changing Focal length

[graham.hobart]

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:thumbsup:

[graham.hobart]

Bump?

[mithrandir]

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

[graham.hobart]

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.

[mithrandir]

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.

[Peter.M]

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.

[gbeal]

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

[graham.hobart]

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