Size vs f/stop in astro imaging

If you have the choice between a larger but slower scope and a smaller but faster one, which would you choose?

e.g.

80mm f/4 or 124mm f/6
6" f/8 or a 10" f/10

Do the numbers end up offsetting one another like halving the ISO but doubling the shutter speed? Does it depend on the target? What else factors into it?

124mm f6.

why? more difinition. Better resolution it is base on Raley's law in physics.

However betwen a cheap 300mm and a very good 200m ....

Depends on it's purpose . If for imaging WF images then the 80mm f/4 throught the 6" f/8 . However the bigger the better if your using a FR with the instrument . If for viewing then any size is great but the bigger the more light gather capacity it has . Being a faster scope is really a helpful part of the mathmatics to help increase the speed of absorbing signal from an object to the filming plane . Other words the faster the scope the less time you have to expose for the same amount of signal . So , like i say it depends what your reason for the scope is . So whats your reason ??

Celestron wrote in post #8791604
So whats your reason ??

To take as many cool pictures as I can.

I've got the deep sky stuff covered (hopefully) with a 10" LX200
I have a 6" f/8 refractor from a package deal on a mount. Semi-widefield.
I might soon be picking up a 5" f/12 Mak for planetary stuff.

I've just been seeing a ton of different sizes, fl, makes, and models of scopes and that got me wondering if a smaller yet faster scope can equal a larger & slower one.

I think I'd eventually like to get to:
10" SCT - Deep sky
5"-6" Mak - Planetary, Clusters
Small APO refractor - guiding, widefield, grab-n-go

I have the most fun with my FSQ85. Why? There are plenty of targets that warrant the degree of coverage it gives and I can do 4 minutes subs that dont stress the capability of the mount or my laziness at accurate polar alignment. Results MORE KEEPERS. Its quick to setup with its baby 70mm guider, easy as chips to balance and cools down fast. When I run out of targets to cover well with it I might move up again but if I went SCT I would want to add Faststar!

chris.bailey wrote in post #8794727
...if I went SCT I would want to add Faststar!

I've only used my C11 XLT once, but that's already on my mind. I did see a Fastar one for just £300 more than I paid for the XLT, (though that price didn't factor in the imaging part of the C11 Fastar.) However, would be a great combination, switching from a 2800mm f/10 to a 500mm f/1.8! Can't wait to see some shots from such a scope.

If I had to only have one scope though, I would probably go smaller and faster. I can see that in time my 70mm will get more use than the C11, due to its portability. Plus I can chuck a Barlow on the 70mm and up the focal length if need be.

I've image with 2 10" SCTs (f/6.3), an 11" SCT, 8" Newt (f/4), 90mm TAK (f/4), 60mm TAK (f/5.9), and have come to the conclusion that the hobby is a lot more fun and satisfying with the smaller and faster scopes. Like Chris, on any given night with the Sky 90 I can rest assured I'll be bringing home a CF card loaded with usable 4-5 minute data that can be had unguided. If I use the 8" Newt, I need to autoguide which adds the laptop, guide scope and camera, extra marine battery/power inverter to the gear list.

A night out with the small and fast refractors is pure fun. Go larger and even with the added plus of focal length, the extra work and non-imaging time consumed during the night makes me want to leave the larger scope home unless the object I'm after requires it. The smaller scopes make the night more relaxing, nice to be able to sit back during exposures.

Nighthound,

What mount do you use for your easilly transportable rig ?
Having to take a computer out to the field is crossing the line of enjoying a dark sky for me.

Steve.

SteveInNZ wrote in post #8797306
Nighthound,

What mount do you use for your easilly transportable rig ?
Having to take a computer out to the field is crossing the line of enjoying a dark sky for me.

Steve.

I use a Losmandy G-11 with Gemini. It's not what most would consider to be an "easily transported" rig but at least it breaks down in enough sections to easily pack in with my accessory cases, marine batteries, etc. The computer is only a laptop and is housed in a light contained unit. It doesn't interfere with the darkness much at all. It's more an issue of just needing an added amount of gear in support of auto guiding, and only when using my RS200 as an imaging scope. Laptops are common practice for autoguiding with the help of software, I use PHD Guide. I drive about an hour to darker skies in the country.