How to calculate focal length for imaging?

[Droogie 2001]

Hi

I am interested in how my current setup will work with my soon to be delivered Celestron AVX mount in the sense of exposure times.

I have a 8SE SCT which has an aperture of 203mm and focal length of 2000mm at F10.

With my Celestron F6.3 focal reducer what does the focal length become? I think it is either 1261mm or 1163mm or something else?!

I used a Focal length calculator from the web but I am not entirely sure how it works...

Also I believe it is favourable to have a shorter focal lengths for long exposure photography so how will the focal range in that range fair?

In the future I may look at faster and shorter scopes but I really just want to get an idea of what I am likely to experience with my current setup.

Many Thanks.

[Pete Presland]

the 8SE SCT would make an excellent Lunar/planetary imaging scope! but would be much trickier for deep sky objects. due to being rather slow, having a very long focal and being on a ALT/AZ mount (field rotation). with a Celestron F6.3 focal reducer, the focal length of 2000mm becomes 1260mm and f/10 becomes f/6.3 for a significantly wider field of view and faster scope. DSO imaging is not impossible with it, but it would be easier with a short tube refractor on an EQ mount.

[Droogie 2001]

Thanks Bunnygod1.

I have the Celestron AVX EQ mount on its way to me so I am really trying to get an idea what my 8" OTA can do with the focal reducer and the new mount. What kind of focal lengths do your scopes offer. I assume you do not use your C9.25 for imaging DSO's then?

No doubt I will be looking for a faster scope further down the line but just trying to plan my DSO targets etc.

[Pete Presland]

its not that you can not do DSO's with the longer focal lengths, you certainly can it is just more demanding of the mount regarding alignment/guiding. i have not used my C9.25 for any imaging other than Lunar/Planetary imaging with a dedicated webcam.

i mostly use my 200PDS which is 1000mm focal length but very fast F5 so as you can see there is not a huge amount of difference, when you use a focal reducer on your scope. my ED80 600mm focal length and F7.5, so shorter FL but slower as well. also for the smaller DSO'S a smallish image scale, great for bigger objects though.

[Uranium235]

Here is where you would go to calculate the FL or FR for any given scope/reducer/camera combination:

http://www.12dstring.me.uk/fov.htm

One of my most visited places.

[ollypenrice]

You just multiply the native focal length or the native F ratio by 0.63 - in theory. In reality it is a bit more complicated because the distance between reducer and chip will have a significant effect on what really happens. But as a rough figure the multiplication works for focal length and focal ratio.

The need for accurate tracking at long focal lengths is really just a shorthand for the truth. The true truth (!) is that the fewer arcsecnds of sky you image per pixel the more accurate your tracking has to be. 'Tolerant' might be 2 arcseconds per pixel or more. 'Very diificult' might be less than 1 arcsecond per pixel or less. These are just rough estimates for your average amatuer imager. My experience lies between 0.6 and 3.5 arcseconds per pixel.

Speed of exposure is also geared to arcseconds per pixel. SImply put, the more arcseconds of light source landing on a pixel the faster that pixel will 'fill.'

What camera are you going to use? One of the many advantages of the monochrome CCD camera is that it can be 'binned' to make larger pixels if the native arrangement gives too few arcsecs per pixel. One shot colour cameras have to work unbinned at their native pixel size.

In general fast F ratios ae best for imaging but there is a caveat, much dreaded in these discussions, called the F ratio myth. This only applies to smaller targets which will fit on your chip at native FL and reduced.

We should leave this till later!

Olly

[swag72]

.....I assume you do not use your C9.25 for imaging DSO's then?....

I am using my C9.25 for DSO imaging and I have to say that it's possible. Many in imaging, myself included, say go for a fast scope, imaging at f10 is madness!!! Well, it is ..... slightly. I've learnt that it's very time consuming and as a very rough figure I need 8x the amount of data on the C9.25 to get a comparable image in terms of noise and detail as from my refractor.

Normally I use 5 hours of data in Ha from my refractor ... so that means about 40 hours of data on the slower C9.25 to get similar data. I say similar as I'm not really sure that SCT data will ever be up there with refractor data. I'm not sure that I have the patience for 40 hours of Ha followed by 40 hours of OIII to make one image!!!