Camera adapter hack? (hoping for a quick answer)

[StutzJr]

So I recently inherited a second hand Canon DSLR camera (EOS 400D) and I was wondering whether there is any remote chance of using it to take planetary / lunar shots with my 6 inch dob.. I dropped by Bintel the other day (unfortunately it's a fair way for me to get there) and I picked up an Orion T-ring adapter for my canon EOS and a 2" T2 adapter to fit into the scope. The guy at the store suggested that I may not be able to get an image to focus with a dob using the "prime focus" technique, and that I might have to consider using the "projection" technique with an eyepiece + spacer in front of the camera. He said that if I wasn't able to focus the setup with the 2" adapter then they would happily exchange that for a 1.25" projection adapter.
So I tried the 2" setup tonight and unfortunately I wasn't able to bring it into focus - the camera sits a bit too far back from the prime focus point even at the "fully in" position on the focuser. Fair enough, it was to be expected but I wanted to try first. At least now I know the camera is not as heavy as I anticipated and I had no issue keeping the dob stable, plus I have a remote trigger button for taking shots.

Anyway, I had an idea that I wanted to get some feedback on -

I had a look at the 2" T2 adapter and I thought, hey there's enough space inside that for either my 10 mm kellner or 15 mm plossl eyepiece! Of course there's no supports inside that tube at all, but the space is big enough for the EP to actually be there.
Would it be worth trying to actually temporarily hold an eyepiece in the inside of the 2" T2 adapter tube, either with some wadding / tape / blu-tac / ? and attempt to focus a projection shot like that - at least until I have a chance to exchange the adapter?
Is there anything I would want to be careful of if I attempt this? I won't try to attach it unless I am sure there is no chance of the EP dropping out or falling on the the mirror or touching the inside of the camera.

At this stage I'm really only interested in taking lunar shots and it would be nice to get an occultation shot if I can get a working setup in time.
If down the line I catch the photography bug I might need a different scope but that's a decision for another day.

[mill]

Just move the primary mirror up the tube with some longer bolts and some stronger springs and you can get focus.

[StutzJr]

Apart from the fact that sounds like a tricky job and needing extra parts, wouldn't that then preclude me from using my dob for visual observing?

I found that when I replaced the skywatcher "shaky" standard rack and pinion focuser with a GSO 10:1 crayford, because it is shorter I have to clamp the 2" to 1.5" adapter back from the bottom stopper and likewise with the EP's to get in focus. If I push them all the way in, they are too close even at the end of the travel of the focuser (basically the opposite issue of the camera).

[ZeroID]

Then you just get an extender for your EP. HINT: Unscrew the lens from your Barlow if you have one and instant extender.

I've done the shift the mirror bit, it's not too hard if you can get all the bits and just a matter of keeping the mirror stable and collimated. I used longer bolts and solid spacers so it wasn't wobbling round on long bolts and springs. I do have to use an extender for some EPs but my primary aim is photography anyway.
I've just bought a 2" 2x Barlow so that is also an 'extender' now when required.

[StutzJr]

I might consider that option in future but for now I'd like to try the projection method first and see how it goes.
Looking at the products of offer I don't understand why the 2" T2 adapter is $10 more when sold without an internal 1.25" ep holder.. Does the claim "Minimises vignetting by providing the widest clear aperture possible using t-thread connectors" really amount to much?

[Regulus]

Why not just a 32mm or 40mm GSO projection lens for $59 from Andrews.? It comes with a 2" to T-Thread adapter too. That should give a reasonably flat field and high mag to play with at a reasonable price.
I do understand the fun of the challenge you have set yourself though, and I only mention these incase you are not aware of them.
Trevor

[StutzJr]

Ok for the third time writing this reply,

I did see those lenses on andrews but with a 6" dob I wouldn't call a 32mm lens high mag, but I might consider them later for field shots (can't really attempt long exposures with a dob). Only the 2" versions of those lenses include T thread adapters - I don't have a 2" barlow - if I did I wouldn't have had the issue in the first place :)

[StutzJr]

On a side note I am looking at this article here:
http://www.astronomysource.com/2011/10/13/eyepiece-projection/

Why is this additional multiplier not in effect when you hold a phone camera up to the eyepiece?

[Merlin66]

When using an eyepiece projection, it's not just the focal length of the eyepiece which gives the magnification but the spacing between the eyepiece and the CCD. The further the distance the higher the magnification.
Re. The camera with lens fitted
This is the "afocal" method. The eyepiece and the camera lens must be both focused at infinity.

[ZeroID]

The 32mm 2" gives a lovely widefeild view. I bought one as an experiment and it just about sucks your eyes into the sky.
This is the fun part of astronomy, figuring out ways to do what you want.

[Regulus]

Is the lack of the multiplier effect because at the point of focus the light cone is narrow to the point of being equal to the size of the sensor?

[StutzJr]

I'm not exactly clear on the difference in optics between the prime focus, projection and afocal methods, but I guess the prime focus method only works if the observed image is virtually at infinity?

Anyway I did try mounting an eyepiece inside the 2" T2 adapter with a strip of packing foam sheet wrapped around the EP and taped in position. It worked quite well to hold the ep in position and central/parallel, however I was only able to get the setup to focus with a 10mm EP right up to almost touching the inside of the DSLR and then I had the 2" adapter gripped at the very end of the travel of the crayford focuser. With other EP 's I was only able to get close to focus holding the camera a few centimetres further back. I guess that's why long spacers are required when using the projection method (which I don't have). I guess it wouldn't be too hard to make a spacer from pvc for a trial run.
Later on I tried taking some afocal shots at an eyepiece with the camera held on my binocular mount but I found it too frustrating trying to get everything aligned before the moon moved out of frame.

[tlgerdes]

A x2 Barlow will often allow you to bring your dob to focus.