Posts Made By: William Earl

Hi,

Well I was finally able to spend some time on this. It appears that the width of the long edge has been reduced to about 1", and a hill has been created in the center. I'm not worried about the hill. I can take that down easy. I am frustrated with how long it takes to reduce the edge. I have been using a small 6" & 8" laps with accented pressure very near the ridge between the two radii, and am now very near to working right near the edge. For a while I used my 12" lap with the center 8" of pitch removed in order to stop working the center. Anybody have an idea aboout what to do next?

Bill

I guess I'm probably going to elected as the poster child for bad mirror making but here is the latest. Please give me your thoughts... good and/or bad.

I have been using a small 6" lap (which really is about 5") right at the edge with a short to medium length chordal stroke while walking around the barrel. Probably about 120 strokes per revolution. Have been around the barrel about 100 times since last update. Arms are getting stronger.

Looks like I have an problate spheroid developing and the long edge is beginning to blend in. TDE hasn't changed much. Surface is pretty smooth.

I was going to continue this approach and test again after ~30 minutes (~20-25 times around barrel). I don't have the resources available right now to make a bigger lap, but if that's what I need to do let me know. A 16" lap would be a mother to work with! If I buy some more pith what kind should I get?

Thanks again and clear skies,

Bill

Well here is a photo of the latest progress. The Ronchigram outside of ROC (right side) looks a little chopped up because I have to piece together multiple images to make it whole. My thoughts on it are as follows:

1) Its evolved into an oblate spheroid (an ellipse rotated about its minor axis). Edges high relative to center. Undercorrected everywhere.
2) Surface roughness has smoothed out some since last photo, but still a little rough.
3) TDE down to about 1/4" but still no diffraction ring all around.

OPTION #1 -
I've read that this type of figure can proceed directly with parabolizing. Deepening the center will shift the average ROC in (make it shorter), which will bring the edge down (relatively speaking). There is a gamble in this that the amount of undercorrection at the edge is about equal to the the correction required once the center is deepened. If it is, then the TDE will remain and need to be masked. If it is not, then further work on the edge will be required. Work on the TDE can continue at that time.

OPTION #2 -
The alternative (which is what I've been doing) is to continue working the edge downward to match the center. The attempt being to return the surface as close to a sphereiod as possible. At that time the surface can be parabolized. This approach should also remove the TDE in the process of bringing down the edge.

I think that if the mirror didn't have a TDE I would definitely proceed with the first option. That would be the quickest way to bringing the mirror to completion. Because of the TDE I'm not totally sold on that approach yet. I am however, getting anxious to finish and the second option means a lot more work. What do you think?

Michael,

I have used plain old Dap plaster of paris and it works fine. Good luck and clear skies,

Bill

Howdy folks,

Check out the latest. Images are set up with inside ROC to left, outside ROC to right. 150 lpi Ronchi grating. Looks like I cleaned up the edge about as well as expected. Still a small TDE and no diffraction ring all around, but tremendous improvement. I see a figure beginning to appear. My interpretation is that the ~70% zone needs to be smoothed some. Looks like a pronounced break in slope as opposed to a smooth transition. Also, center ~40% needs to go deeper. What do you think?

Mark,

When did you receive your scope? Is Burgess finally shipping them? I was one of the first 100 to order and am anxiously awaiting delivery. Bill Burgess told me he was thinking of shipping the first few orders directly to the buyers himself to reward the people who were first in line (perhaps the first 5-10). The rest will be sent to Anacortes for shipment based on order. Glad to hear it is all it is expected. Enjoy.

Bill

Kurt,

Do you live near FW? I live in Joshua. Always looking for someone to astro view with.

Bill

Doug,

Go Blue! Nice looking scope. Great job. Clear skies,

Bill

Mark,

I removed the rear baffle and was very pleased in the additional brightness of the view. I'm really not sure what Bill Burgess was talking about in terms of a "yellow haze" around things. I've looked thru many acro's and it really doesn't look a whole lot different. I did have to break in the focuser a little and recollimate it as well. However, that took all of 10 min to discuss with BO and then about 10 min to fix. That and removing the baffle made the view of saturn go from fair to excellent. Best of luck with yours and clear skies.

Bill

David,

The best way to get the curve out to the edge and slow the focal ratio down is to flip things over tool on top. Continue to work the entire mirror surface though so you don't end up with two different radii. Best of luck and clear skies.

Bill