View Full Version : DMK21BF04 question

June 3, 2005, 03:36:01
There doesn't seem to be a "camera" forum, so I chose this one as maybe a good alternative. I hope a moderator will be able to help me out!

I am considering purchase of a DMK-21BF04, but I have a couple of questions before I proceed. Because I live in Australia this would mean an international order, so I don't want to receive the camera and then find that it's not right for my use. It can sometimes take >30 days to get equipment through customs in this country so the 30 day return policy might be tricky.

I've been corresponding with Shane Jennings about this camera for about 6 months, but I think he must be away for long periods because I rarely get a reply, and then my questions are not answered. This is probably my last try at getting this information before I give up and go look at cameras from other manufacturers.

I will be using this camera for planetary astronomy. You can see examples of the images I'm getting with my current fire-i camera on my website:


I will be using this camea as a replacement for my fire-i. The fire-i has a number of problems that I want to overcome by replacing it with a more sensitive camera with better s/n. The DMK-21BF04 seems to be a good choice, but I need to find out the answers to a couple of questions:

1. How does the sensitivity of the DMK21BF04 compare to the DMK21F04? I have asked this question before, and seen other people ask it, and so far I have not seen any useful reply. The specs on the website are not comparable because they quote the sensitivities of these cameras at different gain values. I know the BF04 is a newer model, and I have been told by Shane that it should be more sensitive because it has a new AFE, but I'd really like to know how much more, so I can decide if the extra cost compared to the 21F04 is worth it, and also how this camera compares to other manufacturers equivalent cameras using the 098BL ccd..

2. If I set an exposure of 40ms (1/25 second), how many frames per second of useful data will I receive? Will the camera send me 25 fps, or will it send 30fps with some of those frames blank?

If the latter, how close to the "real" 25 fps of useful data will I be? This is very important for imaging Jupiter, where imaging runs are limited to 90 seconds by its fast rotation.

I could imagine a worst case where every 2nd frame is blank, leaving me with an "effective" 15fps even though the camera is running at 30fps. This would rule out this camera for my application since I wouldnt get enough useful frames in the 90 second window.

I saw a reply in another thread that indicated the 21F04 camera could not be programmed for arbitrary fps, does this also apply to the 21BF04?

Apologies for the length of this post, I'm just having a hard time finding out this information, and I've been trying for about 6 months ;-)

regards, Anthony

Johannes Vogel
June 3, 2005, 09:59:59

Our DMK 21F04 provides better image quality than the fire-i camera because the chipset of the DMK 21F04 is adjusted to the CCD.

1) At the moment, there is no way to compare cameras by its technical data with regard to the SNR or sensivity. This is because there is no defined procedure for measuring this data.
The EMVA (european machine vision association) is in the process of defining such a procedure which will make technical data of cameras comparable. Please read the current version of the paper defining which physical and mathematical aspects are involved if we talk about SNR od sensivity: (http://emva.org/home/content/view/456/)
If you read this paper, you will see how complex this question is.

For the time beeing, we can state that the DMK 21BF04 is A LOT MORE sensitiv compared to the DMK 21F04. There are poeple using this camera for astronomy. Here is a link to example images:

2) If the exposure time exceeds the interval between two frames, the actual interval between two frames can be calculated as follows:
interval according to the selected frame rate + exposure time.
For your example: 30ms + 40ms = 70ms.

The camera allows the following fps to be selected: 30, 15, 7.5, 3.75 fps.

June 3, 2005, 11:08:03
Thanks Johannes. So, am I reading you correctly - if I set an exposure time of 40ms and a rate of 30fps then I will get 14 frames per second of usable data?

regards, Anthony

Johannes Vogel
June 3, 2005, 14:41:54
that is correct.

June 3, 2005, 15:29:43
Thanks again Johannes, and now I have one final question :-)

My fire-i camera has a register that I can set to change the base rate from 30fps to 25fps, and then the camera speeds become 25, 12.5, 6.25 etc fps. I believe that this was originally intended to eliminate flicker in countries that use 50hz lighting.

By setting this register and disabling the electronic shutter (simply cutting the track between the driver ic and the ccd) I have been able to achieve my goal of 40ms exposure and 25fps. Is there an equivalent setting to this available on the DMK cameras?

If you use the CXD1267AN driver IC then I guess the same simple hardware mod could be used? I have this track reconnected via an external switch so I can selectively disable this behaviour and revert to normal electronic shutter control.

I understand what this would mean for my warranty :-)

regards, Anthony

Johannes Vogel
June 6, 2005, 13:50:28

The BF camera line (Dxx 21BF04, Dxx 31BF03 etc.) is a complete new design and does not use the TI chipset. Therefore, this modification ist not possible.
Anyhow, we highly recomment not to modify the hardware of our cameras because any modification will cause a loss of warrenty.