Trouble with focus length

[Swithin StCleeve]

I'm having fun with my new SLR and the Moon, (and my Skywatcher Newtonian). I have one problem though. I can only get it to focus through the 2X Barlow. I bought an extension tube the same length as the Barlow, hoping to take some shots without the Barlow's lens in the way, but again, it wouldn't focus.

To get the shot attached below, I had to get the camera right up to the telescope, so close that I wouldn't be able to get any fitments in to actually hold the camera, if you know what I mean. The shot attached was taken by hand, (I'm amazed it turned out as well as it did).

Any suggestions? Or an I talking gibberish?

[Demonperformer]

I believe this is a common problem with Newtonians. Options include replacing the focusser with a shallower one, or moving the primary mirror slightly up the tube, to bring the point of focus far enough away from the OTA. I will leave details on how to do this to someone who has actually done it .

You are able to focus with the barlow because it alters the light cone into a longer, narrower one.

[ronin]

As said by DP it is fairly common - not helped by the manufacturer.

I guess it is a 200P of one variant or another, but the same holds true for the 150P as well. The scope is built/setup to operate with an eyepiece. The eyepiece has to be set such that the prime image plane from the mirror sits usually at the base of the eyepiece or a bit further away from it. With the mechanics of a DSLR the image plane has to sit on the sensor that is inside the DSLR body. So the DSLR needs to get further inwards, and there just is not generally the travel available in the focuser.

As you have found you can use a barlow to move the prime image plane out and so it reaches the sensor. That works for the moon but is not good on DSO's as they are faimter, and the barlow makes them fainter still.

The "solution" is to move the main mirror up the tube a bit, 25mm to 50mm depending. You then recollimate. And then you buy a short extension tube. You will need the extension tube so that you can view with an eyepiece as with all the adjustments the focal plane is now too far out for an eyepiece to work properly. So you use the extension tube to move the eyepiece outwards for visual.

I think that ES now produce the majority of their scopes to have a DSLR attached and reach focus and then they throw an extension tube into the supplied accessories. This tends to mean that people do not add the extension tube then say they cannot get an eyepiece to focus.

As said at the start "not helped by the manufacturer" as they say you can attach a DSLR, well you can attach it but not get the result you expected. The presumption is that "attaching" means "attach and get images" but that is not what they state, just "attach".

[Swithin StCleeve]

Thanks for the replies. Very interesting, and obviously, a bit irritating. And yes, it's the 200.

Seeing as I can actually get it in focus by holding long bit of the T-mount on the camera 'inside' the eyepiece, I might look at making something to hold the camera in place first, before I start moving the main mirror. It kind of needs a flange that goes inside the eyepiece, rather than sticks further out.

 

 

[Cornelius Varley]

Your telescope's focuser should be the 1.25"/2" variety. If you unscrew the eyepiece adapter you will find a t-thread on the part that goes into the focuser draw tube. Attach the t-ring directly onto this and then attach your camera body to the t-ring.  See video

 

[Swithin StCleeve]

Brilliant!

Yes, my Skywatcher has the 2 inch hole. As I said, I could hand-hold the camera to get prime focus. Looks like that video's given the answer.

WhooOO!!

Clouds permitting, will try it tonight and hopefully get back to you with some shots tomorrow.