Re: 3-4" RA wiggling while PHD2 guiding with EXOS2-GT PMC-eight #EXOS2 #astrophotography

Walter Vinci

Thank you all for your help,

I will try switch guiding from my guide scope (162 mm) to my imaging scope (480) and see if guiding improves next time I have clear skies. That will give my a pixel scale of about 2.2 arc-seconds.


On Jan 14, 2021, at 11:23 AM, Andreas Volz via <bassliner303@...> wrote:

Hey Doug,

The formula is:
pixel size (microns) x 206.3 /focal length in mm = image scale in arcsec/pixel

206.3 results out of:
3.6 * (180 degrees / pi) = 3600/1000 * (180 degrees / pi) = 206.26

Doug Doonan via <> schrieb am Do., 14. Jan. 2021, 19:09:
Where does the 206.3 in your formula come from, and what are its units?

-----Original Message-----
From: Jerry Hubbell - Explore Scientific VP Engineering <jrh@...>
Sent: Wed, Jan 13, 2021 8:34 am
Subject: Re: [ESPMC-Eight] 3-4" RA wiggling while PHD2 guiding with EXOS2-GT PMC-eight #EXOS2 #astrophotography

Hi Walter,

Yes, I would suggest a 2x Barlow or tele-extender instead of a 3x. The formula for pixel or plate scale is:

Pixel Scale = (Pixel Size x 206.3) / FL, So for your combination: Pixel Scale = (5.2 um x 206.3) / 162 mm = 6.6 arc-seconds/pixel

BTW,  your calibrations look spot on, no problem there,
Jerry Hubbell
Vice President of Engineering

Explore Scientific, LLC.
jrh at
1010 S. 48th Street
Springdale, AR 72762

Author: Scientific Astrophotography: How Amateurs Can Generate and Use Professional Imaging Data
             Remote Observatories for Amateur Astronomers: Using High-Powered Telescopes From Home

Mark Slade Remote Observatory (MSRO) IAU MPC W54 Equipment
Wilderness, VA
: ES PMC-Eight G11 + Telescope Drive Master (TDM)
Scopes: ES 165 FPL-53 ED APO CF, ES 102 FCD100 ED APO CF
Cameras:  QHY174M-GPS + FW, QHY163C
Misc: 3-inch 0.7x Focal Reducer Field Flattener, Filters: Luminance, 
Red, V-band Photometric, Diffuser, 200 lpmm Spectral Grating

Software: MaxIm DL 6, Cartes du Ciel, Astrometrica, AstroImageJ, AutoStakkert!


Doug Doonan
HomeBru  6 inch Reflector  f 6.6
HomeBru  85 mm Refractor  f 5.6
Long retired EE.

Clear skies,


Mount: ES PMC-8 EXOS2, ioptron SkyGuider Pro
Scope: SkyWatcher Explorer 150/750 PDS
Cameras: Canon 6D, Canon 7D Mk2
Guiding: ZWO ASI120Mini mono, ZWO 30F4 Guidescope, PHD2
Msc: TS Optics Coma Reducer Corrector 0.95x 
Software: N.I.N.A, APT, Pixinsight, DeepSkyStacker, Photoshop

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