imaging optimised Newtonian

[slowflow (Barry)]

What is the difference between an ordinary newt. and an imaging optimised Newtonian please?

[philipheaven (Philip)]

Even though I don't do astro photography, I'll throw in my brief 2 cents.

An imaging Newt usually has a short focal length, say f/4. Focal lengths such as this are often referred to as being 'fast.' They allow wider fields of view with shorter exposure times. If you tried to use an f/4 Newt for observing you'd get quite a bit of coma. You'd need a paracorr or something along those lines to get a cleaner image.

An observing Newt has a longer focal length.

In my opinion the dividing line seems to be f/4 and lower for imaging, and f/5 and up for observing. I use an f/5 for observing and I barely notice any coma (though that may be a result of the Televue eyepieces I use). I have looked through some f/4 newts and the coma is quite noticeable.

[RobF (Rob)]

The imaging newts will generally have the mirrors and focuser configured that reaching focus with a DSLR is straight forward.

On the down side, they require more careful collimation and tend to lose it more easily due to the shorter f ratio and the fact the secondary and primary assemblies are rarely much better engineered than the standard scope, which can cause frustrations. Carbon fibre tubes can minimise tube movement and temp changes.

[doppler (Rick)]

There are GSO f5 imaging newtonian ota's available now at Andrews comuniations. I use an 8" f6 newtonian (fiberglass tube) for my astro snapshots and it seems to be fairly coma free. I optimised it by moving the primary mirror forward 50mm to achieve prime focus with a dslr. If I was going to use a full frame sensor camera I would have to fit a larger secondary mirror to avoid vignetting in the image.