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Posts Made By: Allen Tracht

December 8, 2003 12:25 PM Forum: Equipment Talk

Barska 80mm F/5 as finder

Posted By Allen Tracht

Has anyone tried the Barska 80mm F/5 scope as seen here:

I'm thinking that, with one of my spare 32mm eyepieces, it might be a decent finder for my 8" dob. At half the price of the sort-of similar Orion product, it seems worth trying.


April 16, 2004 09:41 PM Forum: Equipment Talk

Remounting my dob. Equipment suggestions welcome.

Posted By Allen Tracht

I've loaded up my 8" f/6 Discovery DHQ with a Barska 80mm f/5 "super-finder" along with a Telrad and 2 pounds of adjustable counterweights (from ScopeStuff, which also provided my with the finder mount). Now it weighs (w/o eyepiece) just under 25 lbs. It's about 50" long and 9.5" in diameter.

The optics are very pleasing to me, though I lack the judgment to know how "perfect" they are. Last night, Jupiter and Saturn were exquisite at 200x (6mm). M81 and M82 were fine, faint, fuzzies at 40x. I’d love to have used even higher magnifications on each planets but, by the time I switched eyepieces, the planet would have left the field of view and, even if I could recenter it, the mount won’t track easily at these higher powers.

This, with the desire to try a little astrophotography, and to avoid near-impossible star-hopping in these suburban skies, suggests that I switch to the antithesis of my melamine alt-azimuth manual mount: a computerized, “goto”, equatorial mount.


Mount: The Losmandy G11 w/ Gemini GOTO looks attractive. Are any more appropriate? More economical? How far will I be able to upgrade the OTA before the mount is the limiting factor: C14? Mewlon 250?

Accessories: How do I attach the scope to the mount? Are rings that allow the OTA to rotate valuable? Manufacturer suggestions are welcome.

I understand that it is strange to put a $3K mount under a $500 scope, but I’m fixing the most obvious problems first. I’ll upgrade the OTA as experience and budget allows.


May 11, 2004 10:29 AM Forum: Equipment Talk

Does an inch matter?

Posted By Allen Tracht

Is there any significant (for either visual or photographic use) difference in light-gathering ability between the 10" Mewlon 250 and the 11" Celestron C-11?

The C-11 with its central obstruction of 34% and XLT coatings (average 400-750nm transmission of 83.5%) should give the equivalent of a perfect 9.45" diameter scope.

11 * sqrt((1-.34^2)*.835) = 9.45

The M250 has a smaller mirror, relatively smaller (29%) central obstruction, no corrector plate, and different mirror coatings. Does anyone know how it compares in light througput? I'm looking for a quantitative answer, if possible.

I'm sure that there are many other considerations, such as optical and product quality, price/performance, and f-ratios, but I'd like to keep this discussion limited to the physics of light throughput.



July 23, 2004 09:05 AM Forum: Telescope Making


Posted By Allen Tracht

Has anyone ever considered using Richlite as a structural material for a dob? It's a paper/phenolic material. It's used in applications from commercial kitchen countertops/cutting boards to boating to skateboard park ramps.

Advantages: waterproof, homogeneous, machines well, holds screws well.

Disadvantages: density (1.26), strength/stiffness?, anisotropic

Perhaps a mechanical engineer could compare it to plywood or aluminum for density/strength considerations.

August 6, 2004 08:45 PM Forum: Equipment Talk

Please help me identify....

Posted By Allen Tracht

I recently purchased, through the Astromart Classifieds (thanks Herb, et al.), a modified 8" F/6 Discovery Newtonian OTA. It has been flocked, and equipped with a JMI DX-3. Also, it came with a dovetail base for a finder, but no finder. Unfortunately, the seller hasn't responded to my request, so I'm asking for your help. Can you identify this dovetail finder base? It's about 1-1/2" long, and slightly wider, painted very dark green (virtually black) with a hammered finish. It's attached to the OTA with two countersunk Phillips-head screws and has a silver-colored knurled screw to lock the finder.

I've attached a photo that includes a tape measure for scale.

Perhaps this is a stock Discovery base. I don't know. Regardless, even if you can't identify the source, can you point me to finders that might fit?



August 13, 2004 08:49 AM Forum: Telescope Making

JMI Wheely Bar Upgrade

Posted By Allen Tracht

A JMI Wheely Bar is a y-shaped base that fits under my G-11 tripod and allows me to roll it around my driveway and, with a simple ramp, in and out of my garage. At each end of the Y is a locking caster along with a leveling screw which is a length of 1/2-13 threaded rod with a rubber foot on the bottom end and an approximately 2" wide x-shaped knob on the top.

Now, to get into my garage, I have to lift the leveling screw by 2 or more inches, turning it perhaps 30 revolutions. That's a lot of turning with the fixed knob, so I've tried replacing the knob with a hand wheel with revolving handle, McMaster-Carr P/N 6033K71. After heating the rod with a propane torch for a minute or so, the loctite holding the original knob loosend so the knob could be unscrewed, and the wheel could be attached with its provided setscrew.

Everything fits well, but when I try to use the wheel's handle to turn the leg, the sideways force causes the leg to stick and bind so I can't turn it any more quickly or easily than with the original x-shaped knob. It turns out the the guide tube has an ID of about 0.6" which allows the 1/2" leg to wobble.

Has anyone had any success with this sort of upgrade? Any suggestions?

August 19, 2004 08:12 AM Forum: Equipment Talk

Antares/ScopeStuff Laser Collimator Opinions

Posted By Allen Tracht

Does anyone have an opinion about the 1.25" laser collimator sold by ScopeStuff? It looks to be the same as the Antares.

August 20, 2004 07:22 AM Forum: Equipment Talk

ETX90 corrector baffle shift

Posted By Allen Tracht

I took a look at a friend's ETX 90 that had been stored on it's side and there is a cylindrical object attached to, but somewhat off center of, the corrector plate. I'm guessing that the this is a baffle that is normally centered around the silvered secondary-mirror portion of the corrector.

Has anyone else encountered such a problem?


August 30, 2004 01:35 PM Forum: Equipment Talk

Solar Filter Help

Posted By Allen Tracht

Along with my 2nd-hand scope, I got a solar filter that looks remarkably like the off-axis one pictured on Thousand Oaks web site:

However, it has a somewhat wrinkled film instead of the glass filter.

I understand that they did sell film filters that looked like this in the past, so I can assume that that's what it is.

Still, when I put the filter on my scope and then pointed it in the general direction of the sun, I was surprised to see a significant amount of orange light coming through the plastic surrounding the filter area. Has anyone else experience this? It doesn't seem reasonable.


October 1, 2004 12:39 PM Forum: Equipment Talk

Collimation Surprise

Posted By Allen Tracht

I modified my ScopeStuff/Antares-style collimator by adding a diagonal target (like the more expensive brands). I also aligned it (thanks, Floyd). Now I can easily see the returned beam while adjusting the collimation of my Discovery 8" F/6 Newtonian.

To my surprise, small adjustments of the wing nuts makes a large, almost random, motion of the returned beam. I'd guess that the friction between the wing nuts and the cell is twisting the mirror position. Has anyone else seen this?