Monday, December 26, 2016

Focal Reduction

I respect visual astronomers and I understand the intimacy of having ancient photons from countless miles away hit your retina.  That works fine for bright stars, but but viewing faint grey fuzzies just isn't my thing.  I am active in astronomy because my Mallincam XT-418 ( upgraded Extreme II) allows me to see deep sky objects in greater detail and in color from my light polluted backyard.    

A challenge with using a video camera with a small chip is the small field of view (FOV).  On my VRC-10 (Focal length 2032 mm) and Mallincam XT-418 at prime focus provides a 14.24 arcmin FOV which is roughly equivalent to a 6 mm eyepiece.  This offers great magnification for small objects but the FOV is too small to capture many cosmic objects.  To obtain a larger FOV I could change to a shorter focal length scope, or change to a camera with a larger chip, or use a focal reducer - often called a telecompressor.   Another benefit of using a focal reducer is that it effectively reduces the focal length of the scope, thereby decreasing the F ratio and increasing the speed of the system.

I own several focal reducers, all sold by Mallincam

  • MFR-6 (short half of the MFR-5) - an ~ 0.80 reducer,
  • MFR-3 - an ~ 0.64 reducer,
  • Mallincam 2" reducer - an ~ 0.75 reducer,
  • MFR-8 (long half of the MFR-5) - an ~ 0.68 reducer, and
  • MFR-5 - an ~ 0.44 reducer. 

These reducers can be used singularly or in many combinations.  To understand how best to employ these focal reducers, I've conducted experiments where I've measured the FOV with the plate solving website   The software provides a solution of your image that includes the FOV size, as shown below.

The results from my experiments are shown below.  This data has been very helpful in planning observing sessions so I can match my optical system to the size of objects being viewed.    It also shows the required extension rings, tubes, and reducers and where the resulting focus point falls.  This greatly speeds up my setups.

Mallincam XT-418 Setups on VRC-10
Focal Reduction 1” Focuser Ring Blue Fireball = 50 mm Blue Fireball = 80 mm Badder 2” - 1-1/4” adapter = 8 mm Filter Focus position arcmin, horizontal arcmin, vertical diagonal dimension, arcmin Calculated Reduction Resulting F Ratio
prime Focus x
x x Lumicon DS 25 11.4 8.54 14.24 1.000 8.00
MFR-6 (shrt hlf MFR-5) x
x x Lumicon DS 26.5 13.1 9.85 16.39 0.869 6.95
MFR-3 x
x x Lumicon DS 24 14.2 10.6 17.72 0.804 6.43
MFR-8 (lng hlf MFR-5) x
x x none 15.5 16 12 20.00 0.712 5.70
MFR-5 x x
x Lumicon DS 35 20.6 15.45 25.75 0.553 4.43
M 2” 0.75x x

x Lumicon DS 31.5 14.8 11.1 18.50 0.770 6.16
M 2” 0.75x + MFR-8 x

x none 12.5 21.4 16.1 26.78 0.532 4.26
M 2” 0.75x + MFR-3 x

x none
18.6 13.9 23.22 0.613 4.91
I experienced no comma or vignetting with any of the above systems, however I suspect that if I couple the 2" 0.75 reducer and the MFR-5 that I likely will see some vignetting.  My results are similar to those of others, although not exactly the same due to differences in the spacers used.   I plan several more experiments to try different spacers to alter the distance between the focal reducer lens and the camera chip.  NOTE: Additional experimental data on numerous focal reducers is available at Jim Thompson's  highly informative Abby Road Observatory website (look under Test Reports).

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