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

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#101 Jeff Morgan

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Posted 22 November 2017 - 03:57 PM

As an update, I have been experimenting with narrowband O-III filters, and they do work.  Different structure is seen compared to H-alpha, but the line seems to be fainter in the large objects that I have viewed.  However, I expect that may be different for smaller planetaries.

 

Great. Now I have to buy the O-III back.

 

wink.gif



#102 Mike Lockwood

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Posted 07 February 2018 - 10:29 PM

I wanted to revive this thread - I have not had time or good weather to do any observing lately, so I am getting ready for the next chance.

 

I have 2" h-alpha filters currently, and I didn't want to wait for the special adapter required to use a 1.25" filter on the monocular, so I made an adapter to use a 2" filter on it instead.  I wrote a short addition to my nightvision blog/article to show what I did:

  http://www.loptics.c...ision.html#Feb7

 

I hope to get some observing in soon and see how it works.



#103 PEterW

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Posted 08 February 2018 - 11:37 AM

I use a 3D printer to make push fit adapters... yours looks more professional ;-)
How fast have you managed to go with fast Morris and the long focal length afocal method?? Is there a limit?

PeterW

#104 Mike Lockwood

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Posted 08 February 2018 - 12:10 PM

Peter, my fastest remains my 14.5" f/2.55, as discussed in the article, operating at f/2.9 with the Paracorr 2.  I don't have anything faster to try.

 

I'll try to do more O-III testing soon if I can get some clear skies.



#105 Gavster

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Posted 08 February 2018 - 01:05 PM

Peter, my fastest remains my 14.5" f/2.55, as discussed in the article, operating at f/2.9 with the Paracorr 2.  I don't have anything faster to try.

 

I'll try to do more O-III testing soon if I can get some clear skies.

So using the 55mm plossl with the f/2.9 dob gives an effective f ratio of 1.45? Now that’s fast!

I really like using NV afocally with my tnvc adapter. I’ve got refractors rather than dobs so can’t go as fast as this but I really like the ease at which the afocal approach can speed the system up significantly. I’ve also just received a photo visual telecompressor, so using this in conjunction with my 55 plossl afocally, I think I get the speed of my NV system down to f 2.2, which im lookIng forward to using.



#106 PEterW

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Posted 08 February 2018 - 03:18 PM

We’ll convert Gavster to a reflector user one of these days...
Would a 55mm in the f2.55 yield an f1.3... ?!

Peter

#107 alanjgreen

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Posted 11 February 2018 - 05:07 AM

 

Because you have access to some uncommonly fast mirrors, you do indeed have a case where the use of 2" filters may be almost essential, but for some guy with an f/5  reflector, f/7 refractor, or f/10 SCT, do you think it will be essential to use 2" filters in front of the eyepiece, and if not, what is an effective focal ratio (or native focal ratio) where the benefits of moving to the front of the eyepiece become worthwhile? You may be one of the only people that can answer that for the rest of us, because few of us have mirrors faster than f/4.

To answer your question and give some suggestions, I added another installment to my NV page:
  http://www.loptics.c...ision.html#Nov5
 

There is no doubt that detuning is an issue as speeds increase, but the question is at what focal ratio is it likely to damage the view enough to cause great concern.  
As I mentioned in my post to Mike, many of us may never encounter the speeds he can achieve afocally because he has the great fortune of having some super-fast mirrors.  Most of us either can' afford or don't want a 20" f/3.3 telescope.   For those that can though, I have zero doubt that Mike is correct and that the filter will give the best result at the front of the eyepiece.  For a guy using a 10" SCT, I am not sure that this is the case, but hoping Mike or Glenn can provide insight.

I find that a lot of people (who don't mind transporting something larger) do want a 20" f/3.3 telescope, especially after they use my 20" f/3.0 or something similar.  Yes, they are expensive, however, properly executed, you have 20" of aperture that you can observe with seated and which can provide both very wide-field views or great planetary views at high power.  They also work superbly with NV.

 

Doubting myself for some reason: I assume a 40mm TV Plossl -- which is also compatible with Dioptrix -- would be an adequate substitute for the 41mm Panoptic for afocal use with an NVD? A lot cheaper and lighter ....

Good suggestion - I have not used the 40mm Plossl, but it will work, and requires only a 1.25" filter.  It may be a nice option for some.  I am not sure if there would be any actual vignetting noticed due to the smaller field/field stop.  I had a lot of fun with the 17mm Delos and a 1.25" filter at Okie-Tex, but I would have had even more fun with something with a wider field.  wink.gif

 

I'm sticking with 2" eyepieces and filters personally, since I already have the eyepieces (41mm Panoptic), because I intend to use a 2" filter slide someday, and because they offer the wides fields.  The 55mm is a bit addictive, and I'll be buying one.

 

I'll also point out that TeleVue posted a new NV page to show how afocal works and explain it a bit more:

  http://www.televue.c...id=36&Tab=_work

 

I'll have some more updates when the moon is past full a bit.  Should be very interesting.

 

Hi Mike!

I am looking into NV with a 20" f3.57 dob and having read your comments on 1.25" and 2" eyepieces it appears that you are saying that I may get a benefit sticking with 2" eps with the big dob (using the tnvc afocal approach), is that correct? Could you restate the benefit in simple terms for my simple brain smile.gif

 

i would have to buy a 55mm plossl which is 2" but for example what would be the advantage of a 2" 27mm panoptic over the 1.25" 24mm panoptic ? With the NV only using the centre 40 degree of the eyepiece, does it really matter.

 

i live in a dark place so was thinking that I may get away with a 2" 12nm Ha filter attached to my paracorr2. Is your point that I need 2" filters further down the lightpath rather than need 2" eyepieces?

 

Thanks,

Alan


Edited by alanjgreen, 11 February 2018 - 05:23 AM.


#108 Busguy

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Posted 13 February 2018 - 12:19 AM

How about this solution for filters?  I've been screwing in a couple filters - sliding, then refocusing as required.  Seems to work well and one less thing to mess with.  2" only.

Attached Thumbnails

  • Filter slide.JPG

Edited by Busguy, 13 February 2018 - 12:27 AM.

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#109 Mike Lockwood

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Posted 10 October 2019 - 04:25 PM

Time to revive this thread.....

 

I have updated my nightvision page with some filter comparisons that I did at Okie-Tex 2019:

  http://www.loptics.c...lterComparisons

 

I know there's a thread elsewhere discussing various h-alpha filters.  The entry above compares two of them, as well as a more generic UHC/LP filter.

 

There are more NV photos (of deep sky objects, people and some gravity waves!) in the Okie-Tex 2019 article which is linked to in the update.


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#110 Mike Lockwood

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Posted 18 October 2019 - 11:13 AM

I have posted the filter transmission measurements that I have done:

  http://www.loptics.c...ra/spectra.html

 

I may reorganize these in the future and put similar types near each other, but at least they're finally posted.

 

I hope to add some mirror coating reflectivity measurements in the future, these may be very important for NV.  Again, if you want something measured, let me know, I will probably do it for free as long as I can post it.  (I will not participate in product disputes, though.)

 

(Edit:  I will note here that the two filters compared were both measured and the results are on the page above.)


Edited by Mike Lockwood, 18 October 2019 - 12:58 PM.

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#111 GOLGO13

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Posted 18 October 2019 - 11:20 AM

I have posted the filter transmission measurements that I have done:

  http://www.loptics.c...ra/spectra.html

 

I may reorganize these in the future and put similar types near each other, but at least they're finally posted.

 

I hope to add some mirror coating reflectivity measurements in the future, these may be very important for NV.  Again, if you want something measured, let me know, I will probably do it for free as long as I can post it.  (I will not participate in product disputes, though.)

Good stuff there Mike. Interesting to see the differences in the HA filters!



#112 spereira

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Posted 18 October 2019 - 12:53 PM

... I have updated my nightvision page with some filter comparisons that I did at Okie-Tex 2019:

  http://www.loptics.c...lterComparisons ...

Mike, thanks a million for posting these comparisons.  

Finally, I can see that there may be little visual difference between a 6 nm and a 7 nm H-alpha filter.  That's what I was looking for.

 

smp



#113 Clutch5150

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Posted 13 November 2019 - 03:33 PM

Good stuff there Mike. Interesting to see the differences in the HA filters!

Agreed, great comparisons.



#114 GOLGO13

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Posted 14 November 2019 - 03:51 PM

Anyone else notice aberrations when doing Afocal on the edges. I personally believe what I am seeing is curvature of the field from my Envis lens. The middle of things are quite good, but the edges are pretty messy. I don't get these when doing prime focus and using a reducer. 

 

I'm using the TV 40mm plossl for this as I only have 1.25 filters at this point.

 

I'm considering the 55 plossl sometime, but with the issues I question if it's worth it. The other option (and this does work) is using the .7 reducer with afocal and the TV40mm. 

 

I don't like the super tall stacks with afocal, but I can deal with that if the image was cleaner on the edges. 

 

What I can't tell is if the image in the center is better with afocal or prime. Still have to test more I guess.



#115 Jeff Morgan

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Posted 14 November 2019 - 06:26 PM

I'm considering the 55 plossl sometime, but with the issues I question if it's worth it. The other option (and this does work) is using the .7 reducer with afocal and the TV40mm. 

 

The 55 Plossl also suffers from this.

 

You learn to live with it. The resulting brighter images help quite a bit with that wink.gif

 

GeezerGazer has been experimenting with a Russell Optics 50mm Plossl that has 5 elements. The thread is still current, his early results appear encouraging.



#116 chemisted

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Posted 14 November 2019 - 07:25 PM

Anyone else notice aberrations when doing Afocal on the edges. I personally believe what I am seeing is curvature of the field from my Envis lens. The middle of things are quite good, but the edges are pretty messy. I don't get these when doing prime focus and using a reducer. 

 

I'm using the TV 40mm plossl for this as I only have 1.25 filters at this point.

 

I'm considering the 55 plossl sometime, but with the issues I question if it's worth it. The other option (and this does work) is using the .7 reducer with afocal and the TV40mm. 

 

I don't like the super tall stacks with afocal, but I can deal with that if the image was cleaner on the edges. 

 

What I can't tell is if the image in the center is better with afocal or prime. Still have to test more I guess.

After getting the best focus position with your focuser have you changed the diopter adjustment of the intensifier to try to improve star shapes?  Many times I have found this to be useful when doing afocal observations (but never when I do prime focus).  I use quite a variety of setups and it is not unusual that the optimum diopter position changes from one arrangement to another.



#117 Mike Lockwood

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Posted 14 November 2019 - 09:30 PM

I put some masking tape on the side of the lens and the side of my NV unit, I carefully focused the 1X lens (TNV-14, PVS-14 type) while looking through it, and then I made a mark so that I knew where exact infinity focus was.

 

When doing afocal observing and taking photos, I made sure the lens was focused to infinity.  It has a tendency to be inadventently defocused, resulting in poorer images at the edge, so you have to keep an eye on it.

 

I don't touch the diopter adjustment.  That is an adjustment for eyes, and people can simply wear glasses when viewing.

 

I use the 55mm for widest-field observing, and the 41mm for when I want better field quality.  The ENVIS/PVS-14 lens does a great job for what it is and how fast it is.  Long focal length eyepieces are not going to be perfect in fast telescopes, but I am pretty happy with the 41mm Panoptic for taking photos through.



#118 Jeff Morgan

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Posted 14 November 2019 - 10:34 PM

I put some masking tape on the side of the lens and the side of my NV unit, I carefully focused the 1X lens (TNV-14, PVS-14 type) while looking through it, and then I made a mark so that I knew where exact infinity focus was.

 

When doing afocal observing and taking photos, I made sure the lens was focused to infinity.  It has a tendency to be inadventently defocused, resulting in poorer images at the edge, so you have to keep an eye on it.

 

That tape is a pretty good idea.

 

When I set up for afocal, I start with just a little break to do 1x viewing, so I know I am focused.

 

However, given the threaded connection it is entirely possible that focus could drift when I put it together. Tape would give me an indication of drift.



#119 starzonesteve

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Posted 15 November 2019 - 12:13 AM

I finally got around to using my PVS-7 with the 20" scope. I attached it to my 55mm Televue and placed that in an Orion filter slide and nested that in my paracorr2. The Orion filter slide has 6 slots and comes with inserts so that you can use 1.25" filters as well. It made comparing the filters pretty quick and painless, although there was some vignetting with the 1.25" 7mm Ha filter. I liked using the 12mm 2" Ha filter best. I was blown away at what I could see. Here was the scene in the vicinity of Alnitak:

 

thumbnail_IMG_2271.jpg

 

The photo is just me holding up my iPhone to the eyepiece. I took what the phone gave me and didn't modify or process the image at all. Incredible! Nice as the photo looks the live view was even better, as it was a two eyed image. Although I enjoyed binoviewing with my PSV-7, I can see where one might be attracted to leaving behind the green cast and having the ability to adjust the gain to individual circumstance.

 

I may have to save up my pennies for a future call to Televue...

 

 

 

 



#120 Clutch5150

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Posted 19 November 2019 - 07:55 PM

starzonesteve nice work!


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#121 Mike Lockwood

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Posted 19 November 2019 - 10:45 PM

However, given the threaded connection it is entirely possible that focus could drift when I put it together. Tape would give me an indication of drift.

Jeff, it drifted more than I expected, and I would never have known if I didn't have some reference marks.

 

Steve, welcome to the intensified dark side.  :)


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#122 Jeff Morgan

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Posted 20 November 2019 - 12:09 AM

Jeff, it drifted more than I expected, and I would never have known if I didn't have some reference marks.

 

Steve, welcome to the intensified dark side.  smile.gif

 

Your tape solution gave me another idea: would painters tape have any holding power?

 

I just picked up a Nova Grade cell phone adapter, which I like quite a bit. But it is on the heavy side, so much so that if the long axis of the phone is not aligned with gravity, it can rotate the diopter ring.

 

Still getting some decent shots with it. Here is the Cocoon Nebula, 16" f/7, Mod 3C, 55 Plossl afocal combo, 12nm h-alpha filter:

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  • IMG_3178.jpg



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