# Calculating effective focal length of an EdgeHD

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Hi there folks, wondered if you could review the logic in my calculation of EFL for my 8" EdgeHD.  My revised outfit incorporates the following:

Baader Steel Track Focuser - Light path from Baader specs given as 120mm when fully extended, focus range of 30mm.

Baader Click-lock Star Diagonal - Light path 112mm

Technique for focus is to bring the scope to rough focus with the stock SCT Focuser, with the Steeltrack at the midpoint of its focus range, so set to 15mm.

Recommended back focus by Celestron 133mm

Therefore revised back focus would be:

120 - (30 - 15) = 105mm

+  112 = 217

giving a delta of 217 - 133 = 84mm

General rule of thumb recommends 10mm of focal length for every additional  3mm of back focus, therefore:

(84/3) * 10 = 280mm

giving an approximate revised EFL of 2032 + 280 = 2312mm with the scope now operating at F11.3.

Kind Regards

Paul.

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My approach to this is to take an image and to get http://astrometry.net/use.html to tell me exactly what I've got.

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AKB

Thanks for your feedback, unfortunately I have no imaging capability as yet.  The afore-mentioned exercise had a two fold aim, to provide me with a revised EFL but also to allow me to come to a better understanding of how my revised configuration integrates into the light path of the scope.

Kind Regards

Paul J.

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I thought the EFL would stay the same and only change if you used a barlow or reducer

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I image with a deforked CPC1100 Edge, nominally at 2800mm focal length.  I use a Moonlight focuser, motorised with a Lakeside focuser.   The back focus given by Celestron for my scope is 146mm.  When using the Moonlight focuser, I am effectively changing the back focus.  So ignored to set the system up for imaging, I first set the back focus distance to 146mm (using the Moonlight) and then focus the scope manually using the stock focuser.  In this case I know that when I am in focus, I also have the correct back focus.

Stars imaged from a Celestron at f10 are always big, even with correct focus.  Whether I am at f10 or some other figure close to it does not concern me.

Hope this helps

Chris

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7 hours ago, Debo said:

I thought the EFL would stay the same and only change if you used a barlow or reducer

Evidently not.  When I was doing research into acquiring the steeltrack, it became quite clear that this is not the case.  There are any number of documents/posts on the various astronomy forums discussing the impact of increased light path on backfocus.  This is also proven in practice, objects definitely appear larger in my 40mm Plossl In the new configuration, than they did with the stock visual back and star diagonal.  I don't pretend to understand the ins and outs of the optical theory, but the scope definitely is no longer operating at its original focal length.  Anybody out there who can explain the optical theory behind this effect in laymans terms?

Edited by DeepSkyMan
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12 minutes ago, DeepSkyMan said:

Anybody out there who can explain the optical theory behind this effect in laymans terms?

You're changing the distance between two optical focusing elements: the primary and secondary mirrors. This will inevitably change the overall, combined, focal length of the system.  A thin lens approximation formula can demonstrate this quite simply, IIRC.

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There was a post on this subject a while back explaining the effect on the focal length when moving the mirror and how to work it out.

Dave

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@YKSE has done quite a lot of research/experimenting with this and may be able to help

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Thanks for the confindence @Stu, I've just read a little and did some measurement with my C8.

Unlike a refractor or newt, a catadiotric telescope (SCT, Mac-cat, etc), when the focusing is done by moving the primary mirror, the focal length change, the added backfocus length(an external focuser, eg.) will need to move primary mirror more forward to reach focus, thereby increase focal length, the main drawback as I understand is the increase spherical aberration to lost aperture when backfocus is too much(200mm more is too mcuh I know). More precise explanations can be read in

Op's estimation seems correct to me, I'd however to do a simple daytime measurement with yardsticks to get it verified.

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With my C8 Edge I use a visual  back , & 2-- 80mm extension tubes . This give me a f/l of 2500. F15. If U download

fire fox, this works out the f/l U are using. U would need a camera to be connected through. I find this gives me the

best results , when imaging with this scope.  Don't know if this helps U at all.

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Thanks everybody for your input, all valuable 'grist for the mill', though the @YKSE hyperlink to the Optical Theory is enough to give this novice a severe headache :-)  Like I said earlier this exercise was mainly to provide me with a better understanding of the effect of the new configuration on the overall light path and EFL of the scope.  Don Pensack ( from Cloudy Nights ) has very kindly provided me with a procedure/calculation for working out the EFL based on the timing of a star across the field of view of an eyepiece, I plan to test this at my next clear skies opportunity (may be a while...) :-)

Some feedback from Cloudy Nights has thrown a little wrinkle in my earlier calculation, according to this theory, the EFL of the stock 8" EdgeHD is in fact 2125 (f10.456) rather than 2032 (f10).  This approach would give my scope a revised EFL of 2385 rather than 2312.  Would appreciate everybody's thoughts on this latest development.

Quoted from Cloudy Nights:

"...

One thing you maybe missed?   I did not check the math, but at 133mm of back focus, the Edge HD 8" is not f/10.   At 133mm of back focus the EdgeHD 8" is actually working at 2125mm focal length and with a focal ratio of f/10.456.

Your light path from the back flange would be (105 + 112) 217mm or about 117mm longer than the nominal 100mm back focus of the C8 (which would give 2032mm) so the actual focal length will be around (117 x 3.1mm) 2385mm.

This is the thing.   EdgeHD 8" is sold as f/10, but as configured with the standard visual back and diagonal, it is actually f/10.4 and 2125mm of focal length.

It is only f/10 if you use it with one of the older, shorter visual backs and 1.25" prism diagonals.

You can find this in the EdgeHD White Paper and if you do the math from the stated focal length at 133mm of back focus (take 33mm off of the 133 and then subtract the 33 x 3.1mm or about 102mm from the 2125 and you get about 2021mm).

So, I come up with a bit higher than your math suggests and this number is about consistent from with my own measurements when using binoviewers.

Key though is that at 133mm of back focus, the EdgeHD 8" is not f/10.  It is f/10.456 and 2125mm of focal length.   For the EdgeHD 8", focal length estimates still needs to be done from the flange with the old 100mm back focus as being f/10 and 2032mm.  This will give only an approximate result of course, but when used with the supplied visual back (which is much longer than the old SCT visual back) and supplied diagonal, the EdgeHD 8" is working at 2125mm of focal length

..."

Kind Regards

Paul J.

Edited by DeepSkyMan
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Ed is one of those CN members who have done extensive measurement and ray-tracing on his many SCTs, he's written this excellent guide for SCT newbies

If I were you, I'd take his coments as a good starting point, and do the measurements to have a confirmation.

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2 hours ago, YKSE said:

Ed is one of those CN members who have done extensive measurement and ray-tracing on his many SCTs, he's written this excellent guide for SCT newbies

If I were you, I'd take his coments as a good starting point, and do the measurements to have a confirmation.

The link you provide also points out the image degradation of miscollimation illustrated by the clipping of the PSF which directly relates to the contrast transfer (MTF). This in combination with a moderate central obstruction can really hurt an other wise good scope.

I must investigate the SA introduced by extended backfocus, good info Yong.

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Well I just learned something...again lol!

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