Horizontal Heavens Observatory "...From  A Galaxy Far, Far Away"
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This page was last updated on 08/28/14.

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Not So Par-Focal Filters -

 When I first started monochrome filtered imaging, I purchased an entire LRGB, Ha, SII, OIII set of Astrodon's Tru-Balance "Par-Focal" filters in order to make my transition from DSLR to chilled CCDs much easier and to shorten my learning curve with my new ST-10XME w/CFW10 imager.  For the most part I have to admit, I was impressed with my newly acquired level of imaging and using what I was told were "par-focal" filters.  After 2 years of experience with CCD imaging, I continued to look for ways to improve each of my images.  The more I paid attention, the more I began to realize that not all of my color filtered sub-frames were as par-focal as I first assumed.  Understanding that not all refractors were equal in color correction, I replaced my Orion 80ED with high-end APOs (a .990 Strehl Ratio TMB80/480 and a Televue NP-127 Apochromatic Refractors).  However, I continued to notice the focus disparity and finally decided to try and quantify it this during August 2007.

Below is a comparison of TWO focus routines I used to record a set of RGB images of the Trifid Nebula M20:

  1. Par-Focal Focusing Routine - Focused only using the Luminance filter and set CCDSoft to record a color series of 5min RGBs (Sequence = RGB, RGB, RGB, etc.).  Refocusing after every one degree drop in Celsius.

  2. Filter Specific Focusing Routine - Focused using Blue filter, image all Blue sub-frames refocusing as temperature dropped, repeated focusing and imaging procedure with Green and then Red filters.

Conditions:

  •  Seeing was roughly equal at 3/5 on both nights (8/6-7/2007 and 8/8-9/2007)

  •  Transparency was slightly poorer during second imaging session of 8/8-9/2007

  •  Target imaged at same time, location and with same equipment (ST-10XME w/CFW10 on NP-127 on MI250 mount) as well as same temperatures, both ambient air temperatures and equal CCD cooling set-points.

Results:

  •  Blue sub-frames went from an average 4.82" FWHM with Luminance par-focal focusing to 3.0" FWHM with filter specific focusing

  •  Green sub-frames went from an average 3.3" FWHM with Luminance par-focal focusing to 3.08" FWHM with filter specific focusing

  •  Red sub-frames went from an average 3.57" FWHM with Luminance par-focal focusing to 3.0" FWHM with filter specific focusing

  •  SEE TABLE BELOW FOR DETAILS

Conclusions:

  • There appears to be a filter induced focus shift across the visible spectrum moving from lower frequencies (Red) through higher (Blue) frequencies when using these purported "Par-Focal" filters when using the par-focal focusing routine referenced above.

  • Current filter off-sets with Lum = 0, R = +9, G = +1, B = -23 when using my FeatherTouch motorized focuser.

  •  FWHM measurements of filter specific focusing routine are nearly identical at 103% greatest variation, however, the par-focal focusing routine shows 146% greatest variation when each imaging night is looked at separately.

  •  BOTTOM LINE - The "par-focal" filters I purchased are NOT par-focal on my NP-127. 

  •  It is possible that this may be partially telescope design induced, due to color correction characteristics of Apochromatic Refractors

  • On the other hand, one would think that filter sets being marketed as "par-focal" would take into account the varied color correcting abilities between telescopes -- Either that or quit marketing them as "par-focal."

  • UPDATE -- It looks like my filter's manufacturer has finally updated his marketing to explain why his filters are not "par-focal" in all telescope designs.  Thank you.  It sure would have been nice to know this when I bought my FIRST filters for CCD imaging.  I have at least 2yr of poor imaging from trying to use them as par-focal filters that show my frustrations.  Now that I finally know better, I focus each filter independently.