TheSkySearchers

My telescope happens to be an astro imaging Newtonian at 254 mm f/4, this was originally chosen for dual use as in some imaging and some visual. So apart from the weight reduction, what are the pros and cons of a smaller refractor versus a larger reflector?

Not only do we consider the f/number, to me the more important number is the focal length. The focal length determines your image scale (based on which Imaging camera you use) and your guide scope suitability. A small ish refractor with a corresponding short focal length is going to show a bigger chunk of sky than your Newtonian. We like these because the smaller refractors are great for imaging the bigger nighttime objects, like Andromeda, the Pleiades, and other large astro features.

JayTee wrote: ↑Fri Jan 14, 2022 6:17 pm Not only do we consider the f/number, to me the more important number is the focal length. The focal length determines your image scale (based on which imaging camera you use) and your guide scope suitability. A small ish refractor with a short focal length is going to show a bigger chunk of sky than your Newtonian. We like these because the smaller refractors are great for the bigger nighttime objects, like Andromeda, the Pleiades, and other large astro features.

How wide-field are we referring to, as I thought my scope was wide at f/4 with a 1016 mm focal length, maybe slightly less with the coma / reducer? I am so out of the technical loop these days, having paused the hobby for some seven years.

Th f/number is a function of speed. It determines the duration of your image. An f/4 scope will function twice as fast as an f/8 scope. The interesting thing about astrophotography is that exposure duration has many different factors involved, more than just the f/number. I will get some links for you to read on the trade-off between f/number and exposure duration.

Quite significant actually, a 254mm f/4 with a 25mm eyepiece has a TFOV of 1.34°
My 80mm f/6.9 using a 25mm eyepiece has a TFOV of 2.5°

The maximum FOV of your 254mm is 2.69°, my 80mm is 4.9°

Lady Fraktor wrote: ↑Fri Jan 14, 2022 6:34 pm Quite significant actually, a 254mm f/4 with a 25mm eyepiece has a TFOV of 1.34°
My 80mm f/6.9 using a 25mm eyepiece has a TFOV of 2.5°

The maximum FOV of your 254mm is 2.69°, my 80mm is 4.9°

I am starting to grasp it once again, something I need to look into more.

Here is my favorite field of view calculator. Make sure you select Imaging mode. Input your numbers, all of them, to see what your FOV is for any astro object you have selected. Go ahead and try different focal lengths and you will see how the imaging scale changes and how imaging scale is not directly dependent on your f/number

https://astronomy.tools/calculators/field_of_view/

Hmm, I see how I could be missing out as most DSO are usually large, and it saves the effort of having to think about mosaics if I wanted to image a large emission nebula for instance. A smaller refractor was also a consideration as it reduced the effort of set up and of course the total weight on the mount.

We almost always recommend a small refractor as the "beginners" setup for AP. Besides the benefits you list above, more importantly, a small, short FL imaging frac allows for larger guiding errors before they are visible on the image itself, this is a function of imaging scale. They also have the advantage because of their lower f/number they can use shorter exposure times creating a smaller window for errors to occur. Meaning, you keep more of your light subs for processing. They "cut" you some slack so you can "cut" your teeth on AP procedures without losing too much hair!

You could easily use your TS 80 as an imaging scope. Go on craigslist and find an "El Cheap" 50 - 70mm frac as your new guide scope. Now you have short FL needed to learn the ropes. Technically, you could use your Newt as a guide scope but that introduces more problems than it solves. Typically you'd like your guide scope to have a shorter FL than your imaging scope.

Let's see if I can remember. Check Wikipedia "eyepiece". If

F = focal length of objective
f = focal length of eyepiece
d = sensor diameter (AP) or eyepiece field stop (visual)
D = objective diameter
FR = focal ratio (x in F/x)

with everything in mm, then

FR = F/D
Magnification M = F/f
TFOV (in radians) = 2*atan(0.5d/F) or, approximately, d/F
AFOV (in radians) = 2*atan(0.5d/f) or, more approximately, d/f
Dawes limit (arc seconds) = 116/D
degrees = (180/pi) * radians

The light flux is inversely proportional to the F ratio squared, the exposure time is proportional to the F ratio squared so a F/4 we're pretty good compared to many fracs at F/7.5 or so. Not good when compared to a C11 with hyperstar at F/2 or a Celestron Raza at F/2. A factor 4 is pretty significant. But those reflectors can't handle filter wheels.

Generally for a 10" Newt compared to an average refractor, it has an advantage of F ratio, resolution based on Dawes limit and magnification. The resolution is deteriorated by the secondary obstruction and bumps in the mirror surfaces. Aside from that, the improved resolution according to Dawes maxes out at about 8" aperture due to turbulence on average nights. So a smaller frac might have an equivalent resolution on most nights.