Astrophotography - Prime Focus

[engstrom]

I have an Explorer 200p and a Canon 450d and I tried prime focus imaging last night for the first time. I popped the t- mount on and the adapter on the camera and tried a few shots at various settings. None of them showed much of a hint of sky...

The closest I got was a couple of feint offset images of the vanes that were bigger at one end of the focus than the other.

What am I doing wrong? I'm not a bad photographer and I've taken some images of the night sky from a tripod before.

[LAW]

I found this when I put my DSLR on for the first time last night. You need to work out where the 'prime focus point' is, take all the eyepieces, holders and adapters off the focuser and point it at something bright like the moon, then measure how far back from an arbitrary point your hand needs to be before an image of the moon (or whatever) forms on your hand. Too close or too far will give you a lovely picture of your secondary mirror romantically backlit

Once you've worked out how far back to go then order the relevant 2" or 1.25" tubes. You'll probably need to go closer rather than further out, which can sometimes require a 'low profile focuser'.

[Astrosurf]

It sounds to me like you weren't focused. I focus first on a bright star like Arcturus with an EP (do all my alignment with EP and finderscope as usual). Then when I attach the cameraI have to focus the camera quite a bit (can't remember if out or in). I use live view and zoom function (if you have it) onto the bright star. I then use a Bahtinov mask as my eyes are awful! You might be able to just use your eyes.

Alexxx

[Cornelius Varley]

Don't use either of the eyepiece adapters that are supplied with the telescope. The "flange" part of the eyepiece holder has a t-thread, you can screw your t ring directly to it and you should be able to achieve focus.

Peter

[astrofox]

With my DSLR I dont need any extentions at all on my 150mm PDS. The T2 screws directly on the primary adapter sitting on the end of the focus tube. That way I can get prime focus.

Always best to try stuff like this out in daylight on some far distant object.

[Astrosurf]

Don't use either of the eyepiece adapters that are supplied with the telescope. The "flange" part of the eyepiece holder has a t-thread, you can screw your t ring directly to it and you should be able to achieve focus.

Peter

Yep, that's how I do it.

[Velvet]

I found with DSLR, no extension or anything on, then the focus is realing hard to find. Evetually I found I was moving the focusers to quickly and passing the point without realising it. Takes time and minute moves to find it, once you have it, then it becomes easier next time.

Velvet (beginner too)

[boabsta]

With my DSLR I dont need any extentions at all on my 150mm PDS. The T2 screws directly on the primary adapter sitting on the end of the focus tube. That way I can get prime focus.

Always best to try stuff like this out in daylight on some far distant object.

The pds is a different setup to the p so that focus is acheived more easily.

With the p, as above, remove anything that extends from the focuser and take it slow. Find a bright star and use that for focus. All the better if youve got a mask.

Good luck and post some of your pics up here when you get them

[Tinker1947]

As said your out of focus, when you find the sweet spot Liveview will get it exact, Liveview will also see right down to mag 9 stars but as these are really faint the focus point is less than half a turn on a 10-1 focusor and does take some patience.

[engstrom]

Thanks for the advice. I shall try while it's light and take it step by step.