Posts Made By: Allen Tracht

Scopestuff has a "Solarscape" that appears to use rather different optics and cheaper construction to achieve a similar result for $99. I have not used either and can't say how their images compare.

Ron,

I'm impressed by the results you've seen. Do you have a reference as how to best use the Chesire?

Thanks,

Allen

The abstract of their paper, along with links to the full text is at:

http://arxiv.org/abs/astro-ph/0408421

I note that this "bright" star is 11.79 visual magnitude.

Is precision pointing of this scope important? If so, then the optical axis of the scope must coincide with the rotation axis of the rings. Otherwise, the aim will shift whenever the OTA is rotated.

I use Parallax rings with my 8" Newtonian. The rings are slightly oversized (by about 0.2") so I can adjust the mounting screws to aim the scope within the rings to compensate for differences between the optical and mechanical axes of my scope. Of course, the collimation mavens will insist that the axes can be aligned, but I find it easier to have this extra degree of freedom.

Allen

According to

http://physics.nist.gov/cgi-bin/Star/compos.pl?matno=169 and
http://physics.nist.gov/cgi-bin/Star/compos.pl?matno=171

Pyrex and plate glasses have the following composition (by weight):

Pyrex

Boron -- 0.040064
Oxygen -- 0.539562
Sodium -- 0.028191
Aluminum -- 0.011644
Silicon -- 0.377220
Potassium -- 0.003321

Plate
Oxygen -- 0.459800
Sodium -- 0.096441
Silicon -- 0.336553
Calcium -- 0.107205

My guess is that real proof would require minor damage to the mirror. Either grinding off a small sample for chemical analysis, or testing with a substance that would corrode plate glass but not pyrex. I believe that flame photometry might easily give the answer, if you can find a chemist with the right equipment. Perhaps a glass recycler could help.

Alex,

Simple formula, for an object at least 10 focal-lengths away:

Focuser position (relative to infinity) = FL * FL / d,

where FL is the focal length and d is the distance to the object.

For FL = 22 x 4.2 = 92.4" and d = 1 mile = 63360", the position is 92.4 * 92.4 /63360 = 0.135".

In general, the formula for a simple lens (or mirror) is:

1/FL = 1/d + 1/e,

where FL is the focal length, and d and e are the distance to the object and image, respectively. Solve for e:

e = 1/(1/FL - 1/d) = FL/(1 - FL/d)

We want e - FL = FL^2/d/(1-FL/d)

If FL/d is small compared with 1, then we can ingore the last term and say:

e - FL ~= FL^2/d.

I hope this helps.

Allen

I have a Garman iQue GPS/PDA. Even when the display is set to red, an undersireable amount of the white backlight still leaks out around the edges. However, I've found that a piece of rubylith does a good job of dimming and reddening all of the light without disabling the touchscreen.

Allen

I finally rememebered where I had seen technologies for repairing detented tubes: musical instrument repairs. Search the web and you'll find a number of technologies for removing dents from tubas, saxaphones, trumpets, and the like.

I know this suggestion may be too late for you, as you'e had some success with the pipe expander, but others may benefit. Also, you may find a local musical instrument repair shop that can take out whatever dents remain in your OTA.

Good luck,

Allen

Thanks. They do have a 6" F/1.4, but it's a precision Zeodur $425 item. Not exactly in my price range.

They also have some 40mm F/0.75 steel mirrors for $2 each. The price is right, but the size isn't within reach.

Before I spend hundreds for a mirror, perhaps I'll try some optics software. The free version of ATMOS should be enough. It'll do three surfaces, and I really only need one. Are there any better freeware ray-tracing programs?

Does anyone have any idea how to evaluate the virtual image quality of a device that projects the image to infinity? This seems to be quite different from the typical spot-diagram analysis I've seen.

Allen

I carry my assembled G11 mount w/ 8" F/6 scope, rotating rings and 44 pounds of counterweight on the Wheely bars. Of course, I only use it on my reasonably level cement driveway.

If I were to want to move it assembled over sloped grass I would worry about several things:

1. The system must be securely fastened down to the dolly so it cannot tip relative to the dolly.

2. The dolly must be large enough so that, at the steepest part of the yard, the center of gravity is within the contact area as defined by the wheels. For me, this would be considerably wider than the feet of my tripod.

3. The wheels must be large enough to move the entire weight over the uneven surface.

4. If the slope is more than a few degrees, then the dolly must have brakes so that it can't run away from me. Ideally, the brakes would automatically be actuated if I let go of their control (dead-man switch).

5. Once I arrive at my observing are I must be able to orient the mount and level it.

1, 2, and 3 don't seem that hard, depending on the slope. I might solve 4 by towing the dolly behind my lawn tractor. I might even use the chassis of a lawn tractor cart as the starting point of the dolly.

Item 5 would be quite difficult with an equitorial mount. In fact, it would seem that all of this would be easire with a Dob and its lower center of gravity.

Perhaps these thoughts are useful.

Good luck,

Allen