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Gadget Freak Case #222: Warming Up the Telescope
8/3/2012

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Eric Chesak's telescope thermal focus management system is made up of three main elements: the heater, the power and temperature control system, and the thermocouple for temperature feedback.
Eric Chesak's telescope thermal focus management system is made up of three main elements: the heater, the power and temperature control system, and the thermocouple for temperature feedback.

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Astro-Eric
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Re: Wow, amazing pictures.
Astro-Eric   8/6/2012 6:33:18 PM
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Rob, the GOTO library is usually just text based (M42, NGC 7822, IC1318, etc). So you have to know that you want to image and the name of it.  I usually look through a sky atlas to see what large objects are in the sky, research them on the internet, check the size and fit into my field of view and then go from there.  

I did send some of my images of Dark Nebula to the Barnard project at Georgia Tech (http://www.library.gatech.edu/barnard/links.html). I also submit my images to magazines and such.   But I find that my images don't hold a candle to what others can produce.

Astro-Eric
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Re: Way Cool Gadget!
Astro-Eric   8/6/2012 6:43:36 PM
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Hi Ralphy Boy,

For the starting temperature, I usually check the dew point at the start of the session. I set the temperature a little above this dew point, and go from there.  I rarely need a dew heater, as I image from the desert Southwest.  But on several occassions, there has been a nice layer of frost on my equipment cases, where the scope & objective are completely clear.  As I mentioned in a post below, this system is not as advantageous on scopes with larger f#.  It's also not needed on carbon fiber tubes, since the CF expansion coefficient is very small.

Thanks for your interest.  I appreciate you taking a look.

araasch
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Silver
Re: Iteresting solution, but may not be optimal
araasch   8/6/2012 7:17:50 PM
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The larger the aperture, the more sensitive your system will be to thermal gradients.

This is becase the light rays entering the aperture of the telescope are essentially parallel with one another.  So, you can think of the section of atmosphere that the telescope is looking through as a cylinder with a diameter equal to that of the telescope's aperture. The length of this cylinder extends to the limit of the Earth's atmosphere.  If the refractive index of the air in this cylinder is uniformly distributed and remains constant for the duration of the exposure, you can expect a great photo.

Unfortunately, this will not happen due to two causes, either of which can be dominant.  Firstly, the atmosphere can be turbulent due to weather conditions, local terrain, nearby heat sources that heat a portion of the cylinder of air like roofs of houses, etc. This is termed "seeing".  And secondly, your telescope may be at a different temperature than the air close to the telescope.  If the seeing is otherwise good, having the telescope at a much warmer temperature than ambient can cause the seeing to become poor, especially for large aperture telescopes. The effect of poor seeing from either of these sources is to have the atmosphere act as a time-varying lens which changes the direction a particular light ray enters the telescope by a small amount.  If the shifts are uniform across the aperture, the telescope will likely act as if it was pointed in a slightly different location for an instant, if it is not uniform, the contrast of the image suffers as the telescope is effectively pointed in multiple locations simultaneously.

The higher the magnification that you are using a particular scope at, the worse this effect will be.

This leads to some interesting situations where on some nights when the atmosphere is especially turbulent, people using large telescopes want to pack their stuff up because what they see at the eyepiece is a jumbled mess, while those using smaller diameter scopes can still continue to observe, as they are sampling a smaller volume of turbulent air with their telescope.

For non-local seeing problems, adaptive optics have allowed ground based scopes to eliminate much of this atmospheric turbulence.

But it is still a good idea to minimize the temperature difference between your scope and the local ambient temperature.

I looked at your photos, and I agree that this is not a problem for your scope, however.  The fact that your scope is a short focal length and has a small aperture allows this solution to work fine for you, and in that regard may be quite optimal.

Bottom line, if you are getting round stars that are point-like, you can't be too far off the mark!

P.S. If you want to see the effects of localized heat induced air currents, look through your telescope at a moderate to high power (150x or so) while observing a bright star well above the local horizon (45 degrees or so).  Defocus the star a bit so you see a fat star, then place your warm hand near the telescope objective so the heat from it will rise through the air cylinder it is looking through.  You should see the effect very clearly.

 

Rob Spiegel
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Re: Wow, amazing pictures.
Rob Spiegel   8/7/2012 11:14:02 AM
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What makes the difference in the quality of the images, Eric? Is it the telescope? The production of the image? The atmosphere at the time of capturing the image?

Astro-Eric
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Re: Wow, amazing pictures.
Astro-Eric   8/7/2012 1:13:20 PM
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Rob, there are quite a few factors that make a good image (probably too many to list here).  But here's a short list:

1.  Telescope - Good optics with a large flat field and little/no chromatic abberation, coma or other optical issues. The scope also need a good focuser to be able to hold the imaging equipment.

2.  Good Mount - very Stable, able to track accurately and accept feedback from an autoguider (separate camera used to help lock-on to a star).  Also needs to be able to hold the weight of the scope and imaging equipment.

3.  Good Camera - Many use spectrum-modified DSLR's, I use a cooled full-frame monochrome CCD with filters (RGB and narrowband). A CCD requires a laptop or computer.

3.  Dark Skies - Light Pollution is the enemy of good image quality.

4.  Good weather conditions - "Seeing" - atmospheric disturbance, Transparency - little dust or water vapor in the air.  This becomes increasingly important as the magnification (focal length) increases.  araasch commented on some of these issues below.

5.  Lots of good image Acquisition and image processing software (MaximDL, Photoshop, Registar are the main ones that I use) and a good knowledge of their use and techniques - Image processing is where I'm the weakest and where I concentrate most of my time.

6.  A lot of patience -  acquiring the necessary equipment, climbing the learning curve and acquiring the necessary images.

7.  Luck - with respect to weather, transparency and still & stable atmosphere.  Also with respect to getting everything to work properly, which is always a challenge.

So there's a few of the things to consider.  The is a partial list and is not necessarily in the order of importance. 

What most folks don't realize is that the majority of deep sky, planetary and some other images are made from many single exposures. My deep sky images can be 20, 30 or more, single frames, shot over many nights.  Planetary images, can be many thousands of individual images.  These are digitally stacked to increase te signal-to-noise ratio.  This also allows one to use other techniques to reduce the noise.

Rob Spiegel
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Re: Wow, amazing pictures.
Rob Spiegel   8/7/2012 1:30:16 PM
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That's an impressive list of variables, Eric. I would imagine altitude is another factor, since high altitude would mean less atmosphere.

Astro-Eric
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Re: Wow, amazing pictures.
Astro-Eric   8/7/2012 8:42:57 PM
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Altitude is most definitely a help.  Just as you mentioned, the less atmosphere you need to look through, the better. 

But all the images that you see on my site were shot from my back yard at about 3,000 ft, with light pollution and less than stellar "seeing".  So good shots can still be had in less than ideal conditions, including light polution, and high humidity.  A lot can also be done, on a tripod with a camera, Star trails, scenics and such. 

Rob Spiegel
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Re: Wow, amazing pictures.
Rob Spiegel   8/8/2012 11:02:21 AM
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As for the technique you use to avoid focus distortions due to temperature, have you been sharing this strategy with other deep space photographers?

Astro-Eric
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Re: Wow, amazing pictures.
Astro-Eric   8/8/2012 1:08:59 PM
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Actually, This was the first time some of my astro buddies had heard of my system.  I had eluded to it many times, but was still testing it.  Since the GF article, I've since shared it.  At least 2 of my astro friends are interested in giving it a try. 

It may not be a catch-all solution, but for small f# refractors (where the critical focus zone is small), it really seems to help.

Rob Spiegel
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Re: Wow, amazing pictures.
Rob Spiegel   8/8/2012 1:36:49 PM
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Well, this is a good forum to share your solution, Eric. Those who are interested get a video demonstration and full build instructions if they decide to give the solution a try.

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