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Old 09-11-2012, 08:54 AM
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graham.hobart (Graham stevens)
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changing Focal length

Just a simple question I hope-
When running a focus run with TCF (optec) in Maxim DL - the focal length/ratio has to be spot on. I am curious as to whether this is as advertised on the side of my scope (f7/900mm) or should I be changing this as I have the added length of the focuser between the scope and sensor?
I wondered whether this would affect then the accuracy of my focus and also image scale for calculations in Maxim as I would soon like to plate solve?
Cheers

Graham
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Old 09-11-2012, 01:44 PM
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graham.hobart (Graham stevens)
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focal length question /(focal ratio)

Bump?
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Old 09-11-2012, 02:43 PM
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Graham, your best bet is to take an image and plate solve it with something like astrometry.net which does not need an estimate of focal length, or UniMap for which a ball park figure will get it started.

That should either directly give you the arcsec/pixel or the FOV from which you can derive the ASP since you know the pixel size.

Then the true focal length is given by:

focal_length = 206.265 * pixel_size / arcsec_per_pixel

where:
focal_length in mm
pixel_size in um
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Old 09-11-2012, 03:15 PM
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graham.hobart (Graham stevens)
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focal length question /(focal ratio)

Thank you sir, I have a suspicion that my calculations are wrong because of the added length of the focuser assembly. Will do what you say when the clouds are gone.
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Old 09-11-2012, 04:30 PM
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mithrandir (Andrew)
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Quote:
Originally Posted by graham.hobart View Post
Thank you sir, I have a suspicion that my calculations are wrong because of the added length of the focuser assembly. Will do what you say when the clouds are gone.
Graham it's not the physical length of the bits. It's the optical length. eg a C8 is physically ~450mm whereas it is 2000mm optically.
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Old 09-11-2012, 05:15 PM
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Just use pinpoint LE in maximdl, if it knows your RA and DEC you can solve an image in there and it tells you your focal length.
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Old 09-11-2012, 06:42 PM
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Yeah, do the solve in Maxim, you'll need a few things to do this, but once you master it, you'll never look back.
That will then lead to auto-centering etc, all good fun, really.
Gary
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Old 12-11-2012, 09:56 AM
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graham.hobart (Graham stevens)
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focal length question /(focal ratio)

Thanks chaps- did a solve in Maxim and it came out as 899mm! Many thanks for help.
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