The role of focus on the guide cam

[nicholas disabatino]

Hello all,

Does anyone have a firm grasp of the role and importance of focus on the guide cam?  Last night I was having horrific guiding for a while, and I was doing all the usual routine to get it back on track. looking at the Phd screen from what I saw on the screen there did not appear to be an issue w focus. I could see stars in the screen as points. But since I tried everything else in troubleshooting the guiding, (GA, re-cal, etc) I finally just re-did the focus (in a rough way) of my QH5Y guide cam ( ST 50 finder scope.) Anyways it is not like  I have a bahtinov mask for the ST 50 so I always just eye-ball the stars. In the case of last night, I had initially just eyeballed the stars and as I said guiding was horrific. But then as I had exhausted all other avenues of troubleshooting, I finally just took the guide cam completely out of the draw-tube and re-did focus (eyeball method). within moments after that, guiding was as pleasant as it ever is. I had previously though that what Phd really needs is just to know where the stars are, and that rough, or even mildly poor focus would be good enough.  So I am somewhat surprised that merely re-doing the focus made a world of difference. is there any possibility my problems could have been elsewhere? logs:

https://openphdguiding.org/logs/dl/PHD2_logs_OzLC.zip

THANK YOU!

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Nicholas DiSabatino

[Bruce Waddington]

Hi Nicholas.  As you have just learned, focus matters. J   The accuracy of the centroid calculation depends linearly on the signal to noise ratio.  With dim, poorly distinguished stars, the SNR will be low.  And with large, “soft” guide stars, seeing fluctuations can result in larger centroid shifts that look like guide star movement but really aren’t.  So I think you definitely need to improve on your guide camera focus.  Even in the later sessions of your log, the focus looked poor to me – the HFD value of the guide star was still above 7 px.  The help guide section on the star-profile tool describes a method for reaching good focus.  You basically want to minimize the HFD value until you’ve hit a floor set by the seeing or by the optics of the guiding setup. But you should use any tool that works for you – a Bahtinov mask, an app like SharpCap, whatever you find easiest to use.  At your image scale, you should probably be able to get HFD values below 4px.  Focusing can be fussy with these finder-scope type guiding setups, but you should generally have to do it once then leave it alone.  If you take the guide camera off between imaging sessions, you need to find a way to restore the correct spacing between the guide camera sensor and the back of the guide scope.   

 

Hope this helps,

Bruce

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[nicholas disabatino]

Thank you Bruce,

 I did a search but did not find any 2" bahtinov mask (which I consider the lazy and relabile method of focus) so I will have to work in dead-reckconing method or SharpCap. 

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[sarg314 sarge]

You can generate a custom bahtinov mask pattern at 

http://astrojargon.net/MaskGenerator.aspx

I actually used this to make a mask for a DSLR with a 200 mm lens.  I printed it on a transparency material.

On my telescope I use an off axis guider so to focus that, I just put a bahtinov mask (from Highpoint scientific) on my 8" Celestron.  Works great.  The trick is to get a star 3.5mag or brighter in the field of the OAG.

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Tom Sargent