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# Focal Length Discrepancy

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### #1 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 Midnight Dan

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Posted 09 May 2021 - 09:39 AM

Actually, the focal length changes quite substantially in an SCT when moving the mirror even a small amount.  I'd believe the plate-solve numbers.

-Dan

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### #3 RJNick

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Posted 11 June 2021 - 05:09 AM

Thank you Dan.  I will do that!

-Robert

### #4 const

const

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Posted 11 June 2021 - 05:24 AM

Interestingly, my Edge HD 8" exhibits the discrepancy in the other direction. By the specs, it is 2032 mm, but at the recommended backfocus it is 2135 mm. With such deviation, they should have not specify the focal length such precisely, rather 'a couple of meters' The .7x reducer does its 0.7 job pretty accurately though.

### #5 alphatripleplus

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Posted 11 June 2021 - 07:08 AM

Even without the reducer you'll see a difference in focal length from the nominal f/10, depending on where the primary mirror is in order to get focus with  your particular placement of the camera.

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