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Posts Made By: Jarad Schiffer

July 20, 2008 02:58 PM Forum: Telescope Making

Low profile spider - any ideas?

Posted By Jarad Schiffer

Craig Combes used this design:
It's a 3-vane T-shape, with the mirror attachesd directly to the vanes at the T (no vertical bolt). Very low profile, also very stable (much less leverage on the spider than a typical design).


August 24, 2008 03:01 PM Forum: Reflectors

Diagonal size question

Posted By Jarad Schiffer

What is the tube diameter?

A Moonlight focuser has 1.6" height. Assuming the tube diameter is 14.5", and you want 0.25" spare in-focus, a 2.14" secondary will give you a 0.7" fully illuminated field, and 1.45" 75% illuinated field, which should be plenty. You could even drop down to 1.83" if you wanted to make it a planetary scope (100% zone would still be 0.3", plenty for planets, but you might see vignetting in wide field eyepieces - the 75% zone would only be 1.1").


September 13, 2008 02:50 PM Forum: Reflectors

Secondary Mirror Question

Posted By Jarad Schiffer

One other thing to consider - are you planning to use a paracorr? Don't forget that the paracorr barrell sticks about 3" into the focuser. If you use a super-low-profle focuser to minimize your secondary obstruction, then you may end up with the barrel of the paracorr sticking into the light path, which will act just like the secondary, only square (so it will produce spikes rather than round diffraction).

On my 14.7" f4.4, I could have just managed a 2.6" secondary by letting the paracorr stick about 1" into the light path. But that adds 2 quare inches of obstruction, which is about the same total as a 3.1" secondary, and the 3.1" will give much better illumination. If there was a commercial source for a 2.8", that would have been perfect, but there isn't. So I went with a 3.1" for the scope. I would recommend the same for your 16".


March 31, 2009 02:56 PM Forum: Eyepieces

Better than Naglers?

Posted By Jarad Schiffer

Part of the problem is that you are trying to trade for eyepieces that are old enough they have become rare.

The original 11 nagler was only made for a short time, there probably aren't a lot out there. The few that are may be kept for sentimental or collector value as much as performance.

The 32 widefield hasn't been made in a long time, again there probably aren't a lot of them out there. It's possible that nobody has seen the offer who has one.

The 55 plossl is a different beast completely. It's generally used by SCT or long-focus refractor owners, who want the low power. The lowest power T4 nagler is a 22, which may be too much power for those scopes on most nights. The people who want T4's are generally fast dob owners, who are unlikely to have a 55 plossl sitting around in their collection.


April 7, 2009 02:39 PM Forum: Telescope Making

How critical? Mirror cell question

Posted By Jarad Schiffer

I don't think it's too critical, but if you are going to redo anything, I would switch to a 6-point cell. They actually provide better support than the old style 9-point cells do according to PLOP.


May 20, 2009 06:13 PM Forum: Equipment Talk

The alt-az "sleeper"

Posted By Jarad Schiffer

I guess I just don't understand why you would want to do this. By leaving it in the normal polar orientation, you get easy tracking (either with a motor, or with a handle for the polar worm gear). You don't have to spend much time or effort on polar alignment for visual use, just plop it down facing roughly north, and you'll be close enough. Why switch it to alt/az mode?


February 18, 2010 08:03 PM Forum: Telescope Making

metallic foam- any ATM uses?

Posted By Jarad Schiffer

I dunno - light weight is good, but elasticity is bad. For telescope parts, you generally want as much stiffness as possible. I think you'd be better off with the carbon-fiber honeycomb sandwich laminates or other similar things with high stiffness/weight ratios rather than this. And since they are intending it for medical use, I suspect the metallic foam will be very expensive - they are going to be trying to get back the millions they are going to spend getting FDA approval...