PiCamCollimator

[paul]

I finished v0.1 coding last night for my RPi camera based Newtonian collimator.  The reason for using the RPi is because it supports direct control of cameras over the CSI and there is a version of the 5MP Arducam that has electronic focus control.

 

Its written in python using pygame and picamera modules.

 

The principle is to align focuser tube with secondary and the secondary with the primary and then square the primary for nice concentric rings (apart from the offset secondary image). Normally you would use something like a Cheshire sight tube for this but a camera is more accurate and in theory could allow alignment sufficient for quality imaging. In the past have used Sharpcap and  webcam and while this works its not stable enough for quick repeated use.

 

The PiCamCollimator app streams the camera image and overlays 6 concentric rings, the set of rings can be moved about the image, the diameters, colour and thickness can be adjusted. A rotatable cross can also be added for each ring and individual rings can be switched on or off. All the settings are saved when exiting the program so quick rechecks of collimation can be made. The focus point of the camera can be adjusted from about 3cm to infinity so you can set the ring position accurately. Exposure control allows for dim a bright conditions.

 

The code runs on a RPi 2W, its currently desktop app so I plan to VNC in to it from my Windows PC. I may push this on a webpage in a future design.

 

The above shot shows a colourful selection (4 of 6) of the rings rendered with thick borders, all with crosses at different angles. The numbers in the top left corner show ring 6 (inner) is selected and the cross is at 39 degrees tilt. The second number is the raw focus position (0= infinity, 16k <> 3cm),   Time to bolt this to an old eyepiece and test it in the tent!

[TerryMcK]

A great design Paul. It looks fantastic.

[paul]

I field-trialled v0.2 this evening:

Zooming in to the edge of the focuser and setting the outer reference circle:

Adjusting camera focus to check the secondary position (green circle) and aligning secondary to be round not oval then verifying the primary mirror is central in the secondary mirror (yellow circle). The red circle is the target for the focuser reflection - which should be central to it = time to adjust the primary knobs:

Adjusted primary to have the secondary reflection central(Note offset focuser inner circle - as expected and symmetric about the horizontal axis - looking good:

 

Verification of collimation - illuminating the doughnut and fine tweaking the secondary collimation to center it on the cross:

Final observation is no evidence of OTA rim in the primary reflection. This means the light path is parallel enough to the OTA tube so stars should always be round. 

 

Findings....

My previous attempt at collimation between secondary and primary after mirror re-silvering was already very good but the secondary was a little high on the focuser, meaning sensor illumination wasn't optimal. A few millimeters only but I was able to make an improvement.

 

I've long suspected that when I upgraded the OTA from its original 1.25" RnP to the 2" Crayford that my hand cut hole wasn't perfect. This means that the focuser doesn't quite square up on the axis through the secondary. Again its only a few mm but with the camera focused right it was to adjust the spider for a good compromise.

 

My laser collimator has just stopped working 😠so I am unable to compare results that way so I resorted to a few 30sec subs on bright stars to judge.  Collimation is as good as it ever was but now I know the sensor is properly illuminated.

 

As you adjust the focus to different depths the previous rings appear to move off their targets but that is an illusion caused by the depth of field altering.

 

Improvements to made to the PiCamCollimator design:

1. Add variable illuminate around the camera - I had to manually adjust torch lighting a lot to get an optimal view of the focuser in the final stages.

2. Don't collimate in the dark 😄.

3. A zoom function would be handy for peeping at the inner circles.

4. A double cross would be handy to place the spider vanes between. 

5. Add a movement lock key. Just so you don't move the rings off the reference by accident.

6. Add fine adjustment for exposure.

 

Will I use it for normal collimation? If I can sort out good illumination to make it quick. I am considering to use the RPi to control my flat panel.  In doing that I also provide a convineient WiFi remote control for my flats production too.

 

 

Edited February 12, 2022 by paul

[paul]

I now have version 0.3 up and running with zoomed view for precise final adjustments of the secondary and primary.  I think I'll be adding an LED illuminator into the barrel to make the last step easier (with PWM control for brightness). Checking with a collimated  laser collimator confirmed good collimation 😉. The result is nicely symmetric vignetted illumination of the camera sensor (making the most of the available photons).

 

When I've finished the hardware build I'll put details of it and the code on my website.