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question

Posted: Thu Feb 13, 2014 8:52 am
by ANuttall
On my quest to find my first telescope, I've noticed that scopes of F7 length offer a focal reducer for a x number of £'s, my question is does adding a F/Reducer work on any scope
i.e. would adding a F/Reducer to say an F5 make a difference.

Re: focal reducers

Posted: Thu Feb 13, 2014 9:59 am
by rwilkinson
I've always used focal reducers with my Celestron C8 f/10 SCT.
My Meade 0.33 reducer can only be used with small CCDs (such as my old MX5C camera and LX webcams), but the Celestron 0.63 is good for DSLRs and even 2" eyepieces (to give wider-field views).

And I recently did some tests with my new Starwave 80ED to see if the 0.63 reducer would work with it, and found that I could indeed achieve a focal ratio of f/4.6: viewtopic.php?f=6&t=605

Re: question

Posted: Thu Feb 13, 2014 12:48 pm
by cstone
Focal reducers do make a difference but if you've not done so already you could also take look at using the Field Of View calculator which would give you a rough indication of what you could see. The drop down menu Barlow/FR: is where the barlow and focal reducers are located.

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

And again if you've not done so already take a look on the Gallery. Above the image there is a green I button. This will display some extra information (which is normally hidden) and providing the owner of the picture has filled in the fields it can tell you what equipment was used to take the image.

Re: focal reducers

Posted: Thu Feb 13, 2014 1:19 pm
by rwilkinson
There's also a handy on-line calculator for focal reducers:
http://www.wilmslowastro.com/software/formulae.htm#FR
which shows how the focal reduction varies with the spacing between the reducer and the CCD.

It also includes the "in-focus distance" required (how far you need to wind in your focus knob to get the image sharp). Beware that this distance is fairly limited with a refractor's draw-tube (whereas an SCT, where the primary mirror is moved to adjust the focus, has a much greater range of focus travel).
For example, using the Celestron 0.63 reducer with my 80ED the furthest I could go was a reduction down to f/4.6, by which time the 'scope's focus-tube was fully retracted.

Re: question

Posted: Thu Feb 13, 2014 4:45 pm
by ANuttall
cheers for the replies, I do look at the fov calculator but i was leaving it on no barlow becuase i could'nt get it to work then i realised i had to press the submit button :oops:
Also i had seen that site with all the calculations on it ross, but with my aversion to maths i quickly switched to another site.