Using Focal Reducers and MMEC software with the Pro Dob II on a portable RVA setup

While testing the MallinCam Pro Dob II (PD2) camera with my latest release of the MMEC software, I looked into how much focal reduction could be reasonably used with my Celestron 130 Newtonian scope. Focal reducers increase your field of view and can shorten the exposure times. A focal reducer works by concentrating the light cone produced by your scope into a smaller, brighter circle.  My Newtonian is an F5 scope which is already “faster” than my other SCT scopes (they are F10).  The following images were captured on different nights, so the image quality varies, but you can see the effects of using different focal reducers. Here is the Orion Nebula (M42) using the Pro Dob II on my 130 Newtonian F5 scope with no focal reducer (No FR = 1).

Here is M42 using a MFR-3 focal reducer with a 5mm spacer (MFR-3 reduction = 0.67).

Here is M42 using a 0.5x Antares focal reducer on the end of a nosepiece (0.5x FR reduction = 0.49).  You begin to see some slight elongation of stars (coma) at the edges of the field of view. Note that even though this is called a 0.5x focal reducer, its reduction as used here is 0.49.  The actual reduction can vary depending on the distance between the sensor chip and the reducer.

Here is M42 using the MFR-5 (MFR-5 reduction = 0.43), which shows the MFR-5 is a bit too aggressive to use with the PD2 on this telescope.

To compute the effective reduction, I measured the x and y pixel position of the same two stars a and b on each of the images. 

Then I computed the pixel distance d between them for each configuration and its effective reduction to produce the following table, where d = sqrt(dx*dx + dy*dy), dx = ax-bx and dy=ay-by.  Reduction is computed by dividing d for each focal reducer by the d of No FR.

With this scope I use either MFR-3 or 0.5x FR, but in general I prefer to use the MFR-3 with the 5mm spacer for good reduction with least side effects.  This essentially converts the F5 Newtonian to a faster F3.3 setup (F5 * 0.67 =F3.3).


The above images of M42 were all captured with the camera sharpness set to 20.  Setting sharpness to a lower value will make the stars appear more round like you see in photographs.  Here is M42 again using the PD2 and MFR-3 with sharpness set to 3.

The design of the Pro Dob II camera enables plenty of inward focus.  It also accommodates standard 1.25” focal reducers rather than having to use 2” focal reducers. Below is the Pro Dob II camera with the 1.25” MFR-3 focal reducer attached.

And here is the Pro Dob II on my SkyProdigy 130 using the 2” eyepiece adapter.

Through these tests, I was able to confirm that the Pro Dob II is 100% compatible with the new version of my MallinCam Micro Extended Control Software. I made use of the new custom presets of the MMEC software during these tests.  The Custom 1 preset I provide with the MMEC software enabled me to easily switch the camera to focusing mode after I changed focal reducers to assist focusing on a bright star with the modified setup. Then I slewed to M42, centered it, and clicked on another custom preset for M42. Since it is easy to create standard custom presets, I was able to quickly create one just for the Orion Nebula that matched my setup for the evening.  Here is the setCustom9.txt preset file (standard format) that I created by copying setCustom1.txt and changing it using Notepad to the settings I wanted to use for the Orion Nebula.

Exposure: Normal - Setting for Orion Nebula
SenseUp: Off
AGC: Off
Lens: x512
Brightness: OK
Gamma: 0.6
3D-DNR: 0
Monitor: OK
DPC: Off

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The latest version of MMEC also supports a new manual custom preset format that allows you to change settings other than just the Exposure and Enhance settings.  Manual custom presets take a little more thought and testing though.  You create a text file containing the button presses needed to navigate to the setting you wish to change and then set it to the desired value.  You need to first adjust the setting to its lower or upper limit so it is at a known value, and then you can change it to the desired value.  Here is a setCustom7.txt preset (manual format) that changes the sharpness to 3 for more photo-like stars.

Manual - Set sharpness to 3
- Display Menu
Enter
- Move to Effect
Right 3
- Set sharpness to 3
Down 2
Left 50
Right 3
- Return to Menu
Down
Enter
- Exit and Save
Right 5
Down 2
Enter

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And here is a setCustom8.txt preset that changes the sharpness to 20 whenever I want increased sharpness.

Manual - Set sharpness to 20
- Display Menu
Enter
- Move to Effect
Right 3
- Set sharpness to 20
Down 2
Left 50
Right 20
- Return to Menu
Down
Enter
- Exit and Save
Right 5
Down 2
Enter

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Now for a little human interest story…

I did a portion of these tests while recently camping using a portable RVA setup.  The temperatures each night were below freezing as you can see by the frost beginning to form on the scope.  I used a battery power tank to power the mount.  I also used a plastic bin to contain a video capture device, camera computer control USB cable, hand controller serial/USB cable, and a battery powered hub at the scope with a single active USB cable run into my camper.

Although it was cold outside, I was a warm happy Astro camper inside.  :-)