Making a DIMM prism

[Stefan Buda]

A Differential Image Motion Monitor is a simple but acurate apparatus used by some professional observatories to monitor astronomical seeing in real time.
It consists of an aperture mask, placed on the front of a 10" or larger telescope, with two circular holes abut 50mm diameter placed as far apart as the aperture of the scope allows. One of the holes contains a prism that produces a deviation of 1 or 2 arc minutes. When the scope is pointed at a bright star, there will be two images of the same star on the sensor. Software monitors the distance variations between the two images and calculates the seeing.
Looks like someone here down under is having a go at producing a prototype, as I was approached to make a prism for it.

I started with a small slab of optical crown from my collection of old war surplus optical glass.
First, I had to reduce the thickness from 16mm to about 10.6 and then make it round. The final size needs to be 10 thick and 52 OD.
After the machining, I used aluminium laps and 240 grit carbo to get the thickness to 10.4, the diameter to 51.99 and lapped the edges at 45 degrees. Next, I will get the angle adjusted as I proceed with the fine grinding.

[bojan]

Interesting.
Would a (slightly offset) pair of periscopes do the same job on smaller scope?

[Stefan Buda]

I guess so, but it would be a pain to align.

[JA]

??? >>> Ahhhh.....




Research paper in which used...
https://www.researchgate.net/publica...Uzbekistan#pf3

They used a wedge angle of 195" on a Celestron 11 with two 80mm holes 200mm apart.

Best
JA

[bojan]

Yes..

Looks like a waste of decent optics for the purpose...

[Stefan Buda]

Originally Posted by JA

??? >>> Ahhhh.....

Attachment 308605


Research paper in which used...
https://www.researchgate.net/publica...Uzbekistan#pf3

They used a wedge angle of 195" on a Celestron 11 with two 80mm holes 200mm apart.

Best
JA

Thanks JA,

They used a camera with 10 micron pixels and the two star images were about 120 pixels apart.
If using a planetary camera with pixels around 3 microns, a 1 arc minut wedge would produce more than enough separation.

When I first measured my finished blanc, I found a wedge "error" of about 50 microns, corresponding to an angle of 206".
During fine grinding, I reduced the wedge measurement to about 11 microns, corresponding to an angle of 45". That should produce a deviation of about 69", or 1.15'.

[By.Jove (Jove)]

Could be an interesting experiment to assess the relative merits of different observing sites using one of these.

[Stefan Buda]

Originally Posted by By.Jove

Could be an interesting experiment to assess the relative merits of different observing sites using one of these.

I'd like to put one on my prototype CDK250 and have it always ready to check the seeing before embarking on lucky imaging with my 16" D-K.
Hopefully this DIMM product will become available on the local market.

Anyway, after about 3 hours of polishing on each side, and a bit of hand figuring, the prism is ready for delivery to the customer.

[AstroViking (Steve)]

I'm always impressed with the amount of knowledge and skill here. I do not have the faintest idea how to do even half of the things described by IIS members.

[By.Jove (Jove)]

Stefan this reminds me of an antique brass refractor I saw once for measuring double stars where sone distance down the OTA (in front of the focal plane) there was an optically flat placed in the beam, but the plate was sawn in half so that one half could be tilted by a micrometer to displace half the beam laterally, crating a double image. By varying the tilt the effect could be adjusted so double stars (appearing as 4 stars) could be overlapped visually with just a plain crosshair eyepiece - instead of a bi-filar micrometer.