Focal length of the SW finderscope used as a guidescope?

[SmokeyJoe]

Trying to work out the FL of the SW finder scope when used as a guidescope as I am using an Asiair Pro I need to input the FL of the guidescope.  I have unscrewed the ep from the finder scope and attached a Zwo 120mm using the appropriate adaptor.  I have read through a number of threads about the focal lenth of the finder scope and there seems to be some discussion as to whether the FL is 160mm or 180mm.   It makes sense to me that the FL would be 180mm with the ep and 160mm with the camera.  Can anyone confirm this please. 

[prusling]

I plate solved an image from mine and it was 176mm which works fine in PHD.

[Jessun]

Is this of interest though? I mean not the thread as such but the FL of a guide scope. PHD and other software don't mind really.

/Jessun

[SmokeyJoe]

46 minutes ago, Jessun said:

Is this of interest though? I mean not the thread as such but the FL of a guide scope. PHD and other software don't mind really.

/Jessun

Don't know to be honest I am a noob as far as guiding is concerned.   The Asiair app needs the FL of the guidescope. 

Thanks Prusling, so from your results it looks like 180mm is nearer.  I will attempt to plate solve and see what happens. 

[michael8554]

16 hours ago, Jessun said:

but the FL of a guide scope. PHD and other software don't mind really.

Having the correct image scale (focal length and pixel size) in PHD2 means the guide errors and graph movements are reported with the correct values.

With the eyepiece or camera removed, focus the sun on a flat surface, and measure from the front element to that point, that will be accurate enough for PHD2.

Michael