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).

Sunday, December 11, 2016

2016 Deep South Star Gaze, part II

Oct 27 - Third night of the DSSG
Heather came up after school so I had an observing buddy.  Set up the Mallincam with the 2” FC + the MFR-8, so I'll be running at F 4.26 with a FOV of 21.4 x 16.1 arcmin.   My observing plan is to continue with the Herschel 400 list I started last month.  At 2245 took a sky meter reading of 21.18 (6.22 mag). It is damp but clear and steady.

     At 0029 took another sky reading, and got 21.11. Has been a great night. I knocked out the 10 Herschel 400 objects in Cetus and saw a few other things as well.  I viewed a few objects through Heather's Edge HD-1100 and also I got to view the Orion nebula through a 25” Obsession dob.  Shut down at 0039.

Oct 28 - Fourth night of the DSSG
I decided to change my optical setup to the 2” FR + the MFR-3. Based on a measurement of an image of the ring nebula, this focal reduction combination provided a 18.6 x 13.9 arcmin field and F 4.91. I'm hoping this milder reduction setup will provide better color rendition.   I'm using the same settings on the Mallincam the past few nights, AGC 6, ATW, contrast 80 – 90, brightness 10 – 50% depending on object. After a view of the Ring Nebula, I continued my hunt of Hershal objects

At 2325 measured sky reading of 21.18 , a beautiful night.  I knocked out many objects in a really nice night although cold and damp. Shut down at 0158.

Oct 29
Fifth and final night of the DSSG. Same optical setup as last night. As I was viewing the first object of the evening, I bumped the mount (hit counter weight with my head – OUCH !!) I reset the scope to CWD position and cycled the GEMINI and cold started but my gotos were off.   The polar alignment was off so I obviously hit the counterweights hard.   I reset the polar alignment and built a Gemini model, but the stars in the Mallincam were not as bright as the past few nights.  I viewed a couple of objects and at 2324 I called it a night as I have to pack up and drive home tomorrow.  I frustrated my last night wasn't as good as the previous two, but I had 2 very good nights observing  I also won a couple of door prizes, including an Explore Scientific eyepiece.   We had a nice time at the DSSG.