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Posts Made By: Mark Bernhardt

November 29, 2002 12:14 AM Forum: Equipment Talk

Mirror Coating problem?!? (small pic)

Posted By Mark Bernhardt

I purchased a used Celestron SP-C6 about two months ago and am trying to optimize its performance. I have had trouble getting a good clean focus at any powers above about 120. From what I have heard, this Vixen made scope should stack up very well if properly collimated. It was built in the mid 80's.

I picked up a cheap (though alignable) laser collimator and used it to no avail. I am still plagued with problems. Tonight I pulled the mirror from the cell to more precisely mark the center and found that my mark was about 2mm off. That I fixed.

I had read that if you hold the mirror up to a bright light or sun that you can detect coating problems. I knew there was a small spot visible on it but until I held it to the light I didn't know how bad the rest of the mirror was. Attached is a pic of the top of the mirror with a 60w bulb on the bottom. I cleaned the mirror so there isn't a speck of dust on it. AS you can see there are hundreds of pin prick sized (though blurry) dots. Had trouble getting the cam to focus and stay steady but you get the idea.

I'm not sure how to proceed. I don't have really deep pockets with the holidays and all. How does this coating effect my image? Will it be noticable? Would it be worth it to construct a knife edge tester to see if the mirror is worth keeping? Or would the coating effect the test?

Thanks for any replies,
Mark Bernhardt

December 15, 2002 08:54 PM Forum: Telescope Making

Building an Off-Axis Newtonian design?

Posted By Mark Bernhardt

I am thinking about starting a telescope project. I have built a pretty large workbench for the task and am considering possible designs to try out. My goal is to build a decent planetary scope.

Leading the list is the off axis newtonian design. ITE sells one of these. The mirror is placed off axis. The mirror is a chunk from a larger mirror to reduce astigmatism.

Mike Palermiti examined a 5 inch model and says:
http://www.itetelescopes.com/report-oa130.htm

"Simply stated, this telescope is at the top of all classes. Whether it is used for low, medium or high magnification visual studies, there is not a closer telescope system to "perfect" than the off-axis parabolic reflective system. The design is truly revolutionary."

This review was by far the best review he's given for any type instrument. So in looking around for alternatives to the $2,700 price tag for this 5 inch instrument I ran across Carl Anderson's site for a home made off axis parabolic reflector.

http://hometown.aol.com/stressedglass/index.html

How close will this system compare to the ITE version? For those who don't want to go to the above link, basically Carl Anderson made a 6 inch F15 spherical mirror and bent it into a lop-sided parabolic shape. He placed his diagonal on the side of the tube. The mirror is tilted about 3 degrees and the bend corrects the astigmatism.

It seems like an easy way to make an ultimate planetary instrument (I was thinking along the lines of an 8 inch model). Why isn't everybody making these? Would I be wasting my time with this project?

Mark Bernhardt

January 7, 2003 12:38 AM Forum: Telescope Making

A question about BVC glass

Posted By Mark Bernhardt

Has anybody here tried BVC glass? Is the material as good as some claim? Does BVC have some unknown quality that people should know about?

And one more question.. If it supposedly grinds easier, don't you have to wait for your glass tool to smooth out anyway?

Mark

July 9, 2003 05:24 PM Forum: Telescope Making

Source for 6 inch F5 mirror?

Posted By Mark Bernhardt

I'm looking for a replacement mirror in this size. How do the discovery mirrors stack up? I know they say 'diffraction limited' which might be 1/4 wave. I would like a little better than that! Are they?

I just want my telescope to be fairly decent without breaking the bank. It probably won't be my main scope in the future but it will serve as a travel scope for camping, etc. I'm looking to spend around 200 clams. Do I have any other options but Discovery?

Thanks for any replies,
Mark

September 17, 2003 05:58 PM Forum: Telescope Making

Two plate primary dob mirror?

Posted By Mark Bernhardt

I noticed an ad on AM from yesterday showing a mirror for sale that is 'superior' to single plate mirrors.

Gimmickry or ??
To me it looks kind of silly. Is this anything new?

http://www.astromart.com/viewad.asp?cid=216470


October 12, 2003 02:53 PM Forum: Telescope Making

Relay Lenses

Posted By Mark Bernhardt

I'm kind of stuck on building my scope because I can't seem to saw a straight line. But that isn't the point of my post.

I noticed an article from S&T in the archives from October 2001 called 'Downsizing Newtonian Secondary Mirrors' by Ernie Pfannenschmidt. In the article he describes how to build a relay lens which is mounted roughly at the tube wall. The light bounces from the primary mirror to the secondary mirror. About half way to the tube wall it narrows to a point then expands again. When it hits the relay lens (a pair of achromats face to face) the light reconverges over a few inches to the eyepiece.

If you play with your favorite newtonian design in NEWT and make your focuser height a negative number 1/4 that of your tube diameter, you can see how small your diagonal can be. On short F instruments an 8% obstruction is easy though a little bit more gives a better illuminated field.

Has anybody tried this yet? I think I can put a .5 inch diagonal on my 8 inch f9, maybe using a pair of 6x30 binocular lenses for the relay. That is a 6% obstruction!

Mark

October 12, 2003 10:02 PM Forum: Equipment Talk

Sombrero galaxy Hubble image

Posted By Mark Bernhardt

For those who haven't seen this and need some new wallpaper, the heritage team has done it again!

http://hubblesite.org/newscenter/archive/2003/28/image/a

December 11, 2003 09:50 PM Forum: Telescope Making

Removing a diagonal from epoxied mount

Posted By Mark Bernhardt

Any ideas on how I can remove this mirror from the hunk of wood its epoxied to? How can I secure this mirror while removing the wood and epoxy without scratching it? Or should I just leave it alone?

Mark

December 18, 2003 08:02 PM Forum: Equipment Talk

My advanced artificial star

Posted By Mark Bernhardt

All the talk of artificial stars lately. Here is mine. The LED is pretty white and very bright though it seems to lean a little towards the blue end.


January 30, 2004 06:54 PM Forum: CCD Imaging and Processing/Solar System

Why no Fastar pics?

Posted By Mark Bernhardt

Is Fastar imaging any good? I have yet to see a Fastar image on this forum. If I have I stand corrected!

Mark