Ditherbox

[wimvb]

In order to minimize noise in my data, I've gone the Tony Hallas way and started dithering between exposures. My setup is a simple one (EQ3 Pro with Pentax K20D, no guiding), and I try to keep everything as much standalone as possible, since I don't want to take my computer out in freezing cold or wet grass.

Unfortunately this also means I have to do dithering by hand, i.e. pushing the Dec/RA buttons on the Synscan hand controller between exposures. Doing this for a few hours in a stretch at -10 C or colder, is definitely not my idea of having fun. So I started thinking of automating the dithering procedure. Searching the internet, I found no solutions for unguided dithering compatible with my setup, so I decided to build my own.

The idea was quite simple:

The RJ12 port of my Synscan hand controller controls the camera, working as an intervalometer. The Synscan mount controller has a guiding port, so all I had to do is find a way to send signals to the guider port in between exposures.

So, I built a simple circuit around an Atmel ATtiny84 which takes the Synscan camera control as an input. It senses when an exposure ends and sends a signal to the guiding port. A counter keeps track of which command to send from a simple string: 214488222111444488..., where 2 means Dec+, 1 means RA+, 4 means Dec- and 8 means RA-.

So here is a pictures of my solution. The ditherbox is the black box below the mount controller. It has three connections: input from the hand controller (RJ12), output to the camera (phono plug type) and output to the mount (ST4, RJ12)

FYI, I put the code and a more detailed description on my Github page: https://github.com/wberlo/AutoDither

I've tested it and it works like a charm. Unfortunately I have no images yet with this setup, because when I tested it, clouds moved in after just 5 exposures. I do have 20 dithered exposures of clouds.

[wimvb]

... and first light.

M44

32 light frames @ 45 secs, ISO 1600 (total time 24 minutes)

bias frames, flat frames, no darks

processed in PixInisght

[lux eterna]

Hej Wim,

That is an awesome project ! What a pleasure for you to sit down with a cup of coffee and watch it working.

I have been thinking along the same path as yours. I like to be really portable, so no pc for me. I need the dark skies more than the pc. But as I have an autoguider (NexGuider) and have to stop guiding, move guidestar and restart guiding between each frame. And since the Nexguider uses a menu based hand controller, there is some tedious button pressing to do. Also, the buttons ar poor quality and does not always respond properly. As the hand controller can be disconnected during autoguiding, I have been thinking on connecting some kind of dithering box. One thing to overcome will be the "wire saving design" of the HC - that is one button controls two wires in a specific pattern.

 

[lux eterna]

I just remembered...

The DEC dithering can be constant all the time, but the RA dithering should be increased with increasing target declination since RA movements near the NCP are hardly noticeable compared to near the equator. I guess it should be a sinus function of the target declination. 

[wimvb]

There probably should be some more elaborate function for correct dithering, but I kept it simple, and it seems to work.

BTW, this scheme won't work for autoguided dithering. Some standalone autoguiders, like the Lacerta MGen, have a dithering function. You still have to recalculate the relation between the dithering amount on the guide camera/scope and the imaging camera/scope.

For my simple setup the box works great, and since I get the power from the handcontroller, I don't need an extra power supply.

[SteveBz]

Hi Wim,

I do have guiding (DIY natch), so in the interval between subs I need to stop the guiding, dither and restart, hoping that guiding won't just take me back to where I started.

I also do use a laptop, but my telescope now has so many wires coming out of it, it's permanently tethered to one place, so I moved into my shed and took the roof off :).

Really I now need to start again with a portable system.  I was thinking maybe a small refractor.

Not decided yet.

Steve