39 replies to this topic

[Gavster]

Before Televue introduced their pvs-14 adapter in conjunction with TNVC in 2017, most night vision use was, to my knowledge, done in "prime" mode. However, unsurprisingly, given Televue's status, afocal night vision observing has become much more popular since then. There's been several threads on afocal versus prime nv use since then, but having been an afocal only user, in October last year I got my first night vision monocular that can do prime, the ovni-m. I thought it may be useful for some to describe my thoughts after comparing directly the two approaches.

 

From a personal perspective, being based in the UK, the introduction of the Televue afocal adapter enabled me to put together an excellent European night vision setup using European sourced PVS-14s which contained a very good Gen 3 German Harder Digital white phosphor tube (FOM 2400+, with EBI less than 1, and gain 60k+). It was a massive breakthrough for my DSO observing, and even though I couldn't do prime I was very satisfied with my results both live visual observing and taking quick phone snaps.

 

I have a range of scopes that I use with nv from a little 40mm askar refractor through a 130mm refractor, a 11inch sct and all the way to a 16 inch f4 dob, which has enabled me to do quite a few different comparisons. My ovni-m has the same make of tube (harder digital) and similar, but slightly superior specs to my pvs-14 (FOM 2600+, ebi, less than 0.5, gain 70k+).

 

As some of you will know, for live nebulae observing, my personal preference is to see as much of the nebulae detail as possible and bright views, I don't mind attenuating the fainter stars since it's the nebulae I am most interesting in seeing). This has meant I mostly observe afocally with the 67mm televue eyepiece to get a effective fast f ratio system (f2.5 or less) and a very narrow ha filter 3nm, to maximise nebulae contrast.

 

With the ovni-m I'm now able to observe in prime mode but this means that my systems have a much slower effective speed of around f4 to f5.5. I have also found using reducers in prime mode problematic because they require quite a bit of in focus which I don't have and so can't get into focus. At these slower speeds the live visual views are nowhere near as nice for me personally as they are much dimmer which means that the gain can't be turned down as much to reduce scintillation. In the fast afocal setups, I can turn gain down to effectively remove scintillation, give great "natural" views, but still get nice bright nebulae detail. Also the afocal setups handle the very narrow ha filter much better due to the faster system speed. I find the very narrow ha filters bring out the contrast better. In prime mode I prefer my 5nm filter to reduce the scintillation a bit but I don't get the same level of contrast. 

 

Where I do prefer prime mode is taking my quick phone snaps. Prime mode gives lovely tight stars to the edge of the field of view and I can easily adjust the iso up a little to increase the brightness with limited impact on noise. In afocal mode it is difficult to get good edge stars due to the fast system speed and the 67mm eyepiece giving edge aberrations (55mm plossl even worse in this regard, 41mm panoptic better). In addition with my dob I do get some reflections on the brightest stars from all the glass in the setup (I think). I can now get some attractive phone pics with my dob which I struggled when just using afocal. 

 

My ovni-m can do afocal observing but from my personal experience, the pvs-14 is better in this regard for a couple of reasons 1) the pvs-14 front lens has a thread in it which means that it can be secured firmly to the televue eyepiece with the televue afocal adapter. My ovni-m doesnt have a thread in its 1x lens so the afocal connection is push fit or with an external bracket attached to the body itself which I don't find as good. 2) The front lens on the pvs-14 is excellent in my opinion particularly as it operates at a very fast f1.2, it was specifically designed for nv usage and it shows. Other 26mm c mount lens I have tried haven't been as good particularlyas you move towards the edge of the field of view. The front detachable 1x lens supplied with my ovni-m is ok but the pvs-14 is noticeably better in 1x and afocal mode. The envis lens (now discontinued, difficult to find in the USA and impossible to find in Europe!) is I understand the same quality as the pvs-14 front lens, but I also have also read that the envis lens requires a large move to twist to focus whereas the pvs-14 is more swift in this regard. 

 

The adjustable gain knob on my pvs-14 is excellent and can be twisted in an analogue way to get to my perfect setting, the ovni-m gain is a digital push button which works but is not quite as easy to use. Finally the pvs-14 has a really nice amount of eye relief which making observing with and without glasses so easy and pleasurable. The ovni-m specifications show a long eye relief in excess of 20mm, but in practice I find I need to push my eye in closer than the pvs-14 to see the whole fov. 

 

So my conclusion is for live visual observing I will be using the pvs-14 with 67mm (and maybe 55, and 41mm on occasion), and for my quick phone images I will swap in my onvi-m with 5nm filter for better edge stars and zero star reflections.

 

Here are my night vision monoculars, 3 pvs-14s (one with photonis 4g tube, 2 with harder digital grr we n 3 tubes) and my ovni-m with harder digital gen 3 tube.

 

 

Attached Thumbnails

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Edited by Gavster, 05 March 2021 - 07:39 AM.

[jay.i]

Excellent write-up Gavin! Are you entertaining the idea of swapping the OVNI-M tube into a PVS-14 to get those buttery specs in a more frequently used intensifier?

[DavidWasch]

Thanks for the contribution Gavin,

 

I just started with afocal myself (last night, actually). What struck me as remarkable was the gain in field of view. Open star clusters now can have context in a 3 degree field. So many more stars in the eyepiece!

 

I have read accounts by Eddgie and others that the brightness gain of afocal doesn't seem to match the decrease in scale (or reduction in f/ ratio). Last night was cold, so I didn't make comparisons yet myself. 

 

Do you find that nebulae brighten in step with the reduction in f/ ratio?

Edited by DavidWasch, 05 March 2021 - 09:37 AM.

[Deadlake]

My ovni-m can do afocal observing but from my personal experience, the pvs-14 is better in this regard for a couple of reasons

1. The pvs-14 front lens has a thread in it which means that it can be secured firmly to the televue eyepiece with the televue afocal adapter. My ovni-m doesnt have a thread in its 1x lens so the afocal connection is push fit or with an external bracket attached to the body itself which I don't find as good.
2. The front lens on the pvs-14 is excellent in my opinion particularly as it operates at a very fast f1.2, it was specifically designed for nv usage and it shows. Other 26mm c mount lens I have tried haven't been as good particularlyas you move towards the edge of the field of view. The front detachable 1x lens supplied with my ovni-m is ok but the pvs-14 is noticeably better in 1x and afocal mode. The envis lens (now discontinued, difficult to find in the USA and impossible to find in Europe!) is I understand the same quality as the pvs-14 front lens, but I also have also read that the envis lens requires a large move to twist to focus whereas the pvs-14 is more swift in this regard. 

 

Is the main dis-advantage of the ovni with regard to the pvs the lack of a threaded connection, you have to use the external bracket? 

I'm presuming the lens is something that can be worked around.

[a__l]

Gavster, but what about DSO? Most are small for afocal use with the TV67.

[GOLGO13]

I find I like using both prime and afocal depending on the situation. Of course if people can only do afocal it will work fine.

 

Biggest difference to me is the length of the stack coming off the focuser. Sometimes Afocal gets a bit silly depending on the scope. It still works but it can get a bit crazy. This is especially true with my 6 inch F4 imaging newt. That scope is meant for needing more inward travel, so afocal works against that scope. But for a regular scope or a scope that doesn't have much inward travel afocal is nice to have. The Antares .7 reducer I have works with near every scope I have though.

[Jeff Morgan]

So my conclusion is for live visual observing I will be using the pvs-14 with 67mm (and maybe 55, and 41mm on occasion), and for my quick phone images I will swap in my onvi-m with 5nm filter for better edge stars and zero star reflections.

 

Nice write up. The cell phone imaging has been fun and in my case led me to purchase a CMOS setup. (And you thought NV was expensive!) In particular I have struggled with a good mounting method for the phone - no strain on the eyepiece and no tilt in the system. And of course, the sensor is quite noisy!

 

I would agree with your visual findings. For brightest image and widest field, the Tele Vue 67 Plossl arrangement is the best game in town. Very superior to my 2" GSO 0.5x focal reducer. I use the 67P frequently and it is my starting point for nebula, faint targets, and large targets.

 

That being said, the marriage of the eyepiece and NV objective lens is not a smooth one. Prime focus delivers the cleanest image, the only aberrations or distortions are what the host scope is delivering.

 

I find the 0.7x focal reducer to be very useful unit. It would be roughly equivalent to afocal with a 40mm eyepiece. Attached are two images of the Helix shot through my 130mm f/7 TMB refractor, SQM 21.5 sky. One is with the 67 Plossl, the other is Prime + 0.7x reducer. The parameters of both shots are equal: ISO 3200, 1/3 second exposure, 45 frame average. I wish I would have shot one more at Prime with no reducer, I think this target was bright enough to support that.

Attached Thumbnails

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[Gavster]

Excellent write-up Gavin! Are you entertaining the idea of swapping the OVNI-M tube into a PVS-14 to get those buttery specs in a more frequently used intensifier?

Unfortunately not possible as the PVS-14 tubes have connector plates and the ovni-m tube has connector wires. However, I have sourced a FOM 2600+ harder digital tube and a PVS-14 body so constructed my own "special" PVS-14 shown 2nd left in the photo above. I now use this in conjunction with my FOM 2400+ PVS-14 in my binoscope as a basically matched pair of PVS-14s, very nice indeed as per my binoscope thread.

 

Having compared directly my 3 harder digital tubes I really struggle to see any difference despite the slightly different specs, I think once you get to a certain level, the observable differences between tubes are pretty small.

[Gavster]

Thanks for the contribution Gavin,

 

I just started with afocal myself (last night, actually). What struck me as remarkable was the gain in field of view. Open star clusters now can have context in a 3 degree field. So many more stars in the eyepiece!

 

I have read accounts by Eddgie and others that the brightness gain of afocal doesn't seem to match the decrease in scale (or reduction in f/ ratio). Last night was cold, so I didn't make comparisons yet myself. 

 

Do you find that nebulae brighten in step with the reduction in f/ ratio?

In short, yes, as I increase the effective f ratio, I do see a noticeable increase in brightness. Good point about the increase in field of view, having the ability to get 4 or 5 degree fovs is really nice for north american, heart and soul, rosette etc..

Edited by Gavster, 05 March 2021 - 03:30 PM.

[Gavster]

Is the main dis-advantage of the ovni with regard to the pvs the lack of a threaded connection, you have to use the external bracket? 

I'm presuming the lens is something that can be worked around.

The threaded connection is a disadvantage for me, others may not mind. Personally I just prefer the solidity of threaded connection with the pvs-14.

 

In addition, the pvs-14 shows noticeably less vignetting and edge star aberrations than the detachable f1.2 26mm front lens that my ovni-m came with. Again this latter point is personally important to me since I do so much afocal observing but may be less important for others.

[Gavster]

I find I like using both prime and afocal depending on the situation. Of course if people can only do afocal it will work fine.

 

Biggest difference to me is the length of the stack coming off the focuser. Sometimes Afocal gets a bit silly depending on the scope. It still works but it can get a bit crazy. This is especially true with my 6 inch F4 imaging newt. That scope is meant for needing more inward travel, so afocal works against that scope. But for a regular scope or a scope that doesn't have much inward travel afocal is nice to have. The Antares .7 reducer I have works with near every scope I have though.

I guess since I started with afocal observing the length of the "stack" never really bothered me. On my refractors and c11 given it just comes out upwards from the diagonal really I don't notice it at all. In my dob, yes its more noticeable coming out the side particularly with the paracorr as well, but with my feathertouch focuser in practice I don't have a problem and am just used to it. My main refractors are petzvals so hence limited in travel, maybe I will try a 0.7x reducer and prime mode on say my AP130GTX but it will still be only f4.7 rather than an effective f1.8 which I get using both a 0.75x reducer and the 67mm eyepiece.

[a__l]

In my opinion, the brightness of objects in TV67 with H-alfa roughly corresponds to the brightness in prime focus without any filter. This can be used for DSOs. Anyway, on my 18" in a dark sky.

[Joko]

To look at small globs, small planterary nebulas, small galaxies or any small DSO, prime use is the best solution IMO.
Most of these targets require at least 100x magnification.
Let’s take an exemple using a 16" F/4 dobsonian telescope. NVD is a 26mm eyepiece, so in prime it gives 1600/26 = 61.5x.
The big advantage is you can add barlow/powermate. So a 2x barlow will give 123x mag and a 4x Barlow will give 246x mag. And bigger magnification is still possible depending on the telescope.

 

Why not using afocal to get high magnifications ? Here are the 3 major reasons :
1/ Well, higher means darker. Additional eyepiece will add more glass so less light. This is not the case with powermate or high grade barlow.
2/ Afocal means very long stack and heavy setup. And afocal means using a front lens, any front lens will add distortion and other optical issues. So it will be better to use a coma corrector. Such a setup would be coma corrector + Barlow + eyepiece + adapter + NVD. While using prime would be Barlow + NVD. I measured and weight the difference
36cm vs 18cm and 1.96kgs vs 0.787kgs
See some pics attached showing those differences (please note i lost my coma corrector at home and use a 2x ES Barlow to simulate the coma corrector so size and weight may slightly change).
3/ In addition to 2/ if you want to attach any additional accessory like a filter wheel, a smartphone or a camera, it will be longer and heavier. This is something to consider.

 

My conclusion :
Unless if you are only interested in large diffuse nebulas and some very rare and very big DSO, you will feel quickly limited by the number of targets.
Prime with or withour barlow will allow you to go well beyond these limits to explore all NGC, Messier and other DSO catalogue.

 

 

Edited by Joko, 06 March 2021 - 06:21 AM.

[bobhen]

 

My conclusion :
Unless if you are only interested in large diffuse nebulas and some very rare and very big DSO, you will feel quickly limited by the number of targets.
Prime with or without Barlow will allow you to go well beyond these limits to explore all NGC, Messier and other DSO catalogue.

 

What you have so eloquently posted is why I use Prime observing. With my C8, many times I don’t even have to add a Barlow to the stack, just the intensifier.

 

And image scale just delivers more detail. One of my best views of the Horsehead Nebula was not through my faster refractors but was, surprisingly, with the C8.

 

With the wider views offered by my fast refractors, scanning the star fields of Sagittarius and Cygnus is fantastic but many more objects become even more impressive with image scale.

 

Bob

[Jeff Morgan]

WRT to Barlows vs. Powermate, i was liking the images better when using a 2.5x Powermate compared to a 2.4x Dakin or a 2.8x Klee. Both of those Barlows were of good quality (I had owned them for years), but it would seem telecentrics play better with a prime NVD. YMMV of course.

 

 

I can use the 4x or even 5x Powermates to good effect, but with a 910mm scope the 2.5x seems to hit a sweet spot on star clusters.

 

I still have a Siebert 1.5x Barlow. It's a good performer, and there is no Powermate in that range.

[Highburymark]

Really excellent and interesting write-up Gavin. Maybe we might even get to go stargazing together again in 2021......

[hoof]

Afocal can do one thing you can’t get with prime: zooming. I had a custom PreciseParts connecter that lets me attach my Mod3c with envis lens directly to my Baader Mk 4 zoom. That gives me 8-24mm focal length with just a twist, the narrow view at 24mm matches the mod3c’s field of view (the usual complaint about that zoom!) And the whole stack is short (for afocal). I use this setup for globs (it’s why I originally bought the zoom), and it works great.

The only downside vs prime would likely be stars near the edges not being as good, and the fact there are 16 elements or so more between the telescope and the intensifier, likely dimming the view a bit.

The irony is I got the mod3c vs a PVS-14 to allow the possibility of prime, but in two and a half years, never even tried it. For most situations, I use the 67p afocally due to the image brightness. If I’m going shorter, I use the zoom. If I want pinprick stars, I go regular eyepieces (non-nv), which go up to 120 degrees AFOV. I do use other lenses on my NV (including a 25mm f/0.85 lens) but that’s for terrestrial use mostly (and 1x h-alpha viewing).

[GOLGO13]

OK everyone. Why can't people using afocal just use some higher magnification eyepieces to achieve the same capability as prime? In this thread there is discussion about the 67mm Plossl setup and 41mm pan. But why not just use a 27 pan and a 13mm delite to zoom in?

 

One thing that seems strange to me with NV is the apparent focus on nebulas only. Nebulas do benefit from faster setups generally. So using afocal with the 67mm or 41mm is useful (.4 and .7 reduction). But that's quite limiting in capability for things other than nebulas. I agree it's generally neat to see nebulas in NV, but for me, it's only a portion of the capability. You'd have to use a very long focal length telescope with the 41mm pan to gain some image scale (since it's causing .7 reduction). 

 

The only eyepiece I can use currently (and it's not proper) is the 24mm Panoptic. I have the attachment for it that allows me to use it afocally. But it's not listed on TeleVue's website probably because the eye relief? Oh...I do have a 4mm Delite now...but that seems like it would be pretty crazy. I could try it next time I'm out.

[Speedy1985]

OK everyone. Why can't people using afocal just use some higher magnification eyepieces to achieve the same capability as prime? In this thread there is discussion about the 67mm Plossl setup and 41mm pan. But why not just use a 27 pan and a 13mm delite to zoom in?

 

One thing that seems strange to me with NV is the apparent focus on nebulas only. Nebulas do benefit from faster setups generally. So using afocal with the 67mm or 41mm is useful (.4 and .7 reduction). But that's quite limiting in capability for things other than nebulas. I agree it's generally neat to see nebulas in NV, but for me, it's only a portion of the capability. You'd have to use a very long focal length telescope with the 41mm pan to gain some image scale (since it's causing .7 reduction). 

 

The only eyepiece I can use currently (and it's not proper) is the 24mm Panoptic. I have the attachment for it that allows me to use it afocally. But it's not listed on TeleVue's website probably because the eye relief? Oh...I do have a 4mm Delite now...but that seems like it would be pretty crazy. I could try it next time I'm out.

I used a 30mm GSO and 20mm Optimus the other night for galaxy hunting. I know Mike Lockwood has written in his blog about using other eyepieces than the 55/67 and 41. I plan to try some higher magnifications even beyond the 20 mm.

[scoale]

OK everyone. Why can't people using afocal just use some higher magnification eyepieces to achieve the same capability as prime? In this thread there is discussion about the 67mm Plossl setup and 41mm pan. But why not just use a 27 pan and a 13mm delite to zoom in?

 

One thing that seems strange to me with NV is the apparent focus on nebulas only. Nebulas do benefit from faster setups generally. So using afocal with the 67mm or 41mm is useful (.4 and .7 reduction). But that's quite limiting in capability for things other than nebulas. I agree it's generally neat to see nebulas in NV, but for me, it's only a portion of the capability. You'd have to use a very long focal length telescope with the 41mm pan to gain some image scale (since it's causing .7 reduction). 

 

The only eyepiece I can use currently (and it's not proper) is the 24mm Panoptic. I have the attachment for it that allows me to use it afocally. But it's not listed on TeleVue's website probably because the eye relief? Oh...I do have a 4mm Delite now...but that seems like it would be pretty crazy. I could try it next time I'm out.

I frequently use 41 and 27 pans in combination with my longer focal length scope (C11) when viewing globulars, smaller nebula/planetary, galaxies, etc.

[GOLGO13]

I frequently use 41 and 27 pans in combination with my longer focal length scope (C11) when viewing globulars, smaller nebula/planetary, galaxies, etc.

I was thinking the 27 pan would be the perfect eyepiece which I think would provide a native focal ratio. So for your C11, the 27 pan should be operating at F10. 

 

In my C8 with prime focus (which I'm guessing is the same as you using the 27 pan), globulars are very good. In my other scopes though, the globs are too small. Still interesting, but too small.

 

I don't think I'll get a C11 though. I find the C8 pretty large as it is. But I imagine the C11 would be very good.

[GOLGO13]

I guess since I started with afocal observing the length of the "stack" never really bothered me. On my refractors and c11 given it just comes out upwards from the diagonal really I don't notice it at all. In my dob, yes its more noticeable coming out the side particularly with the paracorr as well, but with my feathertouch focuser in practice I don't have a problem and am just used to it. My main refractors are petzvals so hence limited in travel, maybe I will try a 0.7x reducer and prime mode on say my AP130GTX but it will still be only f4.7 rather than an effective f1.8 which I get using both a 0.75x reducer and the 67mm eyepiece.

So the only scope I really really do not like using a very tall stack is the 6 inch F4 astrograph. That scope is designed to provide a lot of inward travel capability. I completely gave up on using the 67mm afocal setup with it as it's just absolutely ridiculous. 

 

In my 10 inch F4.7, I find it's much more reasonable, albeit still very tall awkward stack.

 

With my refractors, I don't really mind the tall stack because it can work fine by just setting up your chair to a proper height.

 

So as you say...it really depends on the scope being used. A scope with a poor focuser may have some issues.

[GOLGO13]

I should point out that since I have both prime and afocal capability, It probably makes more sense for me to use both capabilities in the same sitting.

 

For instance, I can setup the 67mm setup and it's tall stack for max reduction. I can then unscrew the Envis lens and place that tall stack to the side. I then can insert my 1.25 inch C mount nose into the NV unit and do prime focus. The only issue here is you would need to re-balance the scope as the difference in weight is gigantic. This could still be the case (balance issues) if you switched eyepieces in an afocal situation.

 

If I'm not needing to use the 67mm setup, staying prime the whole time is probably a better way to go. I can use the .7 reducer for reduction if needed. Use the NV unit at prime focus for native capability, then use a 2x barlow for zooming in. All of these prime only modes do not require any re-balancing of the telescope. Only downside is not getting the .4 reduction capability.

 

Prime focus is very clean.

 

I can setup a few telescopes and go back and forth pretty easy. Such as, setting up a 4 inch F5 refractor paired with an 8 inch SCT. Though for nebulas I really like my 10 inch F4.7 the most.

[25585]

 

To look at small globs, small planterary nebulas, small galaxies or any small DSO, prime use is the best solution IMO.
Most of these targets require at least 100x magnification.
Let’s take an exemple using a 16" F/4 dobsonian telescope. NVD is a 26mm eyepiece, so in prime it gives 1600/26 = 61.5x.
The big advantage is you can add barlow/powermate. So a 2x barlow will give 123x mag and a 4x Barlow will give 246x mag. And bigger magnification is still possible depending on the telescope.

 

Why not using afocal to get high magnifications ? Here are the 3 major reasons :
1/ Well, higher means darker. Additional eyepiece will add more glass so less light. This is not the case with powermate or high grade barlow.
2/ Afocal means very long stack and heavy setup. And afocal means using a front lens, any front lens will add distortion and other optical issues. So it will be better to use a coma corrector. Such a setup would be coma corrector + Barlow + eyepiece + adapter + NVD. While using prime would be Barlow + NVD. I measured and weight the difference
36cm vs 18cm and 1.96kgs vs 0.787kgs
See some pics attached showing those differences (please note i lost my coma corrector at home and use a 2x ES Barlow to simulate the coma corrector so size and weight may slightly change).
3/ In addition to 2/ if you want to attach any additional accessory like a filter wheel, a smartphone or a camera, it will be longer and heavier. This is something to consider.

 

My conclusion :
Unless if you are only interested in large diffuse nebulas and some very rare and very big DSO, you will feel quickly limited by the number of targets.
Prime with or withour barlow will allow you to go well beyond these limits to explore all NGC, Messier and other DSO catalogue.

 

 

 

 

 

 

 

 

 

Joko, are your OVNI telescope UTAs specially constructed for using NV?

[Deadlake]

Joko, are your OVNI telescope UTAs specially constructed for using NV?

Joko

 

What SQM do you observe under?

Is most of your observation with Dobs, say at F4.

 

If you observe at a reasonable dark site I don’t think you will pick up on the need for a fast scope. Moving from my back yard SQM 21 to meet up at another site closer to London at SQM 20.06 I think the scope needs to be a full stop faster to compensate for the light pollution.

 

For portability a fast APO is hard to beat but requires afocal.