DIMM Seeing Monitor

This page presents a dedicated software that computes the seeing from a image of a double star that has been created by adding a prism at the telescope entrance pupil. A basic webcam (like a TuoCam pro) has been used to acquire the data, and thanks to the frame rate (up to 10 frames per seconds), it is possible to perform a real time analysis and compute the seeing based on the method explained hereafter :

A theoritical approach can be found here

The software panel can be seen hereafter and show a user friendly GUI,

!! HOT News !!

July 2008 v2.02 :
 Video For Window has been given up because few cameras appart from webcams support it. The software can now use any Directshow compliant cameras such as Imaging Source DMK/DFK camera, or Sony camera, JAI, IEEE1394, USB2.0 cameras or whatever compliant to it. It keeps the use of Webcams also.

The list of supported camera is huge, so inquire your camera manufacturer about camera Directshow compliance.
The interface has been changed to accomodate 1024x768 resolution and also some enhancement concerning telescope control.

 



CLICK HERE TO GET THE SOFTWARE, v2.02 July 24th, 2008

See here some videos :


This is not a freeware, this a shareware, to get it, you need a licence key, please have a look here. Nevertheless, you can evaluate it for free during 30 days before purchasing.

The html help file can be seen here.


Image 1 : main controls


Image 2 : Telescope control

 


Image 3 : result output

Image 1,2 and 3 shows the software in real operation, when used with a 180mm telescope. A prism has been used, only a two hole mask.
This software is fully compliant with Windows VISTA.

It can be used without no prism, only with a two hole mask and a slight defocus can been applied... the effect of a defocus has been studied here (pdf file).

The following plot has been recorded in July 2008, (X axis = Julian date) , this is the computed seeing with Altair and computed as zenithal seeing

The next plot is again Altair , three days after, the seeing is worse compared to the previous night.

Here is a seeing measurment with Capella star, using an Intes 180mm (two 60 mm holes separated by 125mm center to center).

In March 2005, a combined test has been achieved : Brice Oliver Demory and Marc Sarazin compared the standard ESO DIMM and the webcam based DIMM both in operation, at the same time and place. The place was ESO la Silla Observatory. The webcam DIMM and the official ESO DIMM where 100m appart.

It turns out that the two systems (different in telescope, camera and software) output the same data ! This is a good indicator : the two systems behave exactely the same fashion. The webcam based system presented in this page had higher resolution due to much higher data sampling rate. So this system works !!

Some additionnal tests :

Some tests has been achieved using Intes Mk703, 180 mm F/29 (Barlow), first without a prism, a single two hole mask have been used. Defocus has been applied to produce two separated stars.

Once the mask is removed, the defocus is clearly visible !!

 

More information and tests about no prism operation comparison.

Then, a tunable prism has been installed, allowing perfect focus and larger star separation, the measured seeing seems to be the same : 1.2 arcsec, with or without the prism, at F/29.6


The 2 holes mask, with the tunable prism, installed.


Front view.

Some words regarding the wedge prism issue.

Currently, I have not found a manufacturer that can provide this wedge prism off the shelve... All the wedge prisms I've seen around are custom made...
Some optical manufacturer (such as Fichou) can made them as a custom device, but they may very expensive (may be around 750€).
Most of the wedge prism sold are 25mm diameter which is to small and they wedge angle is far too much.

How this wedge prism shall be ?
- Diameter : 50 mm for a 200 mm telescope, 80 mm for a 350/400mm telescope
- The wedge angle has to be small, 1.5 arcmin, which leads to 48 arsec of beam deviation (ie 131 pixels from a 6.8µm camera with 4m focal length).

Another idea, is to use two 0.5° wedge prisms and stack them up. By rotating one prism with respect to the other, one can obtain a tunnable wedge prism that has a wegde of 0° to 1° , for sure it requires the manufacturing of an accurate mechanical part that holds the two prisms and allow a fine rotation tunning.

Finally, the focal length is reduced to 1800mm, with prism, it seems to be the same measured seeing (around 1.1 arcsec).

Again, on July 2005, M.Sarazin, from ESO, carried out a comparison test at Paranal Observatory
A very good agreement between this system (based on webcam) and ESO standard DIMM has been noticed, as showed hereafter :

This is amazing to get a so close agreement, the 2 plots have the same shape !

By C.Cavadore (April 2005), back to my home page