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This blog contains recent projects, activities, and musings about astrophotography and space, to view my main webpage with prints for sale, final images, and Annie's Astro Actions, please visit: www.eprisephoto.com


Monday, December 5, 2011

Parfocal filter test #1

I always wondered this, but as I didn't have a motorized filter with readable steps (until now) I wasn't able to do any testing on my filters previously. Since the moon is lovely and bright tonight killing most of my imaging plans I figured I would do some focus tests with my new Moonlite focuser. After letting the scope cool to the nice chilly evening (whilst my toes froze nice and solid as well) I ran through my Astronomik set (LRGB and Ha,SII,OIII) and not only did FocusMax's "focus" button but then dug around a few steps each way to find the best focus for each filter. Happy to say that the "focus" button on FocusMax got the focus within 1-2 steps of best focus so gotta say that I am happy to use the automated routine in the future. 

Now for the results. 
Filter set: Astronomik Type IIC broadband and narrowband filters
Scope: Orion EON80ED
Camera for testing: Atik 314L+
Moonlite Focuser
Filter Wheel: Atik EFW2
Camera temp: -20C
Ambient temp at time of test: 3.3C

Position at "best" focus for each filter at time of testing: 
Lum: 10681
Red: 10679
Green: 10676
Blue: 10719
Ha: 10683
SII: 10686
OIII: 10683

All in all, I say that is pretty good (except for the blue filter of course, which I knew already had issues) ... All others were within 5 steps of each other with the Luminance being pretty well centered between them. I am happy to see how close to parfocal all 3 narrow bands are and how close they are to the Luminance. 

With that said, I do have an Astrodon LRGB set on the way and will be interested to see how they perform (at the moment I am staying with the Astronomik NB as I have been happy with them but the Blue on my Astronomik set drives me nuts and if I am gonna change one might as well get the whole set. Once they arrive I plan on doing the same test with them as well and will post those results.

After a few more rounds of testing I will have a reliable focus offset for each filter so I can add that to my automated routine, which should make automated routines easier to setup. 

Addition: I got out today and pulled out the caliper to do some measuring. 
On my system, the Moonlite Focuser moves 16 microns/step (determined by multiple tests by moving the focuser 100, 250, 500, 1000 steps multiple times and measuring the distance moved/#steps and averaged out the #, although almost all of the measurements were right at 16 so any differences were likely user error on the caliper)

Next, some math to determine the critical focus zone (CFZ). 
Formula: CFZ=4.88*λ*f ² 
Where λ  is the wavelength of light and f(squared) is the focal ratio of the system squared. 

So, for my f/6.25 system the CFZ is:
Red: 124 microns
Green: 97 microns
Blue: 91 microns

Based on the measurements I did that showed each step was 16 microns, the CFZ in steps for each color (rounded down) is:
Red: 7 steps
Green: 6 steps
Blue: 5 steps

Not a lot of room for error, only 2-3 steps on either side of the focus point. Makes me even happier that I got the motorized focuser as finding that 91-127 micron sweet spot by hand was exceedingly difficult (even with a Bahtinov mask)

Note: These tests are done on my actual imaging system, not an "ideal" test setup. I find it hard to find results on average setups as "tests" are generally done on apochromatic scopes and very high-end equipment. That is why I like to share my test results on my modest equipment. These are not "scientific" and are done just as I state. I do them for others who have modest setups and would like to find results on setups similar to their own. 

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Monitor Calibration

Monitor Calibration
The grayscale above presents 24 shades of gray from pure white to solid black. If you cannot see all 24, your monitor needs calibration to view the astrophotos correctly: I recommend the site linked in the image