Long back focus

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heyjojos
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Long back focus

Post by heyjojos »

This is going to be a very basic question for a newbie like me looking for a telescope. I read in an article of a telescope having a 'long back focus".

What does that mean and why is it good??: )

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wucheeyiun
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Post by wucheeyiun »

IMHO, long back focus means that the OTA tube is too short, or you will need a longer focusing tube. I cannot think of any benefit if you are encountered with a long back focus. You may not even use your telescope properly. Cheers and clear skys ...

kochu
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Back focus

Post by kochu »


heyjojos
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Re: Back focus

Post by heyjojos »

kochu wrote:I hope this explains.
Kochu/7-6-11

http://www.celestron.com/c3/support3/in ... cleid=2217
Thanks Kochu for your reply. That was helpful.
I'm looking at a refractor with 230mm back focus. So that should mean lots of accomodation to include focus reducer, flattener and a DSLR right?

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cloud_cover
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Post by cloud_cover »

Basically the issue of back focus is one for the imagers: when mounting a camera to a telescope, some advanced users will want to mount various devices in between the camera and the telescope, such as filter wheels, off-axis guiders, filters, focal reducers etc. All of these have a certain width hence will require some amount of back focus in order to come to focus (if you telescope has 0 back focus, that is, has perfect focus without extending the focuser at all, then you cannot use any accessory of any thickness without making it impossible now to focus the telescope since the image is now inside whatever accessory you are using).
If you're intending to buy a telescope to look at things, generally back focus is not a major issue in most telescopes.
Hope it helps :)
DON'T PANIC

heyjojos
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Post by heyjojos »

Thanks all... really helpful :)

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orly_andico
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Post by orly_andico »

long backfocus is also useful if you want to use a binoviewer.

superiorstream
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Post by superiorstream »

Back focus also important if you are thinking of obing the sun.T-scanner require a very long back focus;so is the blocking filter used in most Ha system for the sun and sometimes because of the need to use DSLR and lack of backfocus,you may be force to buy a straight through blocking filter that has a price tag of over a thousand dollar.My 2 cents.

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wucheeyiun
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Post by wucheeyiun »

Also ... if you have a 2" diagonal, will also help some of your eyepieces and binoviewer to reach focus.

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timatworksg
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Post by timatworksg »

Well alot has already been said...and they are all great pieces of advice!
Yes,....longer back focus allows the use of equipment in between the scope and Eye Piece/CCD Cam such as filter wheels, extenders, Off Axis Guiders, 2" Diagonals...etc etc. All these extra in-between's extend the scopes length and the long back focus allows for this.
Shorter back focus, such as newtonians loose out. Mods and improvements have been made such as bringing the primary mirror forward or a shorter focuser body (found on Imaging Newtonians)..however don't benefit from to many in-betweens as it is limited.
Some scopes have an screwed on Focuser section (such as the Stellarvue SV102, which has a screwed on back end which allows one to shorten the scope length for the provision of a Bino viewer).

So as already said...if your an imager....long focus is a plus! Allowing you to add the in betweens needed for imaging (but increases weight and balance). Also larger Refractors have long scope lengths...which with these in-between's increases it a fair bit! Smaller refractors (60, 80, 90mm) are nice and balance out well with the extras, but you get a smaller FOV...trade off depending on your imaging needs (wide field or up-close).
Scopes like the WO ZS66 have short back focus...where as the Megrz is longer.

Hope you get it all in....to play safe...get long focus...lol! Diagonals and extenders are nice bits to have as extra equipment and can benefit in the long run of your hobby!
My wife never complained about how much time, effort & money I spent on my Astronomy hobby!................suddenly I met her!!!

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