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# Focal Length Discrepancy

3 replies to this topic

### #1 RJNick

RJNick

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Posted 09 May 2021 - 07:58 AM

Hi!  I'm very new to astrophotography and wanted to calculate the effective focal length for my setup and have come up with a problem with my calculations.  I sheepishly ask for some guidance or possibly a link to old fora that discuss this.

I have an old Celestron Ultima 11 SCT equipped with a Celestron f/6.3 focal reducer.  I am imaging with a Nikon Z7 mirrorless camera with a full frame sensor and 4.34 um pixels.  My camera sensor sits 106 mm behind the FR (Celestron told me the F/6.3 ratio is for an optimum distance of 105 mm behind the FR).  The FR is on the rear cell of the telescope, the Celestron T-adapter is 50 mm in length followed by a Fotasy T-ring (40 mm) and he flange distance of the Nikon Z7 is 16 mm.

My Celestron Ultima 11 has a published focal length of 2790 mm and a focal ratio of f/10 so I'd calculate the focal length with the FR would be about 0.63*2790=1758 mm.  To double check, I put the same values in the calculator on http://www.wilmslowa...re/formulae.htm and got a slightly different number of 1727 mm which is close.

So I took an image of M44 the other night and plate solved the raw image in Pixinsight and obtained an effective focal length of 1442 mm with a pixel resolution of 0.61 arcsec/pixel. I next plate solved the same image on Astrometry.net and also obtained a resolution of 0.621 arcsec/pixel.  Being the obsessive type, I opened the image and compared it to the photo view in Stellarium and when I visually adjusted the Stellarium image to match the photo. I used a multiplier on the sensor definitions page of 0.51 to match my photo which would give an approximate focal length of 0.51*2790= 1422 mm as well which is close to the plate solved values.

Why the discrepancy?  I know that FL changes as my primary mirror changes during focusing, but can't imagine it's that much.  Could someone please explain so I can head-slap myself and continue on with my life?  Thanks for the help!

### #2 dbrb

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Posted 09 May 2021 - 08:25 AM

The FL of a primary mirror changes as you focus?

Hi!  I'm very new to astrophotography and wanted to calculate the effective focal length for my setup and have come up with a problem with my calculations.  I sheepishly ask for some guidance or possibly a link to old fora that discuss this.

I have an old Celestron Ultima 11 SCT equipped with a Celestron f/6.3 focal reducer.  I am imaging with a Nikon Z7 mirrorless camera with a full frame sensor and 4.34 um pixels.  My camera sensor sits 106 mm behind the FR (Celestron told me the F/6.3 ratio is for an optimum distance of 105 mm behind the FR).  The FR is on the rear cell of the telescope, the Celestron T-adapter is 50 mm in length followed by a Fotasy T-ring (40 mm) and he flange distance of the Nikon Z7 is 16 mm.

..................snip

Why the discrepancy?  I know that FL changes as my primary mirror changes during focusing, but can't imagine it's that much.  Could someone please explain so I can head-slap myself and continue on with my life?  Thanks for the help!

### #3 kathyastro

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Posted 09 May 2021 - 08:54 AM

The FL of a primary mirror changes as you focus?

Yes, all SCTs and Maks that focus by moving the mirror have a variable focal length.

It's not the focal length of the primary mirror per se that changes (since that would require changing the shape of the mirror), but the net focal length of the primary-secondary combination.  The net focal length depends on the spacing of the two mirrors, since the secondary is not flat.

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### #4 Benschop

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• Loc: Eastern Ontario

Posted 09 May 2021 - 07:25 PM

The focal reduction of an f6.3 reducer is specific to a distance between the CCD and the reducer.

If your distance is less than or greater than the spec (around 105mm) , the focal ratio reduction will be either less than or greater than 0.63x

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