the focal length, double the radius of curvature.

Now take these and do a bit of geometry to figure out what the off-axis star image will look like.

Rolando

Here's mine taken with AP 10" Mak-Cass and ST10XE camera. (Image is half of original size).

Rolando

Does it mean a ragtop is not invited to the club?

RolandoMustango

You don't look so old...

You may find that converting your fork mounted scope from Alt-Az to Polar will make the entire system even more unstable. Yes, theoretically it will be easier to guide, but this fork system is in balance when mounted in an alt-az configuration. Once you polar mount it, the upper bearings in the Dec axis will have to take both radial and tangential loads (in alt-az they only take radial loads, which is what they are primarily designed to do). It's the tangential loads that will cause your scope to shift and twist in different parts of the sky.

Remember, you are trying to aim your optical system to the arc second level, and that means that there is no room for any deviations during tracking and guiding.

What many people fail to realize is that yes, larger apertures have the potential for higher resolution, but this also placees greater demands on the accuracy of the mounting system. You would be better off to place this tube assembly on a large GEM. If you do decide to get a wedge system, and you want to approach or exceed the resolution of a mere 6 inch instrument, you will probably need to use an AO7 or AO8. (remember, I said RESOLUTION - light grasp you have plenty of, and you can fill any CCD chip with lots of light with that size scope, but is the image quality going to be there also?)

Rolando