5D3 and odd problem focusing telescope

[PeterJ]

I just bought a cheap 2nd hand Dobsonian 6" telescope with a T-mount and Canon adapter. Anyway just set it up during daylight hours to have a play around and all fine through the eye-piece but couldn't get anything to focus through the camera. So I pushed the focuser on the telescope in so the 5D was pretty much as close as it would go to the telescope body and moved it around to find out where it was focusing. It turned out I could focus on a power pole a little under 20 meters away, so now I have a nice photo of an insulator (actually was suprised how well it turned out).

Anyway I decided to remove the T-mount and hold the camera towards the telescope, then realised it would attach directly to the thread on the telescope without the T-mount. When mounted that way now I can focus out to about 4KM. I've never used a telescope before so maybe it's all normal and it will focus on very distance objects OK because the light will "cross over" one more time or whatever the right term is? Will find out tonight I guess.

Another interesting thing was to get a decent live-view image I could use to focus I had to select a 1/15 shutter speed whereas the metering reported 1/250 was correct which turned out to be about right.

 

[bkorcel]

This is common for newtonian telescopes. For astrophotography you need to move the main mirror further up the tube by what ever distance you need maybe 1 or 1.5 inches. Depending on the design its often just a matter of drilling three more holes EXACTLY the same distance in the tube up from the existing ones. Mark a spot on the back of the mirror holder and on the inside of the tube so you can get it put back in without rotating it. My 8" f/6 scope came with the holes pre-drilled.

I just bought a cheap 2nd hand Dobsonian 6" telescope with a T-mount and Canon adapter. Anyway just set it up during daylight hours to have a play around and all fine through the eye-piece but couldn't get anything to focus through the camera. So I pushed the focuser on the telescope in so the 5D was pretty much as close as it would go to the telescope body and moved it around to find out where it was focusing. It turned out I could focus on a power pole a little under 20 meters away, so now I have a nice photo of an insulator (actually was suprised how well it turned out).

Anyway I decided to remove the T-mount and hold the camera towards the telescope, then realised it would attach directly to the thread on the telescope without the T-mount. When mounted that way now I can focus out to about 4KM. I've never used a telescope before so maybe it's all normal and it will focus on very distance objects OK because the light will "cross over" one more time or whatever the right term is? Will find out tonight I guess.

Another interesting thing was to get a decent live-view image I could use to focus I had to select a 1/15 shutter speed whereas the metering reported 1/250 was correct which turned out to be about right.

 

[PeterJ]

Thanks bkorcel. I just took a better look and on the main mirror there's some alignment screws and I'd hoped they might do the job but they don't have much play in them. Because it's more like an end-cap I wouldn't be able to just re-drill the screws without cutting down the tube. I'll pull it apart tomorrow and take a better look at how it works, I've got a small mill so might be able to make up something like some threaded spacers to hold the mirror further up the tube.

The odd thing is he said he used it with a 5D2 and had a sample shot of the moon so I thought I must have been doing something simple that was wrong. I know he could have been speaking crap but apart from seeming like a nice guy it was a local sale and I know where he works etc. Anyway in a few hours it'll be starting to get dark so might have a further play around and see how I go.

 

[bkorcel]

I see. The screws in the back are used to align the primary mirror. There is a process for that. I will see if I can find an article for collimating a reflecting telescope. It's been years since I had to do mine, I forgot the basics!

In your case if the mirror is affixed as an endcap then you can only cut the tube and remount the mirror...not likely something you want to do...all kinds of things can go wrong.

Other options are to replace or re-machine the focuser to be more low profile to get the camera closer to the tube. You can buy an eyepiece projection adapter which allows you to use an eyepiece in the middle to increase magnification or use a barlow lens which is kind of like a teleconverter for camera lenses but will also move the focus point out as well.

Reflecting telescopes can produce EXCELLENT image quality...often better than most camera lenses. However they are primarily only good for stationary objects and a pain to get focus exactly right. Good for the moon, landscapes, and paparazzi.

Thanks bkorcel. I just took a better look and on the main mirror there's some alignment screws and I'd hoped they might do the job but they don't have much play in them. Because it's more like an end-cap I wouldn't be able to just re-drill the screws without cutting down the tube. I'll pull it apart tomorrow and take a better look at how it works, I've got a small mill so might be able to make up something like some threaded spacers to hold the mirror further up the tube.

The odd thing is he said he used it with a 5D2 and had a sample shot of the moon so I thought I must have been doing something simple that was wrong. I know he could have been speaking crap but apart from seeming like a nice guy it was a local sale and I know where he works etc. Anyway in a few hours it'll be starting to get dark so might have a further play around and see how I go.

 

[PeterJ]

Thanks for the offer but the instruction sheet that came with it has a fairly decent description of aligning the mirror, I checked and it was properly aligned to start with so that side of things seems OK.

The focuser doesn't look like it would be easy to modify, the various parts of the mechanism run pretty much along the full length of the tube. But thanks for the hint on the eyepiece projection adapter, I found a local supplier so just shot them off an e-mail to see if they think it will solve it and if it's OK to return if not.