Night Vision and telescope with dielectric coatings

[janek]

I am considering getting a 3rd Gen NV device and stumbled upon some CloudyNights discussions how dielectric coatings are optimised for visual wavelengths whereas NV is most sensitive in near infra-red. I found that my 16 inch Dobsonian's mirrors have dielectric coatings with reflectivity decreasing dramatically outside the visible spectrum.

For nebulas this is not an issue but one of the reasons I wanted the night vision was the better view of edge-on galaxies. Does anyone have experience with how much worse the dielectric coatings could be for this particular target?

[Sporadic Dobstronomer]

My experience is that using an IR blocking filter, the gain of a night vision device is about 35% of without it -but this is on Earth.  That should give you an estimate of the worst that can happen if you are not troubled by light pollution.

The actual results will also be affected by things such as the actual IR range of the material used in the intensifier and also the proportion of IR in the light you are interested in.  Also IR might pass through the shroud, if your telescope has one, and ruin everything!

So my real answer is you might have to borrow one and try it unless someone else already has.  Sorry! 😬

[Deadlake]

Do you have a response graph of the mirror in question?

All stellar targets are broad spectrum emitters.

While image intensifiers can see visual spectrum they are not nearly as sensitive as they are in near IR.  In fact, the near IR range is the big magic of NV.  An image intensifier takes light that is too far into the red end of the spectrum for us to see and merely amplifies and converts and it to a color that we can see. 

For nebula, you need good system transmission at 654nm, but for stars you need good transmission from where-ever your long pass filter kicks in to 900nm, which is the range where the photocatode is the most sensitive.

I.e. you will boost the spectrum where you cannot see in aided considerably..

As far as dielectric coating many observers using 16” dobs are using NVD’s due to image scale, the contrast boast from the NVD will overcome any loss in infrared light and this is where the multiplication provided by the NVD is greatest.

 

 

Edited July 6, 2023 by Deadlake