Using a 2x Powermate to double the focal length

[Star Gazer]

Is it possible to double the focal length of my FSQ 106 with a TV x2 Powermate.  I thought this would be no problem, but I cannot get anywhere near focus. I've tried hunting the Tele Vue web site but it's not clear from what I can see.

Has any had any experience with this combo or similar?  Any advice is much appreciated.

[SamAndrew]

I believe the powermate should shift the focus point outwards - if you're trying to do planetary, and assuming the camera is in video mode, you can pull it out of the powermate slowly and see if it comes to focus, which would indicate you might need to add an extension tube.

[Star Gazer]

Yep I tried two extension tubes, but never found focus. I even tried moving the camera backwards an forwards in fresh air, but still couldn’t focus. Unless it’s a shallow depth of field or something like that I am at a loss to know what to try..

[Stu]

From a bit of searching, I think the opposite is true ie you need to wind the focuser in to reach focus. Have you tried that, all the way in?

[ollypenrice]

An aside, but Takahashi do the Extender Q which is designed for the task, but I've never used the one I have. There's a big loss in field of view.

Olly

[Star Gazer]

18 hours ago, Stu said:

From a bit of searching, I think the opposite is true ie you need to wind the focuser in to reach focus. Have you tried that, all the way in?

Yep tried that. ..All the way in and all the way out. "as the bishop said to the a..."

[Star Gazer]

8 hours ago, ollypenrice said:

An aside, but Takahashi do the Extender Q which is designed for the task, but I've never used the one I have. There's a big loss in field of view.

Olly

I'm aware of the Tak Extender, but I was hoping the Powermate was going to give me a similar result. The narrow field of view is what I want, with the increased focal length of course. 

Is there a back focus calculator out there, may be that's my question? I don't see one though. I'll keep looking..

[Stu]

42 minutes ago, Star Gazer said:

Yep tried that. ..All the way in and all the way out. "as the bishop said to the a..."

I’m a moderator. I must not laugh…… 🤣🤣

[ollypenrice]

15 hours ago, Star Gazer said:

I'm aware of the Tak Extender, but I was hoping the Powermate was going to give me a similar result. The narrow field of view is what I want, with the increased focal length of course. 

Is there a back focus calculator out there, may be that's my question? I don't see one though. I'll keep looking..

Sorry, I used the wrong term, I'm afraid. Obviously the increased FL reduces the FOV but what I meant to point out is that it reduces the size of the corrected circle as well. I was being careless. I've no idea how the corrected circle will hold up if you use a Powermate.

I can see why you'd want a longer focal length in order to increase resolution, but why would you want a reduced field of view? You can reduce it by cropping the image. When the resolution in arcseconds per pixel is a constant there is no difference between the effects of optical vignetting (ie corrected circle reduction), reduced sensor size or cropping in post processing. They will all give you the same image.

BTW, since the scope is a Petzval with a fixed corrector lens it may not co-operate with a regular Barlow or Powermate.  TeleVue make a similar Petzval so there may be information on their site regarding this combination. 

Olly