'Zooming in' with prime focus images

[Beardy Bob]

Hi folks,

I have a 350d attached at prime focus to a c80ed using a t-adapter and a 2" extension tube, which produces OK images. However, I will hopefully be getting a much sturdier tripod and mount soon to allow much longer exposures than I can currently achieve (and of course greater magnifications).

The FOV using my c80ed and 350d is about 2 degrees by 1.3 degrees, which is great for things like andromeda galaxy and M42, but I need advice on how my setup would need to change to capture, for example, very small planetary nebulae and the like, as I'm guessing I would need to be very particular about any optics that I stick in the lightpath so as to not degrade the quality of the final image.

[RobH]

Hello Bob,

With prime foocus imaging, there are only 2 ways to zoom in....get a longer FL scope, or increase the FL of the one you have by using a barlow type lens.

For tiny planetaries though, an 80mm refractor isn't really the right scope....you need long FL for many of these, I regulary used to use 3500mm with my 14 inch SCT, and a lot of them were still very tiny.

M27 though should be within its capabilities, evn without a barlow.

Bear in mind that as you increase FL, you will rapidly lose light, and your exposure times will substantially increase.

Interestingly, I'm planning on doing the same as you with my TMB152, which has a native FL of 1200mm. I have a 1.6x barlow, which will increase the FL to 1920mm, and should be good for many galaxies and objects such as M1.

I did notice, when I had a quick test a while back, that vignetting was an issue though.

Cheers

Rob

[Milamber]

Personally, I cannot see a problem with cropping. If you are worried about image resolution, remember that the pixel size of, say, an Atik 314L is about 8 x 8um, my 300D is 7.1 x 7.1um - your 350D is 6.4 x 6.4um. I would lose nothing at all by imaging any target the Atik can do - I would just have a bigger picture. The actual size of the target would still be acceptable.

Arthur

[brianb]

With prime foocus imaging, there are only 2 ways to zoom in....get a longer FL scope, or increase the FL of the one you have by using a barlow type lens.

And don't forget that increasing the focal length magnifies the effects of poor tracking / wind vibration / bad seeing as much as it increases the image scale, whilst making exposures longer. I respectfully suggest you master short focal length imaging before trying long focal length deep sky imaging; shoeslaces will be tripped over when you try to start running....

[RobH]

I didn't mention cropping Arthur as it isn't really zooming in....you may end up with a larger image, but you're not going to get any more detail.

Cheers

Rob

[Milamber]

True - but you are not going to get less than with a dedicated astro camera. OP was worried that his FOV would preclude smaller targets - I was pointing out that the FOV had nowt to do with it is all.

Arthur

[RobH]

True - but you are not going to get less than with a dedicated astro camera. OP was worried that his FOV would preclude smaller targets - I was pointing out that the FOV had nowt to do with it is all.

Arthur

Absolutely Arthur, you are correct of course. In this case, it's just about framing the object really.

Cheers

Rob

[dph1nm]

With prime foocus imaging, there are only 2 ways to zoom in....get a longer FL scope, or increase the FL of the one you have by using a barlow type lens.

Actually there is a third way, which is to buy a camera with smaller pixels.

NigelM