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Posts Made By: Steve Vegos

August 26, 2002 02:55 AM Forum: Equipment Talk

dob pic

Posted By Steve Vegos

Do a google Search for Ray Cash, Side Walk Astronomy. He has complete plans for a sonotube Dob on the web.

Steve

August 30, 2002 06:25 AM Forum: Telescope Making

Need 4.5 inch secondary; can't find vendor

Posted By Steve Vegos

Hi Doug:

Here's two to start with:

Newport Glass:
http://www.newportglass.com/angwcat.htm

Nova Optical:
http://www.nova-optical.com/diagonal.htm

Steve Vegos

September 3, 2002 05:43 AM Forum: Telescope Making

PLOP cell designs

Posted By Steve Vegos

Hi Doug:

No one commercially uses PLOP for cell design. David Lewis himself said it wasn't necessary. Everyone uses Chandler, based on Hindle's idea's. David Lewis (creater of PLOP) wrote an article for S&T a year or two ago describing PLOP and it's features. He confirmed that Hindle/Chandler designed cells were entirely adequate and further they more than complied with Lewis' suggested criteria for a good cell.

I just ran a couple of cells in PLOP for a 22"x2" f/3.7 mirror. For instance, an 18 point Hindle cell will produce and error of 1/59 wave p-v at the surface of the mirror, or 1/392rms. A PLOP optimized 18 point cell is 1/67p-v and 1/458rms. Either cell blows away Lewis' suggested minimum criteria of 1/32p-v and 1/128rms at the mirrors surface.

Lewis' confirmed Hindle's ideas are fine, further there is literally NO GAIN AT THE EYEPEICE of this PLOP cell. Why then retool, redesign, and risk unhappy customers for unproven PLOP cells if there is absolutely no gain in performance for the user. PLOP will not make a bad mirror good, no matter how hard we try!

I think there's better places to worry ATM projects. For instance the few companies that user interferometer data to certify thier mirrors use 632nm red light for the wavefront calculations. If a mirror is 1/4 wave p-v on the wavefront in red it will be ~ 1/3.5 p-v in yellow green (550nm) which is the standard for visual use. A similiar reduction in RMS is promised as well.

Couple that with the error of the secondary mirror, the whole system has NO chance to comply even with the Rayliegh criteria let alone more stringent standards, at the itsy bitys diffraction limited central spot before coma takes over, Paracorr or no Paracorr! A Paracorr will not make a bad mirror good either, wish as we might!

This alone will have a much bigger influence over the image at the eyepiece than the literally unpercievable difference between PLOP vs Hindle or just about anything else. Except maybe poor baffling and crappy collimation It's easy to get caught up in numbers and penciling out stuff, but at the eyepiece is where the rubber finally meets the road.

Steve Vegos






September 3, 2002 06:32 AM Forum: Telescope Making

Coulter vs. Pyrex mirrors, Dob feet?

Posted By Steve Vegos

Hi Douglas:

How big of a mirror are you considering? Discovery has some 16" plate mirrors at a heck of a deal right now.

Plate is softer which means it grinds and polishes faster. It also /can/ have a smoother surface than pyrex. Plate expands and contracts with changes in temperture 3 times as much as pyrex. This is not good while the mirror is cooling, but doesn't matter one wit if the mirror is already cooled.

As Jim said, the good one is the mirror with the good figure. Either glass can have a good figure.

Feet?, three wood blocks, teflon right above them.

Steve



September 15, 2002 04:50 PM Forum: Telescope Making

What is a good source for a 10" F/5 Mirror?

Posted By Steve Vegos

Mark:

How you determined the mirror was 1/2 wave will be an important part of your claim to Orion?

Orion would be the place to contact for a replacement. They claim "diffraction limited" which is understood to be 1/4 wave. Use this for a reference:
http://www.nova-optical.com/about.htm

Orion may simply send you a new mirror, the accuracy will be unknown. You'll have to determine it's correction and may have to trade out mirrors a few times to get one that complies with thier adverts.

Is it new, you are the first owner?

Steve

October 13, 2002 04:18 AM Forum: Equipment Talk

Celestron Ultimas vs. Tele Vue Plossls?

Posted By Steve Vegos

Peter:

I think it depends on the scope you use.

I have the Ultimas from 30mm down which are now with an 8" f/6 Dob at my fathers. They are fine eyepieces giving great views of anything you like to observe. But, in my f/4.5 scopes they /seemed/ to me to have more field curvature which makes the off axis image noticably poorer than with either TV Plossls or UO orthos.

I couldn't tell one lick of difference between the TV 32mm and the Ultima 30mm in that scope BTW. Which, I think is a testimony to the Ultima's performance at the better price point.

TV Plossls are really nice eyepieces in the faster scopes and I also use the UO orthos in f/4.5 instruments as well. I have the UO ortho 25mm - 4mm including the 6mm, and the later TV Plossls 32mm - 8mm. These /seemed/ to me to have flatter fields, yielding better correction off axis.

The only way you will be able to see the difference among these eyepieces is in a heads up side by side comparison in your particular scope. I still think there's value in picking a line and working on accumulating those eyepieces. The Ultimas or TV Plossls are parafocal within the line, each eyepiece in that line behaves more or less similiar to each other. UO orthos are not parafocal but they do have similiar characteristics that makes accumulating these as a set attractive.

I like any one of these as a set. Even though there is so much overlap amoung them, I really can't see any reason to get rid of them. The are relativley inexpensive, fine performing workhorse eyepieces. If I have more than one scope out then they willing and ready to work. And work very well.

Incidently I also have a 27mm and 15mm Pan, 9mm and 7mm Nagler and a 20mm UO Super Erfle, and believe it or not a 50mm Axiom. That eyepiece was a prize my wife won in a raffle at a star party. It works great, evenin my fast scopes! The only other eyepiece I would like to have is a 22mm Pan. (The new 17mm Nagler is an outstanding eyepiece, but I like the super wide fields in the higher powers in a Dob. +/-65deg AFOV is really much more comfortable for me.)

The point I want to make is the plain jane Plossls and orthos spend a lot time in the focuser. Despite the fancy stuff in the eyepiece case, in some circumstances they outperform the high dollar glass.


Steve Vegos