Focal length too long for astrophotography

[Shnoz (Sophie)]

I have encountered a problem which I imagine is easily solved but is being quite the obstacle for me. I can attach my DSLR camera to my reflector telescope without any other lens and the attaching adapter is only a couple of centimetres long. The problem is that I can't wind my focus in enough for me to get a clear image. Taking into account the focusing piece and the adapter together the overall focal length of my telescope is too long.

But I don't know where I can find the right pieces to correct this. I have a Skywatcher telescope, 750mm focal length, 150mm diameter, and I need it to fit an Olympus E-420. I don't really think the adapter can get much smaller, maybe I need a smaller focusing piece?

[naskies (Dave)]

There are three main options for dealing with your problem (insufficient back focus):

- Create more back focus by moving the primary/secondary mirrors closer to each other. Easy to do if you have a extendable truss tube Dob, otherwise it's rather drastic with solid tube reflectors.

- Add a telecompressor to magnify the image, e.g. a 2x TeleVue PowerMate or 2x barlow. This will make your focal length longer, but also gives you more back focus.

- Replace the focuser with a smaller, lower profile focuser if you're very close but not quite there.

Second option is generally the easiest. If you search on the forums, there are many discussions about this. Good luck!

[Shnoz (Sophie)]

Cool, thanks mate!

[DJ N]

Hi Sophie,

I previously had a Skywatcher Black Diamond 150mm newt. I did give it a quick try attaching a camera to it and similarly could not get it to focus. I pushed the primary mirror up as far as it could go, then "slackened" it off slightly to give some flexibility for collimation and "hey presto", no problems being able to focus the camera. That 10mm or so made all the difference.

Hope this helps.

Cheers,

Daniel

[Merlin66 (Ken)]

Unfortunately a common problem with the SW Newtonians....
The solutions suggested are the answer...

[Wookie (Ryan)]

My SW 10" reflector has the 1.25" reducer with a thread that unscrews.
If yours also has this type you can simply unscrew it and screw on a t-mount adapter in it's place.
This allows a DSLR to come to focus easily.

[raymo]

Thanks Wookie; I had a look, and both my 8" and 10" Sky newts have this feature. It's a pity Sky don't mention it in their manuals.
Incidentally, has anybody noticed that the Sky dovetail mounts for finderscopes are made back to front. The little flange that limits how far
the dovetail can go in is on the lower end, so that if the retaining knob
comes loose the finder falls to the ground. Celestron ones are at the upper end, so it doesn't matter if the knob comes loose.
regards raymo

[Screwdriverone (Chris)]

Hey Sophie,

Here is how I fixed it. Of course, I am using the 2" MPCC so this may not apply to you.

On my 12" dob and my 8" newt, all I had to do was as described by Wookie, you unscrew the 1.25" barrel on that adaptor and the M42 thread left behind screws into the T ring of the camera.

Check the primary mirror screws aren't also screwed all the way in (meaning the primary is right on the bottom of the tube) as they do this to secure the mirror during transport. If you loosen these, you may get focus as DJ N suggested.

Cheers

Chris

[wulfgar]

Originally Posted by Wookie

My SW 10" reflector has the 1.25" reducer with a thread that unscrews.
If yours also has this type you can simply unscrew it and screw on a t-mount adapter in it's place.
This allows a DSLR to come to focus easily.

Typically the older Newtonian focusers had a small extension tube below the eyepiece collar that could be unscrewed and removed to allow more infocus for camera's.