Raspi astronomy experiment


#1

As some of you are aware, I’m a bit of an astronomy fan. @stealthii very kindly let me experiment with his raspi camera (including removing the glued-in lens). Here’s a quick overview of how it went:

  1. Taking off the lens required scraping out the glue that locks the lens housing into the sensor housing. The IR filter is built into the lens housing.

  2. I needed some way to mount the picam onto the telescope. 35mm film canisters have long been known to be a good fit for a 1.25 inch focuser. I cut a square aperture into the lid of the canister ensuring it was a snug press-fit. (I put the lens back in to keep dust off the sensor)


Here it is in the ultra high-spec ‘housing’

Equally high-spec mounting was employed for the pi:


  1. Took a couple of shots using raspistill (ISO 400, wb off):

First image is the terminator of the moon, some nice detail. Second image is Jupiter with the moons Europa and Io. You should be able to make out the northern (top) and southern equatorial belts (SEB) and the great red spot on the SEB left of the mid-line. Seeing (clarity of the sky) was moderate with a fair amount of atmospheric disturbance.

Overall I’d say that the raspicam has a lot of potential as an astrocam.

Thanks again to @stealthii for letting me take a scalpel to his camera. :slight_smile:


#2

Massively impressed with this. :smiley:

If only I could borrow cameras out of work!


#3

@martinus mind if I compile this into a blog post? (Citations etc of course)


#4

@bolster Sure, I was going to add a few more bits and pieces to the post though (links to source material and related material).

Mind holding off until tomorrow evening?


#5

To remove the lens I took a scalpel blade and ran it around the inside of the sensor housing along the edge where the thread for the lens tops out. I’m not sure if this is the ‘best’ method but it worked and avoided my having to employ brute force. Instructions on removing the lens are somewhat scant but I would suggest that if you’re considering doing the same that a gentle approach is best. At all costs you want to avoid scratching or even dropping debris onto the sensor. When I was finished the lens screwed back into the sensor housing again so it can be used as originally intended.

The ribbon cable has a significant impact on where the Pi is located. In researching previous attempts at astrophotography with the camera-board I noticed that someone had gone to the trouble of attaching a 3D printed telescope adapter to a pi case that had an integrated mount for the camera-board:

Note that you can take pictures without removing the lens if you shoot through a telelscope lens:

An additional discovery (that I expected from previous webcam mods) is that the IR filter is in the rear of the lens housing. You could remove it to get an IR sensitive camera if you don’t wish to pay for the new NoIR version of the camera board.
http://elinux.org/Rpi_Camera_Module#Removing_IR_Filter

Remember that the sensor and housing of the camera are not ‘standard’ M12 size, thus it’s not possible to connect one of the prefab connectors sold on ebay/in astro shops, e.g.: http://tinyurl.com/pg3kefv , this is a pity as, when we remove the lens, we also remove the IR filter. An IR filter is generally recommended for any telescope that includes lenses as the IR will cause the image to lose sharpness:
http://www.threehillsobservatory.co.uk/astro/IR_FAQ.htm

Sorry for the haphazard ordering of all of this, blog at your leisure. :smile:


#6

A bit of a progress report.

I previously mentioned that the picam does not have an M12 mount (standard webcam lens) which forced me to modify a film canister lid. It works but is a friction fit and one cannot easily attach a filter to it, thus it is not ideal. In an attempt to improve the camera attachement I found an old webcam* and stripped it down. There was a detachable M12 which I tried to retrofit to the picam board. As you can see from the previous images the PCB has four mounting holes - two are in line with the sensor. Alas, it turned out that the hole spacing was not compatible with the M12, the picam board has a spacing of 21mm.

I then took a look on ebay and discovered that someone was manufacturing M12 mounts with a 21mm screw spacing! Not only that but they’re only a few pounds (PM me for a link). The seller even advertises the M12 mount as being picam compatible. However, this is not exactly the case. You can see the new and scavenged M12 mounts in Figure 1. Note that there are openings on two of the sides of the scavenged M12 whereas the underside of the ‘compatible’ M12 are flat (Figure 2). Unfortunately this mount does not allow for the sensor attachment method used on the picam. Figure 3 shows a detached sensor and the picam board; the sensor is held on to the PCB with a small circle of double-sided tape, electrical contacts are made with a small, press-fit connector. Whilst the ribbon cable is thin the ‘compatible’ M12 mount does not have an opening on the base that accomodates the press-fit connector, this is evident in Figure 4.

Figure 1. A picam with a scavenged, incompatible webcam M12 mount ® and a mount with the correct screw-hole spacing (L).

Figure 2. View of the bottom of the ‘compatible’ M12 mount.

Figure 3. Picam board with detached sensor.

Figure 4. M12 mount placed on the picam PCB. Note the angle between the mount and the board caused by the mount sitting on the sensor connector.

Turning the ‘compatible’ M12 into a compatible M12 was trivial - I cut a small section of the base away as illustrated in Figure 5. The resulting fit (Figure 6) was good and I was able to attach the M12 to the PCB with two small screws**. Whilst I was modifying the M12 I also cut a section out of the adjacent side of the base, I have a peltier cooler on order and am aiming to put a copper cold-finger on the underside of the sensor at a later date.

Figure 5. Base of the M12 mount showing the cut-away section.

Figure 6. Modified M12 mount attached to the picam PCB.

I attached the 1.25 inch adapter to the picam (Figure 7) and took it for a test-drive. The sky was a bit hazy and I’m not convinced I got good focus however I did take a few still shots of Mars (ISO 1600, auto white balance) just to ensure everything was working. An example of which can be seen in Figure 8.

Figure 7. Complete, focuser compatible picam.

Figure 8. Test photo of Mars.

As a bonus I was driving the mount using INDI (indi_eqmod_telescope driver) from the raspberry pi via a USB to serial cable I built. The raspberry pi was running as a server and Kstars on my laptop did all of the heavy lifting. Whilst kstars is not as feature filled as EQMod it is perfectly feasible to slew and move the mount and the tracking appears to work well.

* The old webcam had low sensitivity making it unsuitable for pretty much anything including astrophotography.

** The M12 mount I purchased was not supplied with screws, I used two spares I had lying around.