Adventures in Nightvision - A work in progress
Introduction, disclaimer, etc.
Consider this my nightvision blog where I can post photos and other things that don't have a place elsewhere.
Typos, poor grammar, strong opinions, and bad language may be present, but I don't care. Proceed at your own risk!
October 12, 2017
On the night of October 12th, I did afocal eyepiece testing. Here are the eyepieces and nightvision unit that I used.
Testing was done in my driveway on a night that was clear, but fairly humid. Transparency was about a 6 out of 10, and Milky Way was visible, but not nearly as distinct as it would be at its best. The temperature was about 55° to 60° F, and there was almost no wind at observing time. I used my 20" f/3.0 (see articles here and here) for this observing session. It uses a Starlight Integrated Paracorr System (SIPS), so including the 15% Barlow effect of the Paracorr, effectively it was operating as a 20" f/3.45. The SIPS was always in place, even when doing prime-focus viewing for comparison. For me, the Paracorr/SIPS is a necessity, even with nightvision, because the coma (especially when added to eyepiece aberrations) is quite objectionable to me.
Before anyone asks - yes you can use a 55mm, 41mm, or 31mm eyepiece with this nightvision unit and get all of the light into the nightvision unit. Even though humans only have a ~7mm pupil in their eyes, the nightvision unit has a ~21mm diameter objective on the front that is used for afocal observing, and this objective can receive the huge exit pupil from the 55mm eyepiece when used with my 20" f/3.0 (operating at f/3.45), and even when used in my 14.5" f/2.55 operating at f/2.93 with the Paracorr 2!
This is a huge advantage, and allows wide fields of view and greater density of photons per unit area falling on the photocathode of the nightvision unit, thus increasing the brightness of images just as a fast telescope/lens does on extended objects with film or a CCD sensor. So, faster = brighter, and exit pupil limitations are essentially gone.
The photo at right shows Peter Wang with the 20" f/3.0 when he came to visit in August to do some observing with the 20". He has a very nice introduction to nightvision on his web site. We had a total of about one and a half clear nights over the course of a long weekend, and very high humidity. I actually had to use a fan/electric heater to heat the primary after it instantly dewed up after I rolled the telescope out of the garage! This worked, and we were able to observe through a good part of the night.
To compare eyepieces, I used a Mod3 filmless white phosphor nightvision device, loaned by Peter. I had also used it in a variety of telescopes (from a 24" f/2.75 to a 32" f/3.6) at the Okie-Tex Star Party about a month prior to this. I set the manual gain at a pleasant viewing level and did not change it throughout all testing in this session. The Tactical Night Vision Company (TNVC)/Televue afocal astronomy adapter was used to connect the Mod3 intensifier to a variety of TeleVue eyepieces.
I used a Canon G15 to capture images. It 's a point-and-shoot camera that can shoot in RAW format with reasonably low noise and a fairly fast lens. The end of its pop-out lens is flat, as is the surface of the nightvision eyepiece with the rubber eyecup removed, so I could put the two surfaces together and hand-hold the camera to get decent images of up to a 1.0-second in exposure, which is the maximum that camera will do. If the camera slipped noticeably during the exposure, I just took another one, but sometimes that was hard to notice. Thus, the images are not all of the same quality, and conclusions should not be drawn about sizes and shapes of stars. We are only interested in brightness here.
I used the camera in manual mode and kept settings constant for the eyepiece comparison and a prime-focus image. I had to zoom in the same amount each time I turned on the camera to increase the size of the image seen by the camera and fill the frame, and I did this as accurately as I could based on the display of the zoom setting. It was not perfect, but I got fairly close.
I tracked M17, which was rather low in the sky. Images at the beginning and end of the session showed that there was no significant change of brightness due to atmospheric extinction, so that was not a factor in the testing.
Photos in the mosaic below were taken of M17 with NO filter. They show the expected loss in brightness as the image is magnified. However, when the intensifier is used visually, this difference is less, because the eye adapts to the brightess of the output of the intensifier. In particular the darker views above looked a bit better visually.
For comparison, below is a mosaic of the view in prime focus mode (left) and then afocally with two different 2" filters used on the 55mm Plossl. The prime focus image was taken at the same camera settings as the eyepiece comparison mosaic above to facilitate comparisons. The center and right image were taken with different camera settings. Refer to the text on the images for exposure information.
It is clear that the prime focus image fits nicely into the sequence of images, in terms of brightness, and is close to the view of the 27mm Panoptic. The view with the 55mm Plossl and 41mm Panoptic are brighter, though, proving that prime-focus is not the best possible view in terms of field of view and brightness of objects. Visually, in my 20" f/3.0 (operating at f/3.45) the 55mm Plossl does distort star images as one looks off-axis, but it doesn't seriously hinder this type of observing, and the wide field is quite worth it, in my opinion. To achieve focus with the SIPS, the 55mm must be pulled out of the focuser a bit, so I may make or acquire a spacer to limit how far it goes into the focuser.
Additionally, I should mention that doing prime-focus with my SIPS-equipped telescope was a royal pain in the ass. I had to remove the lens from the NV unit, install a 2" adapter, and then spin the threaded focuser/SIPS assembly inward about 8 turns in order to simply achieve focus. I also had to take off the Telrad in order to allow the focuser and SIPS assembly to be rotated. This is annoying and time consuming to do and undo, and I found the view in the 55mm and 41mm eyepieces completely removed my motivation to use prime-focus. Be aware that many telescopes will not have enough in-travel to achieve focus.
The two images at center and right above were taken with borrowed 2" imaging filters, an Optolong 2" H-alpha filter (7nm BW) and an Optolong 2" CLS light pollution filter intended for imaging. The latter has two passband regions, one from ~450-525 nm that will not be seen with this particular intensifier, and a region at 625nm and longer which will be seen.
The results of this testing? No major surprises, and finally a definitive set of images showing a comparison between various TeleVue eyepieces used afocally and prime focus with the NV device. The H-alpha filter dramatically improves contrast, but requires more gain. The LP filter improves contrast a bit and also requires a bit more gain. More testing may be done with it later on.
Note: The the prime-focus image is inverted compared to the afocal images. In the prime focus arrangement, the nightvision objective lens is removed. The nightvision tube itself inverts the image with an internal fiber optic "twist", so prime focus images are an inverted image of the focal plane. For afocal, we use an eyepiece and the nightvision lens. An eyepiece only images the focal plane and does not reverse it. The nightvision unit produces images that are non-reversed from what it sees (otherwise walking around with the 1X version would be impossible!), so the whole chain of eyepiece, nightvision objective lens, and fiber optic "twist" produces a non-inverted image of the focal plane.
I hope to do a comparison with my 14.5" f/2.55 in a future session, depending on weather conditions. A drier night should be better. I'll probably use the objects in Cygnus like the Crescent Nebula, North American Nebula, etc.
October 16, 2017
It was a fairly transparent night, and I managed to get both the 14.5" f/2.55 and 20" f/3.0 set up, collimated, and ready to go before darkness fell. I had dinner and poured a beer and headed out for some comparison observing and crude photography of various objects. The goal was to show approximately what one would see with their eye if they were observing with me.
I also did a comparison of the two telescopes. I used the 55mm TeleVue Plossl afocally with the TNVC/Televue adapter, and the same Mod3 on loan as the last installment. I took photos of the same object, M17, to start with, then some other objects. Below are afocal images taken with the 20" f/3.0 and 14.5" f/2.55. No filter was used. All images were taken hand-held against the NV unit top with a Canon G15, 1s exposure, f/2.0, ISO400. The first three sets of images were taken with no filter, all after were taken using a 2" Optolong 7nm BW H-alpha filter. The 14.5" f/2.55 uses a tunable-top Paracorr 2, the 20" f/3.0 has a Starlight SIPS built-in Paracorr 2.
Please keep in mind that the camera was hand held for images, and are not the highest quality. In particular the images taken through the 14.5" f/2.55 vary in quality around the field, and this is due to camera alignment and other issues. Please don't blame this on the eyepiece, there are other issues at work!
Images from the 14" f/2.55 (operating at f/2.93) are on the left below, images from the 20" f/3.0 (operating at f/3.45) are on the right.
Here is M17 (no filter):
M11 (no filter):
M27 (no filter):
After this it was time to install the Optolong 2" H-alpha filter (7nm BW) and see what both scopes could do. The night was better than October 12 in terms of transparency, but it wasn't ideal. I chose objects higher in the sky at first, and then I went hunting for objects that showed well in H-alpha. It turned into an H-alpha binge observing and crude imaging session.
Portions of the North American/Pelican Nebula complex:
Portions of the Gamma Cygni region:
The Crescent Nebula (NGC 6888):
The "interesting" part of the Veil Nebula, NGC 6992, 6995, and IC1340:
Pacman Nebula, NGC281, Cassiopeia:
Part of the California Nebula
Overall I was very impressed with what I could see with these relatively large telescopes. I am not aware of others using 2" h-alpha filters yet, but I think this will be come quite common for nightvision observing in the future.
I have made some inquiries to see if I can get some other loaner filters - these can have blemishes for our purposes, so I am hoping to be able to get some of these and compare many filters in the future and see what works best. I also have a few other tricks up my sleeve......
October 18, 2017
On this night I had invited the friend who had loaned me the Optolong H-alpha and CLS filters over to see what his filters did when mated to an eyepiece and nightvision unit in afocal arrangement in my 20" f/3.0 (again, operating with a TeleVue/Starlight SIPS at f/3.45 with Paracorr 15% barlow factor).
Before he arrived, I had time to shoot a series of eyepiece comparison shots without allowing my Canon G15 to turn off, and thus preserving the zoom setting. (When the camera is turned off, the lens retracts and the zoom goes back to default wide-angle.) I turned up the NV gain all the way and left it, just to make sure I didn't accidentally change it while changing eyepieces.
Please NOTE: These images were taking at a different gain setting than my previous eyepiece comparison, which was at some gain setting that I can't replicate because there is no way to tell exactly where the gain is set. Also, the seeing was much worse on this night than on Oct. 12, so stars are larger. The wind was blowing and it was difficult to get good images with the shorter focal length eyepieces.
Unfortunately I did not have time to go to prime-focus operation, because my friend arrived as I was finishing, and because the camera would have shut off at some point during the 5-10 minutes it takes to do the change-over. The next comparison will have to wait until some appropriate objects is higher in the sky - M17 is just getting too low at this time.
Anyway, here is a mosaic images taken with six eyepieces, all without changing anything at all on the camera, including zoom.
I think that shows the difference in brightness quite well. Sorry I couldn't do prime focus, but I'll do that another time on a different objects.
October 25, 2017
I did some quick testing with 2" narrowband filters placed in a holder, which was the end the cut-off portion of a focuser extension tube that I cannibalized for several parts. This allowed me to hand-hold the filter in front of a nightvision unit and tilt it manually.
I looked at the H-alpha regions in Cygnus, as well as a few others, and with the object centered, it showed up nicely. Moving the object to the edge of the field of view caused it to disappear. However, tilting the filter in that direction brought it back. This is conclusive proof that the flat filters cause band-shift in off-axis portions of the image because the light from off-axis regions travels through the filter at an angle that departs from normal incidence (going straight through the filter) by a significant amount.
The 40° field of a nightvision unit means that light at the edge of the field is coming in at an angle that is 20° from normal incidence. For this angle, the filter allows wavelengths different than H-alpha to pass through, which means H-alpha is blocked, and the object disappears.
So, for afocal viewing, the best place to put a narrowband filter is not in front of the NV unit, it is on the bottom of the eyepiece, the bottom of the Paracorr, or on a filter slide that sits below the Paracorr or eyepiece. The cone of light from the primary mirror or objective lens covers less angle than the cone entering the nightvision unit.
November 5, 2017
So a question has come up about when to use a narrowband filter on an eyepiece (used for afocal imaging) versus over the nightvision device (NVD) objective lens.
To minimize de-tuning, or the filter passband shifting, we want light to pass through the filter at close to normal incidence. This is accomplished with a slower converging light cone, also known as a longer or higher f#. So, we need to compare the effective f# that the NVD lens is working at to the f# of the telescope. If the effective f# of the NVD lens is larger than that of the telescope, then the light cone is converging more slowly and putting the filter in front of the NVD objective will work just as well, if it can be done easily.
The math is pretty simple, so let's define a few things.....
Exit Pupil from the eyepiece (EP) = focal length of eyepiece (FLE) / Telescope f# (TF#), so in variables we have: EP = FLE / Tf#
The f# that the NVD is operating at is easily calculated. A telescope's f# = focal length / primary diameter, and thus the f# that the NVD lens is working at is the focal length of the NVD objective (which is 26mm) divided by the effective diameter of the exit pupil, so: NVDf# = 26 / EP
Now we just substitute in for EP in the second equation: NVDf# = 26 / (FLE / Tf#), rearranged to: NVDf# = 26 / FLE * Tf#
We want to know what focal length of eyepiece (FLE) will cause the nightvision device objective to operate at a higher f# than the telescope itself. So, we just rearrange the equation to get both f#s on the same side -> NVDf# / TF# = 26 / FLE
So, by inspection we can see that in order to have the telescope f# (TF#) be larger than the NVD f#, the eyepiece focal length (FLE) must be larger than 26mm so that the ratio on both sides of the equation is less than 1.00.
So, whenever the eyepiece focal length is longer than 26mm, the exit pupil is large enough to make the NVD lens operate at a faster/smaller f# than the telescope, and it makes sense to put the filter on the eyepiece instead of the NVD.
With the TNVC/TeleVue afocal adapter, it is not really possible to put the filter on the NVD, so I put it on the eyepiece. Since long focal length eyepieces of reasonable quality need to have a large field stop in order to show a wide field of view, they need to be 2" in physical diameter to allow enough of the focal plane to be seen to have a reasonably wide field. Let's pick some established, good quality eyepieces that will work reasonably well in fast telescopes, such as the TeleVue 55mm Plossl or TeleVue 41mm Panoptic. The best way (but not an inexpensive way) to use such eyepieces is to obtain a good quality 2" narrowband filter and simply screw it into the eyepiece barrel, the bottom of a Paracorr, or put the filter in a filter slide below the Paracorr.
For a very nice diagram and some other discussion, see TeleVue's new page here: http://www.televue.com/engine/TV3b_page.asp?id=36&Tab=_work
With relatively slow telescopes, it won't matter where you put a filter unless you are using very long focal length eyepiece. Let's assume that the 55mm Plossl is going to be the longest focal length eyepiece of reasonable quality that is readiy available. So let's figure out, for the 55mm Plossl, at what f/# you might be concerned about de-tuning effects when using a NVD afocally and having a filter in front of the NVD.
This is pretty simple - we just plug our numbers into the third equation above. I have used filters in the barrel of a 55mm Plossl in my f/2.55 telescope and it works quite well with 6nm narrowband filters. The telescope uses a Paracorr, so the true f# going through the filter is 2.55 * 1.15 = 2.93, and let's round to 2.9.
Let's find the telescope f# that produces an f/2.9 light cone entering the NVD. We have: 2.9 = 26 / 55 * TF#, so TF# = 6.13
So, with a telescope faster than f/6.13, it is possible to get a light cone of f/2.9 or faster entering the NVD objective, and when using the 55mm Plossl it could be better to put the filter on the bottom of an eyepiece instead of in front of the objective.
For me, given the complications of using a smaller filter in front of the NVD, I'll just bite the bullet and use a 2" narrowband filter in the barrel of a long focal length eyepiece (until I get a filter slide) and enjoy afocal observing.
More nightvision adventures to come.....
-Mike Lockwood, Lockwood Custom Optics