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NV afocal observing with TeleVue/TNVC adapter and a 20" f/3.0

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#26 Eddgie

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Posted 19 October 2017 - 05:28 PM

Panning also reduces the effect of nonunifr illumination in the view, vignetting insee with some of my setups. Your brain can stitch together the best bits.

Peter

Yes, exactly.  Along the Milky Way especially in the south, it is hard to point at anyplace in the sky where there is no nebula so often what one sees is simply a field that is covered with nebula and often this has very low frequency detail.  If half the field is slightly brighter than the other half of the field, this is not actually easy to see.

 

If you pan the scope across it though, it helps kind of see the graduation as it crosses the field of view.

 

The summer Milky Way from horizon to zenith is the Disneyworld of Night Vision astronomy.  Here you see what no one using conventional eyepieces can come close to seeing.   The amount of detail is nothing short of dazzling.   



#27 Eddgie

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Posted 19 October 2017 - 05:31 PM

Here are my nightvision updates from the last couple of nights:

  http://www.loptics.c...sion.html#Oct16

  http://www.loptics.c...sion.html#Oct18

 

The first update has images taken with the 14.5" f/2.55 and 20" f/3.0 to compare the two and to show approximately what the view looked like to my eye doing afocal observing using a 2" H-alpha filter that I borrowed from a local imager.

 

The second update is a comparison of eyepieces used for afocal observing.

 

Based on inquiries I have received, I have also updated the first installment to make it a bit clearer what is going on regarding exit pupils, etc.

Wonderful pictures if Veil and structure in North American/Pelican!  Even more amazing that this was basically just holding the camera up to the eyepiece.

 

And we all know that unlike most imaging, with NV, the view is almost always better when you actually look though the eyepiece.. These are excellent captures though.   Astonishing what we are able to see in real time.  



#28 Mike Lockwood

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Posted 19 October 2017 - 09:58 PM

Thanks, I was impressed with what I could record with only a 1/4-second exposure.  Of course they're not astrophotos, they are my best attempt to show what can be seen with the eye looking into the nightvision unit.

 

Yes, it looks better better to the eye.

 

And it was a very poor night for transparency, being harvest season around here, it is dusty and there was all kinds of crap in the air scattering light and light pollution.  Can't wait until I get better conditions.  We need rain here, and that will clean out the air and alleviate the dust problem.



#29 GeezerGazer

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Posted 20 October 2017 - 12:52 AM

Very nice results for comparison, Mike; again, thanks for taking the time to test and report. 

 

Regarding H-a filters, there is a big variance in preference among NV users as to usefulness/preference between the 12, 10, 7, 6, & 5nm filters.  It seems to vary not just by the amount and type of light pollution through which the image is drawn, but also by the type of scope used and the emission lines of the H-a target itself.  

 

I have used (briefly) a 12, 7, and 5nm filter and comparing them through my f:7 refractor from my dark site, I prefer the 12nm because field stars remain mostly visible.  But in town, the 12nm is simply insufficient to see much through the light pollution, so I use the 7 or 5nm.  But none of these filters work very well through my tiny TV-60 when used under severe light pollution... at the dark site, the view isn't bad at all with only 60mm of aperture.  It provides a nice wide field but small scale.  Much could be written about H-a filter use when joined to a NVD.  

 

I also discovered that transparency is critical when using H-a/NVD.  Where I live in the central valley of CA, I have been exposed to both heavy covers of smoke from the fires in N. Calif. and from dust from almond orchard harvest/sweeping.  NVDs do not discriminate about which light they amplify... so light pollution bouncing off of dust, smoke or moisture helps to extinguish the light from targets I seek.  NVDs do well in allowing me to see more of our night sky, but a steady, clear, dark sky is still our best friend.  

 

Mike, "shams42" in Tennessee has two of my filters.  We are testing 3 identical filters in his 16" f:4.5 Lockwood Optics dob, and in my 5.5" f:7 refractor.  We will contribute our findings if anything of significance is found.  I look forward to hearing results of your filter tests.  



#30 39.1N84.5W

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Posted 22 October 2017 - 03:38 PM

Great thread, Mike.
I'm not quite understanding how the little adapter mates to the eyepiece or the monocular. Could you share photos of this detail?

#31 Bob S.

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Posted 22 October 2017 - 04:22 PM

" but the real star here is the 7nm filter. "

 

Eddgie, With all of your experience using NVD's, I am very surprised that you would conclude that the 7nm Ha filter that Mike is using is the "real star"? Over that past 13 years, I have tried a plethora of different pass band filters and have always found filters in the 6-7nm range to attenuate too much light and to increase scintillation in NVD's. My optimal experiences overall with NVD and Ha filters has been with 12nm Ha filters. I am wondering what has got you thinking that the narrower band is stealing the show? Bob



#32 Eddgie

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Posted 22 October 2017 - 04:38 PM

Because of the pictures shown,  it showed what I thought was the best view of nebulosity.

 

I use the 7nm and I see more with it than with the 12nm.  I am saving my pennies to buy a second 7nm.  Even in dark skies, I think the 7nm did better than the 12nm.   

A week ago, I was out in dark skies with my 6" f/2.8, and I detected the most faint nebula with the 7nm.  The difference was not as much as I get in the city, but it was in most cases given the best result.   M29 actually sits on a nebula and with the 12nm I could barely make it out, but it was a bit easier to see with the 7nm. 

 

But why I said it was in the pictures he posted, the 7nm easily showed the most nebula and in my ongoing experience, the 7nm is doing better.

 

We are both working at kind of fast focal ratios.  If you are using slower ratios, then that could account for the difference.  When I use the 7nm in the 12" f/4.9 dob, the view is noisier, but even there, I think the 7nm give a bit better view.


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#33 wcw

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Posted 22 October 2017 - 06:32 PM


As it turns out, the the 24 Pan is too small and will not fit the TNVC/TeleVue adapter, unfortunately, so I can't test with that.

 

 

Televue sells an adapter that allows the 24 Pan to be compatible. The part number is the DEA-0001 adapter. It is placed on the 24 Pan first, and then the TNVC/Televue adapter fits onto it. (The original purpose of the DEA-0001 is to make the 24 Pan compatible with DIOPTRX: http://www.televue.c...?id=54&Tab=_acc ).

 

I want to thank everyone contributing to this thread, it is very illuminating!

-Bill


Edited by wcw, 22 October 2017 - 06:33 PM.


#34 Mike Lockwood

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Posted 22 October 2017 - 09:14 PM

Great thread, Mike.
I'm not quite understanding how the little adapter mates to the eyepiece or the monocular. Could you share photos of this detail?

Thanks, it's been a bit of work, but I think it's worth it.

 

See the first photo in my article - the adapter has threads on one end that thread into the nightvision unit objective lens.  The other end of the adapter has a piece that protrudes as it is tightened down, and that piece locks onto the groove in the eyepiece where the rubber eye guard normally mounts.  It is very secure.

 

Eddgie, With all of your experience using NVD's, I am very surprised that you would conclude that the 7nm Ha filter that Mike is using is the "real star"? Over that past 13 years, I have tried a plethora of different pass band filters and have always found filters in the 6-7nm range to attenuate too much light and to increase scintillation in NVD's. My optimal experiences overall with NVD and Ha filters has been with 12nm Ha filters. I am wondering what has got you thinking that the narrower band is stealing the show? Bob

Narrowband works now.  And it works well.  The newer NV units likely have higher gain and lower noise.  In imaging, the narrower the bandwidth, the better the SNR if the sky is not perfect.

 

To be fair, I am borrowing a friend's 7nm, 2" filter, and I don't have a 12nm available to compare, but based on my experience with 1.25" eyepieces, I don't think the 12nm will work as well.  Numerous others also report that narrower bandwidth works better.

 

Televue sells an adapter that allows the 24 Pan to be compatible. The part number is the DEA-0001 adapter. It is placed on the 24 Pan first, and then the TNVC/Televue adapter fits onto it. (The original purpose of the DEA-0001 is to make the 24 Pan compatible with DIOPTRX: http://www.televue.c...?id=54&Tab=_acc ).

Thanks for pointing that out.  However, I just checked, and it doesn't have enough eye relief (15mm) to work at its best in this application.  20mm or very close to that is best for afocal imaging.  I will also test a 22mm Nagler in the future when I have time.



#35 Tyson M

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Posted 22 October 2017 - 09:43 PM

Here are my nightvision updates from the last couple of nights:

  http://www.loptics.c...sion.html#Oct16

  http://www.loptics.c...sion.html#Oct18

 

The first update has images taken with the 14.5" f/2.55 and 20" f/3.0 to compare the two and to show approximately what the view looked like to my eye doing afocal observing using a 2" H-alpha filter that I borrowed from a local imager.

 

The second update is a comparison of eyepieces used for afocal observing.

 

Based on inquiries I have received, I have also updated the first installment to make it a bit clearer what is going on regarding exit pupils, etc.

This is unreal. Look at the details, I'm almost speechless. . All shot afocally ......no stacking. Incredible. 



#36 Eddgie

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Posted 22 October 2017 - 10:53 PM

 

This is unreal. Look at the details, I'm almost speechless. . All shot afocally ......no stacking. Incredible. 

 

And did you catch the usual disclaimer earlier?  As good as the pictures are, the view in real time in the eyepiece is almost always better.

 

Modern image intensifiers give quite spectacular results and it is hard to capture and display the dynamic range on a computer monitor. 

Great pictures, but the actual view is even better. 


Edited by Eddgie, 22 October 2017 - 10:55 PM.

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#37 Jeff Morgan

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Posted 23 October 2017 - 12:31 AM

 


As it turns out, the the 24 Pan is too small and will not fit the TNVC/TeleVue adapter, unfortunately, so I can't test with that.

 

 

Televue sells an adapter that allows the 24 Pan to be compatible. The part number is the DEA-0001 adapter. It is placed on the 24 Pan first, and then the TNVC/Televue adapter fits onto it. (The original purpose of the DEA-0001 is to make the 24 Pan compatible with DIOPTRX: http://www.televue.c...?id=54&Tab=_acc ).

 

I want to thank everyone contributing to this thread, it is very illuminating!

-Bill

 

 

If one has a C-mount device (and no SIPS?) it seems a much better path to fit it with a Scope Stuff nose piece and just use a lightweight lower cost barlow rather than an eyepiece to get more image scale.

 

Perhaps Mike's findings will bring to light new considerations for non-SIPS users I had not considered before.

 

As it is now, I probably will get the TNVC adapter just so that I can have the option for focal reduction. While I am getting good results with a traditional 0.7x focal reducer (only about 10mm additional back focus required), the 0.5x focal reducer is a mixed bag. More testing needed. 

 

And being pragmatic, who knows if any more production runs of the TNVC adapter will be made? Compared to traditional NV markets, astronomy is a pimple on the hind end of the elephant. Even if every astronomer got an image intensifier (which of course won't happen), just not much of a market. Seems the smart move is to grab the adapter while it's available for possible future need. It's only about $60. Duplicating my NV filters in the 2" format will be a little more painful though. Oh, and buying a couple of long Tele Vue Plossls.


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#38 Eddgie

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Posted 23 October 2017 - 08:25 AM

The thing is that you can use almost any eyepiece for afocal and you don't even need an adapter.   Peter and I viewed for a couple of hours and is all we did was hold the Mod 3 up to the eyepiece.    Since the Mod 3 is 1x, it is really no harder than holding your eye over the eyepiece.

 

In fact, the 41mm Pan has an eye guard that almost perfectly cradles the objective lens so that you did not have to worry about it moving around. 

 

There are also other ways to connect afocally.  You can buy afocal camera holders and these would work with most eyepieces. This is just an example but there are many more. http://agenaastro.co...ra-adapter.html

 

Also, for higher power, you can easily convert the Baader Hyperion Zoom to afocal use.  

 

And last, if you go with one of these routes, you don't need to use a 2" filter.  The Televue adatper I think uses the native filter on the PVS-14 objective so this means there is no way to mount the filter between the device and the eyepiece. Because of this, you have to place your filter ahead of the eyepiece and if you are using a 2" eyepiece, this means a 2" filter, which can be many hundreds of dollars for a 7nm H-a filter.

 

Even if Televue had allowed to filter to be placed between the device and the eyepiece, it would be hard to change, and this is an advantage of using the 2" filter.  The Eyepiece/Device can be pulled out and the new filter installed.   If Televue had put the filter between them, the device would have to be removed.

 

Advantages of the afocal brackets though to me makes them pretty compelling:

  1. Mod 3 and PVS-14 both have 1/4 - 20 threaded receivers, and this is still the standard camera thread, so NVD  will mount to any afocal bracket that is designed to hold a camera (sorry PVS-7 owners). 
  2. Work with 1.25" filters, so huge savings on the cost of filters
  3. Easier to change filter because with something like the Baader, the device just swings out of the way. 
  4. Works with eyepieces other than Televue.

See, afocal is nothing new.  Imagers have used Afocal for decades.   There is already a lot on the market.  Also, I think that the TV adapter is unique to the native filter threads on the PVS-14 (which is the same as the PVS-7 and ENVIS) and to the Televue housing, while these other solutions work with other brands of eyepieces.  Most people using NV have "Converted" their objectives to 28.5mm using the RAF adatper.   The 1.2" x 32 thread on the PVS-14, PVS-7, and ENVIS is an odd size thread that has only one use at all for us, which would be to thread on the 3x afocal lens, but of course if you do that, you can't use filters with the 3x afocal.  


Edited by Eddgie, 23 October 2017 - 08:57 AM.


#39 Eddgie

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Posted 23 October 2017 - 08:40 AM

And for people using slower scopes where edge correction is not an issue, an afocal eyepeice holder might be a good option.

 

Stick your 40mm Plossl into this, put a T2 to filter thread adapter on the top, and screw your Mico, PVS-7, or PVS-14 on.  (though this requires that you have the RAF 1.25" filter adatper).  

 

http://agenaastro.co...apter-t-03.html

 

https://www.amazon.c...jection adapter

 

Bottom line is that there are lots of solutions out there.  Televue did not invent afocal projection, and there are a ton of other options already so if TV runs out of adatpers, don't worry.  


Edited by Eddgie, 23 October 2017 - 08:41 AM.


#40 Tyson M

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Posted 23 October 2017 - 08:51 AM

 

 


As it turns out, the the 24 Pan is too small and will not fit the TNVC/TeleVue adapter, unfortunately, so I can't test with that.

 

 

Televue sells an adapter that allows the 24 Pan to be compatible. The part number is the DEA-0001 adapter. It is placed on the 24 Pan first, and then the TNVC/Televue adapter fits onto it. (The original purpose of the DEA-0001 is to make the 24 Pan compatible with DIOPTRX: http://www.televue.c...?id=54&Tab=_acc ).

 

I want to thank everyone contributing to this thread, it is very illuminating!

-Bill

 

 

If one has a C-mount device (and no SIPS?) it seems a much better path to fit it with a Scope Stuff nose piece and just use a lightweight lower cost barlow rather than an eyepiece to get more image scale.

 

Perhaps Mike's findings will bring to light new considerations for non-SIPS users I had not considered before.

 

As it is now, I probably will get the TNVC adapter just so that I can have the option for focal reduction. While I am getting good results with a traditional 0.7x focal reducer (only about 10mm additional back focus required), the 0.5x focal reducer is a mixed bag. More testing needed. 

 

And being pragmatic, who knows if any more production runs of the TNVC adapter will be made? Compared to traditional NV markets, astronomy is a pimple on the hind end of the elephant. Even if every astronomer got an image intensifier (which of course won't happen), just not much of a market. Seems the smart move is to grab the adapter while it's available for possible future need. It's only about $60. Duplicating my NV filters in the 2" format will be a little more painful though. Oh, and buying a couple of long Tele Vue Plossls.

 

I bought a 2" to c mount adapter already to prepare from scopestuff. 

 

No filters yet except the Baader 685nm which may or not be used I simply already owned it. Probably too dim with a small scope. 

 

 My long term plan is to use a TS72 apo f5.5 doublet (with threaded removable cell) to be grab and go and NV.

 

I have 0.5x reducers in 1.25 and 2" and can get a reducer with a flattner from TS as well. 


Edited by Tyson M, 23 October 2017 - 09:22 AM.


#41 Eddgie

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Posted 23 October 2017 - 09:13 AM

 

 

No filters yet except the Baader 685nm which may or not be used I simply already owned it. Probably too dim with a small scope. 

 

 My long term plan is to use a TS72 apo f5.5 doublet (with thread removable cell) to be grab and go and NV.

 

I have 0.5x reducers in 1.25 and 2" and can get a reducer with a flat erfurt from TS as well. 

 

The 685nm is almost essential for use with most non-Apo type refractors because otherwise stars will bloat from chromatic aberration.  For your scope it would not be necessary I think, but if you have to use it, the dimming is not really nearly as much as you might think because NV is so senstive to red and near infra-red that there is still plenty of light. 

 

In heavy light pollution with very fast scopes, the 685nm can be very useful.

 

More and more, for people using faster scopes and higher end devices, I would say skip the 12nm and put your money in the 7nm  H-a filter.  Just my opinion of course, but even under very dark skies, I think the 7nm gives the most nebula of the two.  If your scopes are slower or your tubes are not upper SNR ranges, a 12nm might be a better choice. 


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#42 Mike Lockwood

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Posted 23 October 2017 - 10:09 AM

And last, if you go with one of these routes, you don't need to use a 2" filter.  The Televue adatper I think uses the native filter on the PVS-14 objective so this means there is no way to mount the filter between the device and the eyepiece. Because of this, you have to place your filter ahead of the eyepiece and if you are using a 2" eyepiece, this means a 2" filter, which can be many hundreds of dollars for a 7nm H-a filter.

 

Even if Televue had allowed to filter to be placed between the device and the eyepiece, it would be hard to change, and this is an advantage of using the 2" filter.  The Eyepiece/Device can be pulled out and the new filter installed.   If Televue had put the filter between them, the device would have to be removed.

You wouldn't want to put a filter between the eyepiece and NV objective anyway - the light cone is at its steepest there.  Narrowband filters shift their response as the angle of light passing through them departs from normal incidence (straight through), so that makes it a poor place to put a filter.

 

It still works well with the filter on the front of the eyepiece at f/2.93 (f/2.55 plus 15% for Paracorr), so that's good enough for me.

 

I plan to use a filter slide at some point.  Yes, 2" filters are expensive, but what I'm seeing is worth it, as is the safety and convenience of having them in a slide where I don't have to handle them and risk dropping them or damaging them.



#43 Eddgie

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Posted 23 October 2017 - 10:21 AM

I have used the 7nm filter at f/1.2 in afocal and not had any serious issue.  If it has shifted the bandwidth, it was not enough to keep it from performing better than the 12nm.

 

The afocal lens I am referring to of course is the Mil-spec 3x afocal lens.  This lens is working at f/1.2 and the filter sits directly behind it and in front of the ENVIS. 

 

I think when you get into the 3nm fitler, it is far more of an issue, but at 7nm, I run at f/2.8 and at f/1.2 with an afocal lens, and it seem to work fine.

 

Also, the telescope eyepiece always works at the focal ratio of the telescope (assuming you are not using a Barlow or reducer of course), so if you put a 55mm lens in an f/3 telescope, the lens is still working at f/3 and the exiting beam is really a pencil and not a diverging light cone. 

 

For afocal then, I don't think the slower filters are as affected by fast focal ratios and having used the 7nm behind the  f/1.2 3x afocal lens, I can say that the filter works really well with that lens, and once again, the eyepiece in a telescope is always working at the focal ratio of the telescope, so it should not matter if the filter is ahead of or behind the eyepiece.   Perhaps I am wrong, but we could ask Glenn. 

 

I know that my 7nm works great (at least as compared to the view in the 12nm) even at f/1.2. 

 

Anyone that uses the 3x this way has been doing afocal, so again, afocal is not new.  I have been using it for 2.5 years now, but only with the 3x lens.   

 

It is the 3nm that I think has a more serious issue with focal ratio, but again, if the eyepiece is working at the same focal ratio as the telescope, then it should not matter here what the effective focal ratio is because the focal ratio of the eyepiece is itself no different than the focal ratio of the telescope it is used in.  Only the image scale changes, not the true focal ratio.  All eyepieces used in an f/3 telescope work at f/3.  Just the way it works out.  The "Effective" focal ratio is really just accounting for the change in energy distribution due to the smaller scale, but the focal ratio of the eyepiece does not change the light cone angle. 


Edited by Eddgie, 23 October 2017 - 10:47 AM.


#44 Mike Lockwood

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Posted 23 October 2017 - 11:00 AM

Also, the telescope eyepiece always works at the focal ratio of the telescope (assuming you are not using a Barlow or reducer of course), so if you put a 55mm lens in an f/3 telescope, the lens is still working at f/3 and the exiting beam is really a pencil and not a diverging light cone.

No.  The incoming angular width of an f/3.45 beam (out of the SIPS in my 20" f/3.0) is 16.5 degrees.  However, when you look into the 55mm eyepiece, you have a 50 degree field of view.  You have to turn your head or eye off axis to see things at the edge of the field, so you look into the eyepiece at an angle.  The NV unit effectively does the same thing - it "looks" into the eyepiece at an angle for off-axis objects, and it has a 40 degree angular field of view.
 
The NV unit functions the same way it would on a normal 1X scene with no other optics - light comes in at an angle for off-axis objects.  Thus, the light from the outer areas of the field would have to pass through a flat filter (placed in front of the NV objective) at a greater angle.  While on-axis objects are unaffected, off-axis objects are, and this mainly contributes to perceived vignetting.

 

For afocal then, I don't think the slower filters are as affected by fast focal ratios and having used the 7nm behind the  f/1.2 3x afocal lens, I can say that the filter works really well with that lens, and once again, the eyepiece in a telescope is always working at the focal ratio of the telescope, so it should not matter if the filter is ahead of or behind the eyepiece.

I find that I am quite sensitive to seeing vignetting - for me it is a bit annoying using a filter at 1X with the NV unit because H-alpha objects disappear at the edge of the field even with a 7nm filter.  I think this is proof of what I am saying - I think this is mainly due to off-axis light passing through the filter at a 20 degree angle at the edge of the 40 degree field, where the filter doesn't work properly.  Without the filter, the perceived vignetting is less to my eye.

 

I can only imagine how much more majestic 1X would be with a curved h-alpha filter over the objective and no vignetting!

 

Some filters are specifically manufactured to be more tolerant of fast incoming light cones, but I have only used one 2" filter at this time.  It will take time and work to figure out what filter is best, and which has the best tradeoff between performance and cost.  How the filter responds to fast light cones depends on how the coating is designed, and this is very complicated.



#45 Eddgie

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Posted 23 October 2017 - 04:10 PM

I kind of see what you are saying, but at the same time, the angle of the beam leaving the eyepiece does not change just because the eyepiece focal lenght gets longer and I guess we will disagree about that.

 

I think that the real issue with vignetting here is more to do with the design of he device itself.  If you have a caliper, you can verify my measurements. Here is an MS Paint (Crude, but it is what I have) "Ray Trace" of a C mount device using an ENVIS lens.  It is drawn to scale, though this may not be exact. Again, readers are invited to use their calipers to explore this.

The C mount spec is for a flange to focal plane distance of 12.5mm, but I have measured to the tube window and I usually get between 13.5 and 15mm.  The Photocatode for an image intensifer is about 1.8mm behind the front window, so this puts it between 15mm and 16mm or so from the lens mounting flange minimum. 

 

The rear lens of the ENVIS is about is about 10mm from the front of the  C mount opening so this puts it only about 5mm to 6mm  (at the edge of the field) from the photocatode and this is rendered here to approximate scale in my crude drawing. 

The green lines are about equivelent to f/1.2 light cone. 

 

ENVIS Geometry.jpg

Now what is important here is that the rear lens of the ENVIS itself is only 15mm, so it is actually smaller than the photocatode, which is nominally 18mm (though not quite all of this is used due to shading from the case itself).

 

Now again, please forgive this crude ray trace, but if you look, you can see that with the ENVIS, the entire field is not fully illuminated, and the edge of the tube is actually already being vignetted quite a bit.  This is easy to see even at 1x.  The field will fall off in illumination quite significantly when you use the 1x lens.  I would say edge of field illumination is down by 80% or more at the edge of the ENVIS field at 1x and this is not going to change when doing afocal. Anyone can step into a closet and use a dim LED something and drift it to the edge of the field and easily see that the ENIVS itself vignettes the field pretty seriously.   

 

I use 1.25 filters all the time at f/1.2 and I can find little evidence that the vignetting is enough to hardly notice.  On and off, I have tried this and it is just hard to see.

The reason I tried this was to try to understand why nebula were disappearing at the edge of the field, and my diagnostic lead me to the rear opening and spacing of the ENVIS and not the filter. The ENVIS vignetting is much more severe I would guess because I have put the filter on and off and was not able to see much difference in off axis illumination (not to say that it is not vignetting, but only saying that the ENVIS and fast light cone seem to me to be the primary sources of vignetting in the system.  I do not have this vignetting at prime focus.  

 

Please don't take me as overly argumentative here.  I want everyone to win, and to win, we have to all have a clear understanding of all the components and how they interact. 

 

 



#46 Mike Lockwood

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Posted 23 October 2017 - 05:38 PM

Eddgie, don't worry about it - this is a discussion, not an argument, and the worst I will do is disagree.

 

We do not have the details of the ENVIS lens design, and it is not possible to say what is going on without measuring the lenses and doing analysis.  The rear element may be sized so that it stops down the lens system on purpose in order to allow for more even illumination of the photocathode.  In other words, light that is focused on the center of the photocathode comes from only a portion of the front lens.  Likewise, light that is focused on the outer parts of the cathode comes from an off-center area of the front lens.

 

I just pulled the ENVIS off the NV unit that I have and looked at the image that it forms with a 24mm Panoptic - the two fit together almost perfectly, as it turns out!  I see a very even illumination, so I suspect that what I say above is true.  Also, inserting an object (my finger) in front of the lens produces no visible effect until it is well into the aperture of the front lens, which further supports that.  Still, I am speculating a bit.

 

Someone more skilled in lens design and analysis can do a much better job analyzing this than I can.

 

In any event, my point was that light traveling through a filter on the bottom of an eyepiece barrel passes through the filter at a shallower angle than a filter placed over the NV objective, and therefore there should be less non-ideal effect with the filter on the eyepiece.


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#47 GeezerGazer

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Posted 25 October 2017 - 12:22 AM

During a discussion with "jdbastro" about his 4x and 6x prime lenses for use with NVDs, I remember him stating that the Envis lens was designed to illuminate an 18mm sensor.  So perhaps the Mod 3 sensor IS fully illuminated... explaining why a view through the Mod 3C/Envis prime lens does not vignette.

 

The conversation started because I was curious if a small diameter H-a filter could be inserted between the rear lens of his 4x or 6x prime lenses and the sensor window in the Mod 3C... rather than filtering a large aperture objective.  He said that such an arrangement would cause significant band shift.

 

Is band shift what you are referring to as vignetting of the H-a image when using narrower pass band filters such as the 5nm or 7nm filters?  Band shift, as I understand it, allows the H-a image to be cleanly viewed on-axis, but the closer the object gets to the edge of field, the less effective the H-a filter becomes, until the H-a object disappears completely before actually leaving the FoV.  H-a filtration varies by band width.  The narrower the band width, the greater the possibility for band shift, especially when used with fast optics.  Personally, I do not see band shift when using a 12nm H-a filter at the front of the Envis.  But I do see it when I replace the 12nm with a 7nm, and even more with the 5nm.  It does resemble vignetting.

 

Mike, are you saying that there is less band shift if H-a filtration is removed from the front of the Envis and instead, placed on the barrel of the eyepiece in an afocal configuration?  


Edited by GeezerGazer, 25 October 2017 - 12:41 AM.


#48 Mike Lockwood

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Posted 25 October 2017 - 11:23 AM

To check the ENVIS design one needs to put a wide angle, uniform light source at the focus of the lens and then observe the diameter of the beam coming out the front of the lens.  This would show how much of the lens was being used to illuminate a particular part of the photocathode.  I do not have a source that is wide-angle and uniform enough to do this, my fastest interferometer diverger is f/1.7, and even that is not uniformly illuminated.

 

The best place to have a narrowband filter is where the angle of light passing through the filter is the closest to normal incidence, or going straight through the filter.  (In certain eyepieces, the ideal place might just be inside the eyepiece if the light is nearly parallel there, but this is not going to happen.)

 

This brings up the possibility of some interesting relay lens arrangements, which create parallel light, but to me that is overly complicated.

 

Putting a flat filter over the photocathode is the worst possible place because the angle is quite steep there with the very fast incoming light cone.

 

For larger lenses or telescopes, the best place would be over the aperture because the field of view is smaller and thus the departure from normal incidence of incoming light is pretty small.  Of course this is impractical for larger instruments, but it could work for some camera lenses, and ~2x-6x objectives.

 

For a NV unit with ENVIS objective, the lens system gives the NV unit a field of view about 40 degrees wide, meaning that the light from the edge of the field is coming through the filter at an angle of about 20 degrees from normal incidence.  This is a lot.  I do not expect any narrowband filter to work exactly as it was designed with those angular departures.

 

Yes, band shift is what I am talking about.  As the angle increases, the band shifts, and you effectively move away from the H-alpha wavelength, and this should cause the image to get significantly fainter.  So, yes, I am attritubuting the vignetting that I see to band shift of the well off-axis parts of the field of view.

 

Yes, there is less band shift if the filter is moved in front of an eyepiece compared to putting it over the ENVIS lens, and that is what I said above.  For an F/3 cone coming out of a telescope and into an eyepiece, the light covers a 16 degree cone, and that is a maximum departure from normal incidence of about 8 degrees, much less than 20 degrees for the light coming into the ENVIS.

 

I believe I have a way to confirm this phenomena the next time it's clear and I have time.

 

I will also have other updates in the near future.



#49 Eddgie

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Posted 25 October 2017 - 03:32 PM

Looking forward to seeing more updates on your site.  I find it to be a very promising way for people using slower scopes to get a big increase in speed. 

 

Then band shift is indeed an issue but I think it is more so with the narrowest filters.  I was working at f/2.8 with a 7nm last night and I had almost no light falloff at the edge of the field.. If it is there, it is too subtle to see, but if your arguments are correct, then at f/2, I would think the story would be different.   Most of us will never own a scope that could get to this speed though. 

 

There are always going to be compromises, but your pictures show that there is a real benefit to using these very long focal lenght eyepieces, and most reflectors will not have the necessary inward focuser travel to be able to reach focus with anything but a very mild reducer, so this is attractive to me, and I am sure to many others.

 

(Gary Russel does sell some very long focal length eyepieces.  I think he has a 65mm and something like an 80mm  I can't speak form experience about their quality, but for someone using an MCT or SCT, this could be a great way to get a very agressive effective focal ratio reduction). 


Edited by Eddgie, 25 October 2017 - 03:33 PM.


#50 BJS

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Posted 25 October 2017 - 04:06 PM

Surpluss Shed also has some very long focus eyepieces.  Up to 80mm or so.  I have never seen them in person so I can't say anything about the quality of the eyepieces.

​Brian




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