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.

Sunday, November 13, 2016

2016 Deep South Star Gaze, part I

The 2016 Deep South Star Gaze, the 34th annual event, was memorable for the good attendance and weather. The days were warm with no rain and three of the five nights offered clear nights. This is always a nice, well run event  We had over 90 attendees and roughly 65 telescope setups.    The telescopes included small Dobsonians, a few large Dobs (including a 25"), several 8-12" Schmidt–Cassegrain and Ritchey–Chr├ętien, and quite a few refractors of various sizes.  Here are a few pictures from my walk around the field.

Some hardy folks camped out on the field and employed solar panels
to keep their batteries fully charged.

My tent and 10" RC riding a Losmandy G-11 on the right and my
daughter's Celestron Edge HD-1100 on the left.

Here on the Eastern side of the field you can see several scopes and on the far left
 of the image is 1 of the RVs.  The field has electrical hookups for upto 4 RVs, but
no water or sewer hookups.  

a closeup of my 10" Mallincam VRC on a G-11 mount

Oct 25 - First night of the DSSG
We arrived late in the afternoon.  The first order of business was to get the trailer setup and the air conditioner running as it was in the upper 80s.  Unlike previous years where I set up my scope on the Southeast side of the field, I picked a spot on the North side where the trees receded a way from the field.  Here I would have a view of Polaris to polar align the mount and better viewing to the South. By the time I had the tarp, tent, tripod, mount, and scope all setup, I was worn out. So I decided to just do some visual observing that night. I came out around 8 after eating a good supper.  Skies look reasonably clear but there are high thin clouds making the transparency poor. Humidity is not too bad, no star twinkle.

Polaris was surprisingly dim in the polar scope. After completing the setup of the scope I settled in to do some visual observing. Conditions were not conducive to seeing faint galaxies or nebula but star clusters were good. I was able to see the ring nebula although it was faint and the hole was not visible. NGC 457, the Double cluster, Andromeda Galaxy, were nice targets. I closed down for the night around midnight.

Oct 26 - Second night of the DSSG
The night started out with partially clear skies but the clouds soon filled the sky and the night was over by 9 pm.  I covered everything up and went back to the trailer and Cindy and I watched television.  Nice to have DirecTV in the trailer.   The weather forecast is for clearing skies for the next several nights.

Sunday, August 21, 2016

16.3 V Power Supply for Losmandy Gemini

My G-11 performs quite well and I am very happy with it, but I do note that despite careful balancing I occasionally get Dec or RA Heavy Tr warning messages.  I've read numerous times in the Gemini forum that these mounts are a bit under powered at 12-13 volts from a battery.  The Gemini system can take up to 18 volts, but most users seem to recommend 16 volts.  The Losmandy power supply provides ~ 15 volts from 110 V AC, but I prefer to operate my astro gear from battery power even in the observatory.

After scouring the internet I came across a regulated DC converter that increases ~ 12 volts to 16.3 volts and can handle up to 6 amps.  The device is sealed and is available on Amazon at a reasonable price.     Here is the back of the device.

Here is the front of the device.  I've used wire nuts to connect a 12 v accessory power plug (cigarette lighter plug) to the input wires.  To the converter outputs I've used wire nuts to connect the Gemini power plug with the 2.1 mm right angle plug.

When plugged into my battery, the output to the Gemini unit reads spot on the advertised 16.3 volts.

Here is the finished unit attached to the side of the pier with heavy duty Velcro.

I'll be curious to see if my G-11 performs any differently and if I get the RA or Dec Heavy Tr warning messages.