Beginner question regarding FL and F Stop

[Catanonia]

If I take my ED 120 Pro Diamond from F7 @ 900mm FL and x3 with a Barlow this will take me to 2700mm and a higher F Stop

What will be the F Stop at 2700mm ?

What is the calculation you do ?

How much longer will I need to keep the shutter open to get the same light as at F7 900mm ?

The reasons, I want to zoom in on a distant DSO and take loads of subs every night and need to work out compared to normal subs, how good my giuding needs to be.

Many thanks

Cat

[narrowbandpaul]

you will be working at 2700mm focal length at F21. 3x longer focal length implies 9 times longer subs for the same signal.

good luck

paul

[Catanonia]

you will be working at 2700mm focal length at F21. 3x longer focal length implies 9 times longer subs for the same signal.

good luck

paul

So F stop = the magnification size x original FStop

and

Time to capture = the square of the magnification size ?

[Psychobilly]

It will be quite a challenge ... good luck

[narrowbandpaul]

got it!

but f21 will be very slow....i suggest a larger aperture say at f4-6.

a 200mm f6 will give a good mag at a decent speed.

2.7m is too long a focal length. you will be at the mercy of the seeing, which in the uk can be very variable.

paul

[Catanonia]

cheers guys, with my 120 ED SW refractor.

I like to do test guides of 9 mins before the proper watching to make sure things work.

The plan is to do this on M51 with these settings every single night I start imaging and hopefully after 20 - 30 times of 9 mins at F21 2700mm I will gaver quite a few subs. I believe someone did this recently on a cluster as thier test subject every night.

I know the lens will be slow, but I think I might go for 9 min sub test each night to give a nominal 1 min at 900mm to make the calculations simple.

With 9 min subs should be enough each night to test my guiding kit before main imaging.

It should also give me close to 1/3 a full frame (DLSR frame) worth of galaxy in the image. Ie the galaxy taking up 1/3 of the total pixel count of the picture. (ish)

Sound like a sensible approach ???

[narrowbandpaul]

you sound like you realise it will take lots of subs to get the image quality you desire.

i think your proposed 3-4.5 hours of m52 will certainly give you a good image, and an idea of what is required at f21 with reagrds to guiding and sub length

best of luck, and post your image once the data has been processed.

paul