Will my guiding RMS figures get worse or better?

[Priesters]

So, in an idle moment this afternoon (other half was at work 👍) I came across an article about using Astrometry.net to find out the true focal length of your imaging set up.  I’d been curious about it for a while as I have a Crayford on the back of an SCT so I always knew the ‘published’ focal ratio with the reducer would be wrong.  I was pleased that all worked fine with Astometry.net and discovered my SCT set up is operating at F7.5.

I then put in an image from my guide scope.  My guide set up is a Skywatcher EvoGuide 50 (242mm focal length) with Starizona FIeld Flattener 2 and an ASI 294MC. I can find nothing to suggest the Starizona FF2 has any affect on focal length.  I was surprised to find out that the focal length came through as 266mm making the set up F5.32 as opposed to the expected F4.84.  

Last night (I.e before this afternoons discovery) I enjoyed decent guiding with sub arcsecond RMS.  I’ve now changed the focal length in PHD2 from 242mm to 266mm and am wondering which way that will throw my RMS figures,  if anyone can help?

Thanks

 

[kens]

It wont make any difference in your images. PHD2 works on pixel measurements and the focal length is only used to convert that to arcseconds for display purposes. Increasing the focal length value will make your guiding appear worse (assuming identical circumstances) as the longer length translates the pixel RMS value to a larger displayed arcsecond RMS value

[wimvb]

1 hour ago, kens said:

It wont make any difference in your images. PHD2 works on pixel measurements and the focal length is only used to convert that to arcseconds for display purposes. Increasing the focal length value will make your guiding appear worse (assuming identical circumstances) as the longer length translates the pixel RMS value to a larger displayed arcsecond RMS value

With a longer fl scope and the same camera (pixel size) the angle/pixel will be smaller, so the arc secs RMS will be smaller too. But only by 10% in this case.

[kens]

3 hours ago, wimvb said:

With a longer fl scope and the same camera (pixel size) the angle/pixel will be smaller, so the arc secs RMS will be smaller too. But only by 10% in this case.

Quite right. Should have had my morning coffee before responding 😬

[Priesters]

I like wimvb’s answer more 😂 but with the prospect of worse RMS kens answer did make me rethink my set up and I’m pretty confident my centre of gravity is better now so it’s a win win 👍.  Thanks to both of you @wimvb @kens.

[michael8554]

Neither answer is better for you.

All that incorrect FL will mean is that the reported guide errors will be 10% smaller than reality.

Michael