[LVAS] Star Testing a Telescope

Wed Feb 17 21:16:42 PST 2010

Roger,

Cool story!

I met my friend Tony Labude at my very first Okie-Tex star party. He later said I was the only one that would take the time to show him stuff in my old home-built 16" f/6.4. He said I didn't have an "attitude" and opened his eyes to a lot of objects he didn't even know were out there.

I not only showed him some "tourist" objects, but as I did my deep sky searching, I showed him every one and what to look for. We became buds after that and the next year, he had an 8" scope, which I seem to remember was a Coulter. He was the one person I always looked forward to seeing at each Okie-Tex.

All it takes is a kind word and a little effort and you never know what will happen. That was 17 years ago and he is now one of our Observer's Challenge participants.

Fred

From: drivester at hotmail.com
To: lvas at lvlug.org; trenglish at gtcc.edu; dolive at gardner-webb.edu; jdire at gardner-webb.edu; reskridge1 at carolina.rr.com
Date: Thu, 18 Feb 2010 03:39:28 +0000
Subject: Re: [LVAS] Star Testing a Telescope

Fred, 

You make a good point. 

I remember reading an article in S&T many years ago. It said that if the image snapped into focus this was a good indicator that the optics were OK. I have had scopes in the past that have had problems and they would just, "kinda" roll into focus. Never really getting there. 

I must confess something. Every scope that I have ever looked through at a star party or elsewhere I perform a star test. The owner of the 4-inch Takahashi fluorite did not even know that I had performed the test. I did tell him, but he already knew that it was "textbook" optics. The same for the 5-inch Astro-Physics.

My Vixen would rate as an A(--). The 10-inch f/4.5 would be B+. Pretty much all commercial optics other than the "very" premium high end refractors will have some problem. I think that all will agree with me on this one. 

A quote from Meade Instruments in the late 80's and also early 90's, advertised my 10-inch DS-10A. "The primary mirrors included with the Models 826C, 1060, and 1266 are virtually as fine as be manufactured. It is impossible from a practical point of view, and without great expense, to manufacture the 10-inch and 16-inch short focus (DS-10A and DS-16A) to the same level of accuracy. But that is precisely the point of the Deep-Space telescopes. The DS-10A and the DS-16A are primarily designed for deep-space observations of galaxies, nebulosities, and extended stellar objects. In these cases it is essentially meaningless for an astronomical mirror to be of research-grade accuracy, since mirrors of lower optical accuracy perform fully as well, and for much less cost. 

However, on one special night I was able to use 500x on Saturn without any breakup  with a view that rivaled any photo that I had ever seen. A friend was observing with me also. He has one of the best 14.5 mirrors that I have ever encountered. He was able to use almost 600x with images that could not be imagined. This was one night only in my 30+ years of observing , and never to be repeated. The year was 1992.  I am glad that I had a witness. When I see Bob, the owner of the 14.5-inch, we always talk about that night.

When I first met Bob at an observing session one night I did a star test on his scope. We had never met before. I asked for several of his EP's, and then concluded that I had never seen an f/4.5 mirror this good. I told him. He was surprised that someone in the crowd would be able to test his mirror. He thought that I was just another casual observer. He was already aware of this fact. He did not know me at the time nor did I know him. Some of the expert mirror makers in Charlotte had helped him make the mirror several years earlier, and like the Komorowski scope, all test revealed that it was beyond superb.  

Bob and I became friends that night and spent many, many nights observing together, often till 2 AM or later on work nights. Bob has since moved to Charlotte and I seldom get to see him anymore.   

Roger    

    The moon and stars to govern the night.....   Psalm 136:9

From: rayworth1969 at hotmail.com
To: lvas at lvlug.org
Date: Wed, 17 Feb 2010 20:00:27 -0600
Subject: Re: [LVAS] Star Testing a Telescope

All,

I've used everything from an oatmeal box with a light inside and a pinhole on the side, to the old reliable insulator on a telephone pole, to Polaris with a Ronchi grating. I prefer the Ronchi grating on a bright star, and not usually Polaris as it's kind of dim. In some ways, maybe you don't want to star test your optics either. If you suspect something major wrong, sure. However, if you are happy and just curious, it may not be wise to dash your hopes and dreams by finding your optics are astigmatic, over/under corrected, have a turned down edge. Sometimes it's better to let sleeping dogs lay/lie, whatever...

Fred

From: drivester at hotmail.com
To: lvas at lvlug.org
Date: Wed, 17 Feb 2010 13:45:09 +0000
Subject: [LVAS] Star Testing a Telescope

All,

Hey guys, just some good information. I am sure many are aware of the following information, but it is still some good reading.  

Do you know how to star test your telescope? Have you ever collimated your scope in the daytime using an artificial star? 

1. Artificial star: Any very shiny well polished sphere. I use a round brass cabinet knob that I sit on the top of a hollow metal tube. Some people use an insulator on a utility pole at a distance, or the corner of a chromed auto bumper. There is a formula for the distance, but hey don't worry about this. Just make sure that the sphere is at least a few hundred feet away and you can focus on it. A larger scope requires a greater distance.  

During the early mornings when the wind is calm and the sun is shinning very brightly. Sit your scope is in the shade, to avoid heat on the OT. Observe the disc at very high magnification. Look for the airy disc. Are they of the same brightness all around the star? Now de-focus your scope. Are the rings on the inside of focus exactly the same as the outside? Very few scopes will pass this test. Probably the most sensitive test of all. I have seen "textbook" images in only one scope that I have ever tested, a Takahashi 4-inch refractor, and I think it was a 5-inch Astro-Physics refractor. Of course we all know that these are high resolution, very high quality scopes, and cost as much as a very good used car. I attempted to do a test on a Questar one night, but the seeing was not good enough.       

Are you familiar with the "airy disc" named after 19th century scientist Sir George Airy. This is a critical test of your scope. Have you ever observed the airy disc in your scope?
No, this is not the out of focus diffraction rings, but the very faint ring or rings surrounding the perfect in focus star at very high magnification. This can be almost impossible for a larger scope as it requires at least 30x per inch of aperture. If this test is performed at night "almost" perfect seeing is required.

I had an excellent quality 80 mm f/15 Japanese refractor quite a few years ago. I noticed when working on double stars that the airy disc was brighter on one side as compared to the other (180°). However, it was very slight and I could not see the miscollimation while observing or defocusing on a star. A precise adjustment of the aperture resolved this problem. I am somewhat afraid to mess with the adjustments on a refractor. I would never, let me repeat, never touch an Astro-Physics, Tak, or any other $5,000 (+) OTA. This is not a job for the amateur, wanting to learn.  

I also had an ETX 90 with superb optics. When comparing to a $5,000 90 mm Questar the images were not much different. The ETX was a 1996 model, the first year that they were introduced. I am not sure how well the mirrors are in the newer, made in China ETX's. The same problem as the 80 mm. At very high magnification the airy disc was brighter on one side, again, exactly 180°.

 I could move the test star very slightly from the center and could find perfect collimation within the scope. Being a Maksutov-Cassegrain scope without those primary adjustments screws (Laura was talking about this in an e-mail a couple of days ago, regarding a certain 100 mm Newtonian) I could not put the scope in perfect collimation. It was so slight that the performance of the scope was not really compromised. A great little scope indeed, that would split double stars far beyond its limits, so this would indicate the miscollimation was not that great. 

A good book that I would like to recommend that will explain everything above and much more.

Star Testing Astronomical Telescopes by Harold Richard Suiter. This guy is really a genius at optics. He is an experimental physicist.