My Failed Attempt at a Prime Mount

Why would I post an article on a failed engineering project? Don’t know. Maybe because I haven’t posted anything in a while. Maybe in hopes that somebody will read this and tell me what I did wrong. I began obsessing about a way to prime mount my 60D to my XT10 a few months ago knowing full well that a telescope without a motor is not an ideal situation for astrophotography; I just wanted to tinker with some optics :) Photos with a 25mm eyepiece placed between the telescope and the camera turned out less than satisfactory. Only the very center of the image is in focus. I was able to composite multiple images into one to create a fairly clear image of the moon using this set up, but I felt I could do better. Photos of the sun taken using someone else’s XT8, a 25mm eyepiece and an aperture mask (a cover with a much smaller opening to block most of the light) turned out delightful.

It took me a while to realize the aperture mask made all the difference. To this day I don’t know why reducing the aperture improves clarity from side to side. I tried it on my XT10 with a homemade aperture mask and produced similar results.But I wanted edge to edge clarity without having to sacrifice my telescope’s giant f/4.7 aperture. I thought about buying a more expensive eyepiece, one that boasted “flat field” characteristics. I’ve been told the larger the aperture (smaller f-number) of your telescope, the more important the quality of the eyepiece. But I also knew the less glass you introduce to an affordable optical system, the less chance you have of screwing up the image. So, I decided to eliminate the glass entirely. Prime mounting a camera to a telescope means light reflects off or refracts through the primary optical element and then falls directly on the camera sensor. In the case of my newtonian telescope, light reflects off the surface of a single parabolic primary mirror, is redirected 90 degrees via a flat secondary mirror, and a few inches after focusing, produces a sharp image. The key is to place the camera sensor exactly where the image is produced.

And that is where I failed.

The problem with prime mounting my telescope is that the image is produced inside the telescope which makes it physically impossible to place my camera at the correct position. This isn’t an issue with an eyepiece as it’s glass lenses magnify the image and place it outside the telescope. How then, does one push the image outside the telescope without the aid of glass? You raise the primary mirror. So, with a dozen nuts and bolts I was able to do just that, only, not to the extent I needed. Part of the problem was the bolts and screws my telescope came with are a mixture of metric, imperial and from what I can tell some newly invented unit unknown to the Home Depot. Nothing fit quite right. Nothing was long enough. Everything just sort of fit into each other. In the end I raised my mirror probably just millimeters shy of being able to prime mount my camera and achieve focus at infinity; mount being bolded because if I don’t mount my camera but remove the mounting apparatus and just hold my camera to the focusing tube of my telescope, I can achieve focus. Here is a picture of the moon doing just that. Pretty sharp isn’t it. A hell of a lot sharper than with the eyepiece. But it’s not very stable having to hold the camera up against the telescope and when all is said and done, it’s not worth it. When you move the mirror up, all your eyepieces (which I’d still like to use for normal eyeball viewing) become useless as the focal point is also moved up. I guess you can’t have it all. One of these days I’ll get smart and fork over the money for a motor-driven refractor and make glass work to my advantage.

Check this guy’s post out who wasted invested (see comments below) some money on a low profile focuser if you’d like some more inspiration. He actually got it to work :)

2 thoughts on “My Failed Attempt at a Prime Mount

  • Wasted money on a focuser with an 11:1 reduction and far better stability than the original… Hmm. Well, if you don’t think that the original that came on an XT8 is a useless pile of garbage for mounting a 1Kg camera to a scope, or if you like fiddling endlessly with a focuser that has no gear reduction then I can see why you think it might have been a waste. The simple fact is, it is a huge improvement and IMO was well worth the bucks.

    As you have noticed, if you move the mirror up the tube certain eyepieces won’t focus anymore. The low-profile focuser has a built-in adjustable extension tube so I can easily swap from camera to eyepiece. Yes, you can have it all, but sometimes you have to spend a bit more than the cost of a bag of screws to get there from here. And it’s only a waste if it doesn’t do the job. :)

    • dfjosh says:

      I came off fairly bitter there at the end, didn’t I? Probably because I didn’t get it to work. Anyway, I changed the wording of the last paragraph. Being cheap is something of a vice of mine and doesn’t mix well with an expensive hobby such as astrophotography. I sincerely congratulate you on your success. Maybe I’ll give it another go someday ;)

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