Imaging Train question

I'm new to this telescope, an 8" f/4 Imaging Newtonian with coma corrector. I got the spacing right between the CC and the focal plane of the camera and get nice stars across the FOV, but in order to get to focus I need to use the 35mm extention tube. Is this right? It seems like the camera is too far away from the focuser.

The attached image shows the location of the camera that gives a properly focused, flat image. All the numbers are in millimeters.

Any input is greatly appreciated.

Jeff

When I started attaching my camera to my 12" Newtonian I had to cut 5cm from the telescope tube to shift the mirror up to allow the focal point to meet the plane of the camera sensor.

Now I need to use a 5cm extender when using the telescope with eye pieces for visual observation.

I think that your setup may not be correct.

If the ‘scope is an “imaging Newtonian” then I understand these are designed to produce sufficient back focus to allow DSLRs to come to focus, including the addition of a coma corrector in the optical path to fix the coma from such a “fast’ ‘scope.

Not sure if they use an oversize secondary to achieve this, or maybe a combination of low profile focuser and reducing the distance between primary and secondary.

I have heard of amateur astronomers with classical visual Newtonians resorting to chopping off a bit of the tube to move the primary closer to the secondary.

If the OP is getting pin point stars across the full field, then it looks like he has the correct set up.

Cheers

Dennis

Thanks Dennis,
If one were to use this scope for visual observing, then would a much longer extension for eye pieces be required?

This is the Astro-Tech 8" Imaging Newtonian. It is advertised to be for both visual use and imaging and it does live up to that with the supplied accessories.

For visual there is a 35mm extension tube and it works great. There is noticeable coma though.

The thing I didn't really expect was that to image with a CC (purchased from Astro-Tech) you'd need to use the extension tube as well.

It seems like the tube could have been lengthened to bring everything in towards the focuser a bit.

Jeff

Are you using the 35mm extension with the camera as well (as per the OP picture)? Although my OTA has been shortened to extend the focal point by 5 cm, I only use the 5cm extension when observing visually. The camera does not use the extension tube.

Classic Newtonians for visual use have parabolic mirrors and generally had a focal ratio of F8 which minimised coma and other unwanted optical aberrations. However, these were considered too “slow” for astrophotography and required quite sturdy mounts due to the length of the optical tube.

For a shorter tube imaging Newtonian, a focal ratio of F4 is very “fast” and requires a Coma Corrector to correct the coma in these fast optics. These imaging Newtonians also have larger secondary mirrors so they can utilise the full light cone from the primary.

Cheers

Dennis

Just to kind of close this thread out, I tried a couple of configurations again last night and ended up at the same place each time.

Here's a look without the 35mm extension tube in place, but then the focuser has to be moved out to compensate.

I guess that's just the way it's supposed be.

Thanks for all the input.

Jeff

I wonder if the “extra” space taken up by the 35mm extension tube, is to allow for an “off axis guider” being inserted into the imaging train?

Cheers

Dennis

I find the length of the combined hardware strange. My Vixen R200SS is an f/4 Newt and I replaced the stock focuser with a Moonlite low profile unit for a couple of reasons. One, to assure focus with my DSLR and secondly to keep the camera load as close to the optical tube as possible for balance reasons. My camera sits considerably closer to the tube than yours currently does and I'm sure the Moonlite design is at least partially responsible. I don't have any spacers and my CC slips into the focuser, can't tell how much of your CC does from your photo. So with my hardware being more "compact"(shorter) I'm a bit confused to why your focus range is achieved with your camera so far out from the optical tube. In many cases with Newtonians low profile focusers are installed replacing the stock unit just to reach focus with a DSLR.

Balance on both axis' is very important for long exposure work so I would think that counterweight(s) opposite your camera would need to be rigged on your set up. The most important thing is that you're able to achieve focus so if your current hardware is what it takes then you may have to rig something for balance. I would test it out and see how your mount is behaving during extended exposures.