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Posts Made By: William Earl

August 29, 2002 08:13 AM Forum: Telescope Making

Aperture Fever

Posted By William Earl

I bought a Orion ST80 ~ 4 years ago and pretty much saw everything it could image over 2-3 years of observing. Last year I took one of John Dobsons mirror making classes and built a 10" f6.3 that I eventually converted into a truss configuration. This years project is a a lightweight 16" f5 (actually about f5.4) based on Kriege & Berry's book. Used one of Dan Cassaro's pre-generated 7/8" thick plain glass blanks. Last night had it out finally for some star testing. Happy to report no astigmatism, only the typical malidies of slight TDE and under correction. Plan to continue with star testing for a while before putting it up on a tester. Whenits ready to ship away for coating I'll finish the mount. I wonder what I'll do next year? Clear Skies,


September 3, 2002 05:19 AM Forum: Telescope Making

What the heck did I do?

Posted By William Earl

Hi, I'm looking for mirror figuring advice. 16" f5.3, 7/8" plate glass. Prior to figuring session it star tested as undercorrected (based on secondary shadow) quite a bit, and turned down edge (hairy intrafocal image and well defined rings at extrafocal) somewhat. No astigmatism. Did some figuring (45 minutes probably too much), specifically concentrated on the edge using tangential strokes around the perimeter using a 6" lap, and then W strokes across the whole mirror using an 8" lap. Star test afterward looks horrible! Intrafocal looks like a hairy black hole. Almost like a solar eclipse except that all of the edges are blurry. Extra focal is a bunch of sharp well defined rings. Takes forever to get the secondary shadow to show itself. There is also a noticible astigmatism now. Strange thing is that it was able to handle high power focus on the moon better than before. Any ideas? I used a 12" lap to polish it. I'm thinking that I may need to return to it an try to restore the damage I imparted.

October 1, 2002 07:03 AM Forum: Telescope Making

Foucault test question

Posted By William Earl

Please help me understand something. I've read that the actual center of curvature for a mirror is the average of the distances from your eye to the mirror and the light source to the mirror. Theoretically, if you place your eye at the same distance they are the same. The question is, how do you ensure that your eye is placed consistently in the same position for each reading? It seems that since you are taking very precise measurements, a small movement of your head could result in a huge error. Am I missing something? Thanks in advance and clear skies.


October 15, 2002 06:11 PM Forum: Telescope Making

Mirror figuring question(s)?

Posted By William Earl

I think I started figuring this 16" too soon. Should have worked on the long edge first but didn't recognize it because I only looked at the Ronchi screen inside of focus. The bend in the shadows just sort of blended in with the parabolic center. I kept wondering why the star test was so ugly. Anyway, I've begun using my 12" lap with straight center over center short strokes to try and blend it in with the middle. Also using tangential strokes at about the 70% zone to wear down the area where the focal length changes. Have reduced the width of the long edge so far from 3" to 2". It looks like this action is un-doing what correction I had achieved (note the straight shadows in the center of the mirror) up until this latest revelation. I'm not worried about that though. My question is... Does my approach make sense? Also, what effect on the optics does a long edge have? Before embarking on this latest plan the mirror seemed to work very well. It split the double-double in Lyra extremely well (refractor like), and it had absolutely no focus problems on either the moon, Jupiter or Saturn. Was able to view them all at around 440x (over 25x per inch). Of course the image is a little dim, but it resolves detail better than my 10". My only complaint has been the star test. Is this something that will show itself more once the mirror is coated? Has anybody run across this condition before? Any words of advice and experience to share? Any thoughts are welcome. Thanks and clear skies.


February 16, 2003 09:52 AM Forum: Equipment Talk

Chromacor question

Posted By William Earl

This one is for the experts. Is the chromacor simply take a achromat design and convert it to a Petzval?


February 26, 2003 05:10 PM Forum: Equipment Talk

Barlow Question

Posted By William Earl

Another one for the optics experts. When using a barlow with a refractor (either in front of or behind the diagonal) is the color correction improved along with the increase in effective focal length? Thanks


March 9, 2003 12:16 PM Forum: Telescope Making

mirror making machine parts

Posted By William Earl

I have an old belt driven washing machine that is rapidly approaching the applicance graveyard. Maybe a dryer as well. Can I salvage the motors, etc.. and use them to build a mirror grinding/polishing machine? Anybody have a good refernence for info? Thanks,


March 12, 2003 02:00 PM Forum: Equipment Talk

Another Chromacor question

Posted By William Earl

This is another one for the experts.

I've read here that for an achromat doublet the recommended f ratio is 3*D minimum, and preferably 5*D. I think the Chromacor was designed to work with the Synta 6" f8 and 4.7" f8.3 scopes, but not the short tube f5 versions. These two expressed in the previous terms are 1.33*D and 1.77*D respectively. I've also read that the Chromacor was optimized to operate around f10.

The question is... Would a Chromacor work with a 4" f6 (i.e., 1.5*D), or would it be better suited to say a 4" f10 (Celestron C-102 with 2" focuser adaptation) or 5" f9 (Meade AR-5)?

August 19, 2004 07:26 AM Forum: Equipment Talk

Value of used equipment? Opinions please.

Posted By William Earl

First of all let me say that I have alwasys wanted to own 3 scopes; a 4" refractor; an 8" SCT and a 16" Dob. I use the 4" (Burgess 1026 on Vixen/Celestron GP) for quick looks and low power wide field viewing, the SCT for dabbling in phtography, and the Dob (which I am still working on unfortunately) for deep sky hunting.

Recently a friend offered to sell me his Meade LX50 setup, but due to its age, condition and the additional options I'm not sure what a fair price would be. He needs the $ for college so I would like to be fair, but since I'm in no real hurry to get this equipment, I also want a good deal.

Here's what he has... ~1995 8" LX50 w/ matching dew shield. Tripod and wedge are upgraded to the ones that came with the 10" and 12" models. 2" Meade diagonal, off-axis guider, .63 focal reducer and 12mm illuminated reticle eyepiece. Other upgrades include rechargable 12v battery pack, 2 Losmandy rails (one for counter weight, one for guide scope w/ rings) and a Bogan pan head to mount a 35mm camera to (I have an old OM-1).

Overall condition is good (many scrapes, etc...) but all is functionally excellent, and optics appear to be excellent as well, although corrector needs cleaning (dust).

My best guess based on what I've seen for sale lately was as follows:
> scope - $600
> extras - $700

One more thing is a Televue Pronto w/2" non-Everbrite diagonal, screw on lens cap and soft case (~same condition as SCT). He wants to sell whole package together if possible. Figured that added another $500.

Does this sound like a good deal? Maybe offer $1500 for all? Please share your thoughts with me.

Thanks in advance and clear skies.


September 22, 2004 05:54 PM Forum: Equipment Talk

Fork Mount Polar Alignment?

Posted By William Earl


I recently bought a used LX50. Was wondering what is the best way (is there an after market gizmo?) to obtain rough polar alignment before doing any star pointing or drifting.

Thanks, and clear skies,