Night Vision Devices are now being used with telescopes to increase the image brightness and detail available to the user at the eyepiece. There seems to be a revolution when it comes to this emerging art of “Black Ops Astronomy” with his revealing phone images available on the internet each day.
Thanks to Alan at Stargazerslounge.com for this article.
I decided that I wanted to give this a try especially as I own a very fast focal ratio dobsonian.
Two main factors were forefront in my mind
– the latest night vision technology is very expensive
– guidance of “experienced” people in this domain is vital
So, I have done plenty of research over on cloudynights.com (our US based friends are well ahead of us in this arena!)
https://www.cloudynights.com/forum/73-eaa-observation-and-equipment/ (Look for posts tagged “NV”)
I found useful information on Mike Lockwood’s site and yet more images to build my desire!
I also communicated directly with Reimer at ActInBlack to tap into his knowledge of night vision as to what “specification” I should be looking at for my planned astronomy purposes…
There are two main areas to decide upon:
– the night vision device (NVD)
– the image intensifier tube (which is fitted inside the NVD)
This post will stick to the image intensifier tube.
-The NVD for me had to be the PVS-14 as I will be doing “afocal” observing using my Televue eyepieces and additional TNVC Televue adapter (which allows a direct connection of the PVS-14 NVD to the Televue Dioptrx connection on Televue eyepieces).
http://www.televue.com/engine/TV3b_page.asp?id=36 (The topic of “afocal” observing is covered in a separate post, * LINK AT BOTTOM OF ARTICLE *)
The above is somewhat “simple” compared to the wonderful world of image intensifier tube terminology!
If you are based in Europe then you MUST BUY your image intensifier from countries other than the USA (its illegal to export image intensifier tubes out of the USA). Its even illegal for a US citizen to let a non-US citizen look through their NVD should they be in the USA visiting a friend !!!
Luckily, Photonis (France & Netherlands) make first rate image intensifier tubes (also Harder Digital in Germany) which can be purchased safely and without restriction from non-US based suppliers (such as ActinBlack based in Luxembourg).
Here is an example spec sheet for an image intensifier tube
What is that all about?
Here are the main things for Astronomers to look for on a spec sheet…
Figure of Merit (FOM)
Image Intensification tube specification designation, calculated on line pair per mm x signal to noise. (FOM=Resolution * SNR)
-over 1800 is good, over 2100 is great (the higher the FOM then the higher the price )
Signal to noise ratio (SNR)
Describes how well faint signal may be distinguished from the scintillation noise floor of the device. This metric is particularly relevant to the performance of the device in the “photon starvation” regime of tight narrowband filtration or very slow optics. I quote ” Because SNR is directly related to the photocathodes sensitivity and also accounts for phosphor efficiency and MCP operating voltage, it is the best single indicator of an image intensifiers performance“.
-High S/N – 30 is excellent, 25 is good, 16 is probably bordering unusable
Measured in line pairs per millimeter, this specification describes the finest spatial frequency that can be reproduced or represented on the phosphor screen of the intensifier. Higher resolution will reveal finer details and tighter, sharper star images, particularly when operating at 1x.
-High resolution – 64lp/mm or better
Equivalent background illumination (EBI)
Describes the contrast potential of the tube. EBI describes how dark the “black point” of the phosphor screen can be (at a standard temperature) when no light is hitting the photocathode. I believe that this metric describes how well dark features, such as dust lanes, will be seen to “stand out” (in terms of contrast) from a more illuminated background.
-Low EBI – less than 1.0 desirable, less than 2.5 is mil-spec on a lot of tubes. Luckily Photonis tubes specs have a maximum of 0.25 (But you need to multiply the quoted value by x10, so 0.04 becomes 0.4 in the case of the spec above)
Describes the degree of light amplification that the tube is capable of when operating at full gain. I believe that this is rated in electrons. Gain is measured at different wavelength, but a “standard” seems to be 2×10^-6.
-A value of around 35000 would be nice in a 4G (multiply the Photonis quoted spec value by “Pi”). A tube with a gain of 35,000 means that a single electron generated at the photocathode results in a cascade of 35k electrons striking the phosphor screen.
This is very critical as you want to see the object and not the HALO!
-Low halo – less than 1.0 desirable
Finally, we have the murky world of “dark spots”. All tubes have dark spots! The number of spots, size & location of those spots are a critical measure during quality control and the tubes with fewer & smaller spots in the more central zones are more expensive (surprise surprise!).
The 18mm objective is split into three zones:
and a spot report is produced.
After all that, I went for a “Photonis 4g INTENS” white phosphor intensfier tube and my device is due for delivery on Tuesday
Hope this is helpful to others considering “Black Ops Astronomy”.
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