178MM-Cool or 174MM-Cool for fast refractor and 8-inch SCT

bfonseka

I have a F5.5 refractor 500mm focal length and a 8-inch F10 SCT. I did some image scale caclulations using CCD calc and came up with a 178MM as giving the best image scales.

SCT + 178MM gives 0.25 arcsec/pixel for planet imaging - no barlows needed. Refractor with 178MM gives 0.97 arcsec/pixel for deep deep sky. Adding a 0.5 x reducer covers 67 x 100 arc min field at 2 arcsec/pixel and gives a very fast F2.8 imaging system.

Most of the pics I see around though are from 174MM cameras and barely anything from 178MM cameras.

I am wondering what I am missing in considering a 178MM-Cool. Is it too noisy, not got enough dynamic range, fixed pattern noise impossible to fix, too much amp glow?

riklaunim

178MM is new, and only 178MC was available in the past. As for DS imaging with 178 - I would rather see only lenses and very short refractors for it. A generic 0.5x focal reducer will either eat all of the backfocus or if it will focus the field may not be fully corrected.

firstc8

Yeah, by your own calculation a very fast FR is needed for DSO with 178 small pixels, which is most people do at this price level but very few has such fast scopes. The 174's bigger chip also makes it more popular, again for DSO.

I think for your situation use of the generic 0.5 reducer is fine if you choose 178 because of the small chip capturing only the center. And Hyperstar will work will also but by that time, might as well buy a bigger CCD camera.

riklaunim

firstC8 wrote:
I think for your situation use of the generic 0.5 reducer is fine if you choose 178 because of the small chip capturing only the center. And Hyperstar will work will also but by that time, might as well buy a bigger CCD camera.

Refractor without field flattener will show a bit of curvature without the focal reducer. The sensors isn't that small, and adding 0.5x generic reducers can destroy image quality easily. For small pixels the f/ratio doesn't have to be extremely fast - would be nice, but that's not essential. What is essential is the focal length as resolution depends on that - it's easy to get to high resolution for DS imaging. That's why lenses with their <= 300 mm of focal length and few refractors are the best pick for such pixels. Hyperstar at 300-400 mm (6-8") could be used too, but that's a much more expensive solution than a lens.

firstc8

High resolution is nice but can average backyard with so so seeing conditions take advantage of it? Or is this reserved for a dark site?

riklaunim

You need very good seeing to shoot DS objects at high resolution. Plus very good and stable tracking/guiding. I just found it way easier to shoot DS objects on low resolutions where it's very easy to make everything "just work".

bfonseka

Yes - I understand about the tracking requiremnts. I am on a NEQ6 auto guided with a Lodestar on a separate guidescope. The mount is on a permanent pier in my backyard. The image scale of a 178MM is similar to a DSLR if I used a 0.5x reducer on the scope. I have every confidence in being able to autoguide at that scale - that is what I do now.

The scope is a fairly decent triplet refractor (TMB-92ss) and I do have a field flattener for it. It does produce a well corrected field all the way out to the edges of APS-C sensors and is also quite good even to a full frame DSLR. It also has ample backfocus. I have a removable section in the OTA if I need more.

riklaunim

Then it's up to you to pick the camera ;)

Sam@ZWO

178 has the lowest amp-glow problem among these cameras

very suitable for high resolution imaging