Hi araasch, You're right about the degrading effects of thermal gradients. This was definitely a consideration during the initial though processes. But with an aluminum tube, and careful management of the temperature, the internal volume seems to come to thermal equilibrium fairly quickly. The key is to keep the temperature differential small. I typically keep the system just above dew point. Regardless, I get pinpoint stars from my first exposure through my last. So this leads me to believe that any thermal gradient effects must be less than the resolution of my system (3.5 arc-sec/pixel). I have no problem resolving single-pixel stars, all night long.
You also made a good point about the effects being magnified on tubes with longer length (larger f#). However, this focus management system would not be as advantageous on larger tubes. An f10 scope would have a CFZ 4x that of an f5 scope. So thermally-induced focus issues are much less of a problem. In the build instructions, I mentioned that I was having focus shift issues within the window of my 30 minute exposures. An f10 scope would be far less likely to experience this.
About the initial temperature setting... so you start off with a setting that will maintain whatever the ambient temperature is at the beginning of a session? And the real point to my question... do you still need a dew heater on the primary?
I have made a dew heater for my Canon 20D because I do 2 min. max piggy back astrophotography. I'm upgrading in a year or so to a scope that will allow longer exposures and prime focus work. It would be great to skip the dew heaters for what you've shown us here.
Doing most of my astrophotography from a site near the Delaware Bay means that dew can be a big problem. At times the whole scope gets wet. This might even stop that.
That's fascinating, Eric. So the GOTO already has the object you're seeking. The link you provided actually shows photos of the objects you may wish to capture. When you capture an object, do you submit it to the site (if you get a better shot than they're presenting)?
Rob, You got it. There are some mounts where you enter the coordinates and have to manually push the scope to the proper coordinates. But most modern mounts have some sort of GOTO option. It takes a lot of the time consuming work out of trying to find objects.
The mount I currently use is an Astrophysics Mach1GTO. The "computer" that it uses has over 17,000 objects in its database (stars, clusters, nebula, etc). Once the mount is aligned and calibrated, you can enter an object or coordinate, press GOTO, and it will slew to that object with superb accuracy. If you're interested, you can find details here:
If you speak to any serious astrophotographer, the majority will say that the mount is the most important piece of equipment for getting good, long-exposure, deep sky images.
As you stated, telescopes focus points change over temperature so one way to minimize this is to hold the telescope temperature constant over time. However, as the temperature drops during the exposures, this will cause thermal air currents of increasing strengths to be present around the telescope which are not desired.
This becomes more critical with longer focal length scopes, as they provide more magnification for any given imager.
A different solution to this problem is to use a micro focuser which actually makes minor focus adjustments that are intended to compensate for the shift in the focus point of the telescope. This way, the main body of the telescope is allowed to vary in temperature so it will be close to the ambient temperature and generate fewer thermal air currents.
As you are no doubt well aware, if the objective lens of the telescope is cooler than the dew point, moisture will soon form on the objective lens which is highly problematic. The addition of a heater which only heats the objective end of the telescope to that necessary to keep it just above the dewpoint allows imaging with minimal thermal air currents being generated.
You may find that you would get sharper images with such a setup, at the price of a more complex system.
Wow! Those images are breathtaking. We are big astronomy fans and have made many trips to Fort Davis to go to McDonald Observatory for their star parties. We took our son 2 years ago to look through their 109" telescope for his thirteenth birthday and Dr. Lambert, the director, allowed us to stay after the viewing and observe astronomy researchers at work - my son even got to help and log in on the official viewing sheet for that night's research.
We have been talking a lot on Design News about getting the next generation excited about science and astronomy is certainly a great way to do it! We have a Meade ETX-125 and have been toying around with the idea of astrophotography enough to buy a scope mount for our camera - but never got any further. After reading about this super cool gadget - it makes me want to go at it again. What a neat way to keep the scope temp regulated AND prevent condensation. Way cool gadget!
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