Image of the day

Captured by
Lawrence Parchman

Eastern Veil Nebula

About Astromart

My Account

New to Astromart?

Register an account...

Need Help?

Posts Made By: Alan French

July 8, 2013 02:11 PM Forum: Telescope Making

Help - please?

Posted By Alan French
Kathleen,

I'm very sorry to hear about your loss.

Don't throw them away! Corning no longer makes Pyrex, so such blanks are in demand and there is a robust market for them. A search of Astromart classifieds would help you come up with reasonable prices. Even quality plate glass blanks would have a market.

If you have the original shipping crates, it would also be useful to know where they were purchased.

Clear skies, Alan

July 9, 2013 12:06 AM Forum: Telescope Making

Help - please?

Posted By Alan French
By far the best place to sell such things is here on Astromart. The Classifieds have a "Telescope Making" category.

Clear skies, Alan

July 30, 2013 11:56 AM Forum: Telescope Making

Mirror blank.....what is this material?

Posted By Alan French
I have some older Pyrex blanks, and the molding looks identical to your photo, with a raised rim around the back. One 10" blank has a yellowish tint similar to yours.

Pyrex is less dense than plate (2.23 g/cm^3 versus ~2.5 g/cm^3), so you could weigh it and measure its volume and calculate the density for a reality check.

Clear skies, Alan

August 28, 2013 01:23 AM Forum: Refractors

Pre-1970 Objective with Large Air Space

Posted By Alan French
Here's a view of the push-pull screw arrangement on the front of the tube, with the lens and cell removed.

Clear skies, Alan

August 28, 2013 01:24 AM Forum: Refractors

Pre-1970 Objective with Large Air Space

Posted By Alan French
Here's the cell with the retaining ring removed. The cap at the base of the photo (top of cell) is not part of the original cell, but was added later.

The black cylinder you can see inside the cell is the spacer between the elements.

Clear skies, Alan

September 8, 2013 09:34 PM Forum: Equipment Talk

Imaging: What's the Point?

Posted By Alan French
The point? Some people enjoy it. There are things folks pursue do that I don't understand, but that doesn't mean there is no point to their interests. And there is a lot of creativity out there - it's not all just the same-old same-old.

The issue of excess lights is simply rudeness, or lack of concern for others, which seems to infect a modest segment of any group. (It can also be ignorance.) I understand things happen, so dark skies are sometimes interrupted by brief intrusions of white light (often by today's cars), so I don't get hyper about it. I sure will go ask folks who continually use white light or overly bright red light to please knock it off. It's better than just fretting about it and getting annoyed, and I can't recall anyone giving me a hard time about a polite request.

Clear skies, Alan

October 2, 2013 01:57 AM Forum: Refractors

Under/over correction and width of air space

Posted By Alan French
Robert Apruzese said:

If an air spaced objective has too wide an air space, will this contribute to under correction or over correction? Same question if the air space if too narrow. Final question: For a 60mm f/16 objective, does anyone have an educated guess as to what the optimum air space gap would be ? Thanks in advance.

What sort of change are you thinking about? I designed a 60mm f/16 BK7/F2 doublet that had a 0.5mm air space and a Strehl of 1.00 in green light. With the same design and a 1.0mm or 0.0mm gap and refocusing the Strehl was still 0.99 in green.

Most achromats I've seen a very thin foil spacers. Long ago I believe it was common to use pieces of a postage stamp.

Clear skies, Alan

October 3, 2013 06:11 PM Forum: CCD Imaging and Processing/Deep Sky

Uranus and Moons

Posted By Alan French
This is the satellite orientation given by Stellarium for midnight on October 1, so you can see which is which.

Clear skies, Alan

October 5, 2013 09:10 PM Forum: CCD Imaging and Processing/Solar System

Jupiter 10/05/2013 good seeing conditions

Posted By Alan French
Excellent!

Clear skies, Alan

November 13, 2013 12:11 AM Forum: Astro-Physics

AN unusual AP scope

Posted By Alan French
I found it in an AP catalog with a price sheet dated 9/5/90. The entire scan is too large to post, but here are the "Technical Specifications."

Clear skies, Alan

Funding Member

Sponsors


View all sponsors

Search