For imaging I thought aperture was king?

[swag72]

It all boils down to compromises at the end of the day. One day I hope to have to be making the decision between changing my scope or my camera.

[Whippy]

The reason I asked this question is because I see nice looking RC scopes, saying how great they are for astrophotography and yet they are imaging at f/8. I can't help thinking though that if these scopes are any use at all, I'd quite like one!!

I (unfortunately, only briefly) had the 6" version of this and that works out at f9 which for nebulae and galaxies is painfully slow! My understanding is that GSO who manufacture these do have a reducer in the pipeline to make the 8" & 10" models a bit quicker but I don't know when it'll arrive. There are alternative reducers you can use (reducer that also flatten are useless with these as the field is already pretty much flat) like the Intes and Astro Physics models. John (JGS001) also has the 6" and has had a couple of decent images out of one.

Tony..

[ollypenrice]

I (unfortunately, only briefly) had the 6" version of this and that works out at f9 which for nebulae and galaxies is painfully slow! My understanding is that GSO who manufacture these do have a reducer in the pipeline to make the 8" & 10" models a bit quicker but I don't know when it'll arrive. There are alternative reducers you can use (reducer that also flatten are useless with these as the field is already pretty much flat) like the Intes and Astro Physics models. John (JGS001) also has the 6" and has had a couple of decent images out of one.

Tony..

Ouch, yes, I'd forgotten that the small one was F9. That is asking for a reducer and in that case I think there would be better alternatives like one of the good budget apos with FL around 900mm.

Olly

[swag72]

When I go down the CCD or new scope road - Will I be able to roughly work out the exposure time needed to get a useable image with both different options and then compare that with what I already have?

Does that make sense?

[Grinde]

Of course if I went for the QSI it might work with built in filter wheel but would it work at f3.2?

Olly

Quoted from the QSI homepage: "Unvignetted images below f/5 with standard 1.25" filters, or f/2.8 with low cost 31mm filters"

Since my summernights are too bright to even see stars, I have to make the most out of my imaging nights in the winters, so I'm definately into fast focalratios (f/5 or lower). I've looked at photos shot in sweden with slow 10" rc's, & found the level of detail is the same shot with 10" newts, with half the exposuretime... I bet at +2000mm focal length(if not sooner), seeing starts to limit the detail, so why spend more time & loose alot of fov, when detail won't increase anyways? After watching the stuff Peter Shah produces with his OOUK AG astrographs, I guess you don't need thousands of mm's of focal-length to pull out tons of detail in small targets. & the reduced imaging-time required to pull out noisefree images, I'd definately say focalratio is king in imaging.

[BlueAstra]

Pedants Corner:

I believe F/Number is actually defined as the effective focal length (EFL) divided by the entrance pupil diameter. The EFL is normally quoted by the manufacturer, and fortunately the entrance pupil of a telescope is usually equal to the diameter of the first lens (refractor) or mirror (reflector, note may not be the first element). However, there are some lens systems (eg wide field photographic lenses) where the entrance pupil is buried within the system, and is nowhere near the size of the first optical surface. The actual position and size of the entrance pupil depends on the location and size of the physical aperture stop in the optical system, and the optical elements located infront of it.

[ollypenrice]

Quoted from the QSI homepage: "Unvignetted images below f/5 with standard 1.25" filters, or f/2.8 with low cost 31mm filters"

Since my summernights are too bright to even see stars, I have to make the most out of my imaging nights in the winters, so I'm definately into fast focalratios (f/5 or lower). I've looked at photos shot in sweden with slow 10" rc's, & found the level of detail is the same shot with 10" newts, with half the exposuretime... I bet at +2000mm focal length(if not sooner), seeing starts to limit the detail, so why spend more time & loose alot of fov, when detail won't increase anyways? After watching the stuff Peter Shah produces with his OOUK AG astrographs, I guess you don't need thousands of mm's of focal-length to pull out tons of detail in small targets. & the reduced imaging-time required to pull out noisefree images, I'd definately say focalratio is king in imaging.

Yup, this is pretty much how I see it as well but fair points have been made about binning and pixel size which do complicate the issue.

Olly

[Grinde]

Yup, this is pretty much how I see it as well but fair points have been made about binning and pixel size which do complicate the issue.

Olly

Agreed. When it comes to discussing the light before it hits the sensor, faster focalratios is the way to go in my opinion. & sensorwise I'd want large sensors & lots of tiny pixels, which can be binned to 'simulate' large pixels