What's my resolution / FL?

[codemonkey]

I bought an older GSO RC8 not all that long ago, assuming it was the same focal length as the current gen ones, but I'm getting some unexpected numbers.

PixInsight's annotation script is reporting a resolution of ~0.73"/px, but according to the various online calculators and formula, I should be at ~0.82"/px.

Doing the math to change the f-ratio to 9, giving me a focal length of 1827mm, gives me bang on the ratio that PixInsight reports.

So now my question is... are these old RC8 tubes actually longer than the new ones? Or is PixInsight reporting an incorrect resolution? Or are the calculators/formula only an approximation?

[codemonkey]

A quick google shows me at least two sites referring to an F9 1827mm version of the GSO RC8, so maybe that's actually what I have. One of which is definitely the older model I have.

[photosinferno]

I don't believe these calculators like CCDcalc are an 'approximation'. So your suspicions about the different focal length are probably correct!
John:)

[RobF]

Have you got platesolving going Lee? That's another great way to get a good indication of your actual FL.

[Merlin66]

Lee,
Don't confuse plate scale (arc sec/pixel) with resolution...
Using the Nyquist/ Shannon sampling model the resolution is usually 2 to 3 times the plate scale e.g. a 0.5 arc sec/ pixel may give 1 to 1.5 arc sec resolution.
The actual resolution of your system with depend on the PSF (Point spread function) and the seeing conditions.

[codemonkey]

Thanks everyone :-)

When I was referring to PI's annotation software I was really referring to its plate solving. Here's the actual output. I didn't notice the reported focal length until I went to generate this to output it, only the reported resolution.

=================================== =================================== =========
Referentiation Matrix (Gnomonic projection = Matrix * Coords[x,y]):
+5.82724e-05 +0.000197098 -0.142898
+0.000197143 -5.82725e-05 -0.106898
+0 +0 +1
Projection origin.. [695.733791 519.313793]pix -> [RA:+23 18 52.74 Dec:-42 18 49.79]
Resolution ........ 0.740 arcsec/pix
Rotation .......... 73.537 deg (flipped)
Focal ............. 1797.87 mm
Pixel size ........ 6.45 um
Field of view ..... 17' 9.3" x 12' 48.9"
Image center ...... RA: 23 18 52.746 Dec: -42 18 50.00
Image bounds:
top-left ....... RA: 23 18 06.281 Dec: -42 25 14.04
top-right ...... RA: 23 18 32.720 Dec: -42 08 47.30
bottom-left .... RA: 23 19 12.878 Dec: -42 28 52.47
bottom-right ... RA: 23 19 39.053 Dec: -42 12 24.79
=================================== =================================== =========