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

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#76 Mike Lockwood

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Posted 04 November 2017 - 01:53 PM

Just to emphasize, please read my post #54 above again - it is better to put the filter on the bottom of an eyepiece than on the ENVIS objective if you have a choice between the two positions.  This is because the light cone coming into the eyepiece is slower and the de-tuning effect is smaller.

 

For me, this means I'll just be using a 2" filter most of the time.  Though they are expensive, the results are very worth it, IMO.



#77 SeymoreStars

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Posted 04 November 2017 - 02:06 PM

Just to emphasize, please read my post #54 above again - it is better to put the filter on the bottom of an eyepiece than on the ENVIS objective if you have a choice between the two positions.  This is because the light cone coming into the eyepiece is slower and the de-tuning effect is smaller.

 

For me, this means I'll just be using a 2" filter most of the time.  Though they are expensive, the results are very worth it, IMO.

Thank you Mike you answered a question I had without have to ask. Now how about this , if a 1.25 Ha filter is mounted at the ENVIS lens and then 3X magnifier mounted. Will this achieve the "the best wide-field views" you mention in post# 54?

 

p.s. I don't own any light buckets just 4 & 5 inch refractors.

 

Here's a picture of my device with the magnifier.

20171006 125517

Edited by sink45ny, 04 November 2017 - 02:08 PM.


#78 Jeff Morgan

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Posted 04 November 2017 - 02:11 PM

... keep all lenses wide open for the brightest view. If the fratio is slower than the other parts is will limit the image brightness, but these lenses are generally the fastest component.

Peter

 

Back from my high school days when an imaging sensor called "film" was in use, I did a science project of photographing constellations with a SLR camera. It was a Canon FT-b with the f/1.2 50mm lens.

 

What I found was that the stars showed significant aberration when the the lens was wide open. I found the best balance of quality vs. exposure time was f/2.8.

 

For those using very fast short lenses, some experimentation may be worthwhile.



#79 Jeff Morgan

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Posted 04 November 2017 - 02:15 PM

Can the TNVC/Televue adapter be used with other lenses? In particular, is the thread for the ENVIS 1x lens a standardized one for which step-down/up thread adapters could be purchased?

 

Clutch5150 (the TNVC vendor) cautioned me that not all ENVIS lenses were made with the same thread:

 

https://www.cloudyni...-vue/?p=8173963

 

I ordered the afocal adapter, and it does fit my particular lens. 



#80 Jeff Morgan

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Posted 04 November 2017 - 02:33 PM

For me, this means I'll just be using a 2" filter most of the time.  Though they are expensive, the results are very worth it, IMO.

 

I'm grumbling about this at the moment. Sure, I am going to end up doing it ... already ordered the adapter.

 

And, I can use the 2" filters on my telephotos somewhat more conveniently than the 1-1/4".

 

But this afocal adventure is going to run well north of $600 when it's all done ... and be a beast in the focuser.

 

I'm thinking for those that don't need SIPS shortening truss tubes by 55mm and using a 0.5x focal reducer might be a better option.

 

But I will try it both ways ...



#81 Eddgie

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Posted 04 November 2017 - 03:27 PM

Just to emphasize, please read my post #54 above again - it is better to put the filter on the bottom of an eyepiece than on the ENVIS objective if you have a choice between the two positions.  This is because the light cone coming into the eyepiece is slower and the de-tuning effect is smaller.

 

For me, this means I'll just be using a 2" filter most of the time.  Though they are expensive, the results are very worth it, IMO.

Keep in mind that most people will not have the luxury of starting with sub f/4 mirrors, and for a lot of people using slower scopes like refractors and f/10 SCTs will not achieve effective focal ratio of anything close to what you are getting. 

 

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.


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#82 Eddgie

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Posted 04 November 2017 - 04:07 PM

 

Thank you Mike you answered a question I had without have to ask. Now how about this , if a 1.25 Ha filter is mounted at the ENVIS lens and then 3X magnifier mounted. Will this achieve the "the best wide-field views" you mention in post# 54?

 

p.s. I don't own any light buckets just 4 & 5 inch refractors.

 

Here's a picture of my device with the magnifier.

 

The H-a filters will show detuning at the edges when used with the 3x and the filter mounted on the ENVIS.

For example, if I use it on Barnard's Loop and Anglefish nebula, I can get both into the same field of view, but only the parts in the more central part of the field show bright, and as nebula runs out towards the edges of the field, you can see that they dim pretty substantially..  This means that at 3x, I have to pan across to see things at their brightest.. If I center the Anglefish nebula, then the southern part of Barnards loop will be very dim.  If I center the southern part of Barnard's Loop, then the Angelfish (which is very dim as compared to Banard's Loop) will be almost impossible to see.

 

Now for the Anglefish alone, because it fits more or less completely into the center of the field, the detuning does not matter, and I think the 3x gives the best view of this nebula.  Likewise, as you pan Barnard's Loop with the 3x, you see structure that is not revealed at 1x, but you can only see  this structure when in is in the center 50% of the field. 

 

 

For the best view then of this particular pairing, I find the binocular at 1x actually provides a better view.  Now I can see the entire region and the brightness is very even across the field. 

 

You absolutely get detuning with the 3x ahead of the filter, but remember that the 3x is very fast.  I think it is about f/1.4.  I have heard different figures though.

 

Now at f/2.8 with the same filters, detuning is hard to see.  Nebula at the center of the field do not dim substantially when drifted to the edge of the field. 

 

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. 


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#83 cnoct

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Posted 05 November 2017 - 02:41 AM

Clutch5150 (the TNVC vendor) cautioned me that not all ENVIS lenses were made with the same thread:


Despair not, only one lens type was ever supplied as standard with the ENVIS monocular , no variants or revisions with deviating filter threads were ever supplied as standard with the ENVIS monocular.

 

All ENVIS objective lenses feature the exact same 1.2x32 objective threads .

 

The ENVIS lens is simply an AVIMO PVS-14 objective that was re-threaded with a single start fine focus thread. These AVIMO lenses were all of the same design with no variations so there is no issue threads of fitment from one ENVIS lens to another.

 

ENVIS Objective Assembly.jpg

 


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#84 KJL

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Posted 05 November 2017 - 11:04 AM

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 ....


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#85 Mike Lockwood

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Posted 05 November 2017 - 02:38 PM

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.  ;)

 

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.



#86 Jeff Morgan

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Posted 05 November 2017 - 05:37 PM

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 ....

 

Unless you already own the 41 Pan.

 

But even if I did, I know I would want the Plossl to for the lesser load factor.

 

BTW - a 0.7x focal reducer would deliver roughly the same speed reduction as a 40mm eyepiece. Based on my measurements with two scopes, about 12mm additional backfocus is required (compared to the "naturally aspirated" Mod 3).


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#87 Jeff Morgan

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Posted 05 November 2017 - 05:50 PM

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 just scored a 55 Plossl on the used market, should be able to do some testing later in the week with both refractor and Newtonian. Not quite as fast as your Speed Demon, but hopefully useful data points for others.

 

A 2" 0.5x focal reducer shows pretty heavy darkening in the outer 10% of the FOV with the f/6 AT60 ED refractor. I'm getting the speed, but not the true field I had hoped for. Perhaps I may be bumping up against the limits of practical FR?

 

If the 55 Plossl works better I'll grumble a lot less about having to acquire 2" h-alpha filters! Reinforcing the focuser board for the load should be relatively easy.



#88 KJL

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Posted 05 November 2017 - 07:52 PM

A 2" 0.5x focal reducer shows pretty heavy darkening in the outer 10% of the FOV with the f/6 AT60 ED refractor. I'm getting the speed, but not the true field I had hoped for. Perhaps I may be bumping up against the limits of practical FR?

 

If the 55 Plossl works better I'll grumble a lot less about having to acquire 2" h-alpha filters! Reinforcing the focuser board for the load should be relatively easy.

This is at prime focus, not afocal, right? I've noticed this as well with my 0.5x and 0.7x focal reducers, but the views themselves are poor in other ways. For example, I end up with really poor field curvature: the stars start bloating halfway across the field. Furthermore, when I tried the focal reducers with my 9.25 SCT, the in-focus requirement caused full-on aperture reduction, with the end result that the reduced view was actually dimmer than without the reducer. Yuck.

 

This is why the afocal method is exciting: with a good eyepiece the field can be much flatter than my cheap 2" focal reducers while requiring 0mm of in-focus -- not to mention the ability to change the speed/FOV. I'll try some experiments with longer focal length plossls compared to my larger AFOV EPs (30 and 40XW, 30mm Leitz, etc) to check whether the field illumination is still even. NVDs are pretty light but using lightweight plossls would help reduce the combined weight when used afocally.


Edited by KJL, 05 November 2017 - 07:52 PM.


#89 Mike Lockwood

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Posted 05 November 2017 - 09:08 PM

I'm not considering focal reducers here because they eat up back focus, generally won't work with fast Newts (like the two I am discussing in this thread) and if they did, coma correction is not going to be easy (or possible) to do with them.

 

So, you'll have a major complication simply achieving focus, and the field is going to look really bad off-axis.  They are a non-starter for me, and I can push the limits of the NVD objective without them thanks to good quality eyepieces in the 40-55mm range.

 

KJ points out some benefits of afocal above, and the biggest one is that if an eyepiece works, then afocal will work.  No changes are necessary other than coupling the NVD to the eyepiece and enjoying the view.


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#90 GlennLeDrew

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Posted 05 November 2017 - 09:55 PM

Not only does the final f/ratio 'seen' behind the NVD lens have an impact on filter de-tuning when the filter lies between eyepiece and imaging lens. The field angle encompassed plays a role as well.

 

For example. Imagine using the NVD and its lens alone, with no other optic in front. A narrow band filter is installed. If the lens covers a wide field, and has an aperture stop, even if stopped well down the outer field will suffer de-tuning because the incoming light transits the filter at some beyond-optimal angle. The wider the TFoV, the worse the de-tuning. I found this to occur even with broader filters like the UHC on 50mm lenses back in my 35mm film days.

 

If the NVD lens by itself suffers de-tuning when a narrow band filter is installed on it, then the same will occur with eyepieces that fill the FoV of the afocally-employed NVD. Which is why the filter is better installed ahead of the eyepiece.

 

But if the NVD covers a suitably small field angle, or if the operating f/ratio is not by itself too fast, or if one is not concerned about de-tuning in the outer field, then a filter placement betwixt eyepiece and lens is OK.


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#91 GeezerGazer

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Posted 06 November 2017 - 12:24 AM

Limiting field stop size also limits AFoV.  Do I understand correctly, that a 1.25" eyepiece at 40 mm will effectively produce an image with a brightness equivalent to a 2" 40mm eyepiece (because their exit pupils are identical), in spite of its smaller field stop?  If the 27mm field stop does not limit image brightness, and since NVDs are limited to about 40 degrees FoV, is there ANY reason to use 2" eyepieces for afocal NV?  Especially in view of the cost difference between a 40 Plossl and a 41 Panoptic.


Edited by GeezerGazer, 06 November 2017 - 12:41 AM.


#92 GlennLeDrew

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Posted 06 November 2017 - 07:17 AM

Geezer,
Your surmises are correct; a 1.25" 40mm ocular is essentially as effective as a 2" 40mm ocular. If the eye relief permits to get the NVD lens's nodal point at the exit pupil, and the additional FoV of a big 'un contributes nothing, then a little 'un is not disadvantageous.


Edited by GlennLeDrew, 06 November 2017 - 07:19 AM.


#93 Eddgie

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Posted 06 November 2017 - 02:48 PM

Stupid me.. I see the point now.  The ENVIS itself is the source of debanding when used with afocal.  

Yes, this would dictate that the filter be placed in front of the eyepiece.

 

I just was being blind to this because I work at prime focus at f/2.8 so in this case, it is little  to do with the effective focal ratio of the telescope/eyepiece, but rather the ENVIS.

 

My apologies for my ignorance.  

 

I just did not see any detuning at f/2.8 and I had it stuck in my mind that the effective focal ratio would have to be very fast, but afocally, the speed of the system/eyepiece hardly matters because it the detuning would always be that of the ENVIS in that configuration.   I just don't know why I could not see that. 

 

Thanks once again for getting me on the right track.  

 

Since most of us have been working at prime focus with telescopes, we have not encountered this but those wishing to get best result afocally will have to move to front mounted filters.



#94 GeezerGazer

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Posted 08 November 2017 - 01:28 PM

OK Glenn, this seems to make sense.  Using H-a filtration in an afocal configuration is similar to using it in prime focus, in that, if a reducer is used for prime, the filter must precede the reducer in the light path.  In afocal, the filter must precede the telescope eyepiece (which in longer focal lengths acts as a reducer).  If the filter is placed after the light cone is altered by the pathway optics, the light cone can be changed enough to cause what has been alternately described in this thread as de-tuning, debanding, vignetting or band shifting.  Basically, the light cone angle becomes too steep to be processed "correctly" by the filter, causing band shifting in the FoV at some point between on-axis and outer field, which is the annoyance we see.  In this case "correctly" simply means that the combination of the optical system and the filter's limits have been exceeded.  Additionally, in afocal, the extra optics beyond the telescope eyepiece plays a role in how the filtration performs before reaching the viewer's eye, because the optics (Envis) can also de-tune filtration depending on its focal ratio and FoV compared to what precedes it in the optical chain. 

 

Do I understand this process correctly?

 

In my experience, when it does occur, H-a filter band shifting is quite dramatic within the FoV,.  The on-axis image is well filtered and then suddenly, the line is crossed and the H-a object simply disappears.  But with different optical systems, this shift will occur occur at a different point in the FoV, because of the considerations noted above.  So having a full aperture filter is best for any system (however, impractical $), but having a small filter mounted where the light path is least altered by optics is next best.  


Edited by GeezerGazer, 08 November 2017 - 01:32 PM.

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#95 GlennLeDrew

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Posted 08 November 2017 - 02:00 PM

Geezer,

You understand the issues pretty well. ;)



#96 Mike Lockwood

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Posted 08 November 2017 - 06:14 PM

D a m m i t Eddgie, stop apologizing.  wink.gif  We're just learning here.

 

Not sure why people are asking Glenn questions when it's my thread, but whatever helps people understand.  I'm happy his explanation helped.

 

I haven't had time to put an illustration in my explanation of the band-shifting/de-tuning issue.


Edited by Mike Lockwood, 08 November 2017 - 06:14 PM.


#97 PEterW

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Posted 09 November 2017 - 01:26 PM

Etalons like normalmincidence light... so put your filter in the most collimated beam (that you have access to). It’s great to have new and better ways to help us see what’s up there without having to drive to find dark skies... if they exist.

Peter

#98 Tim M

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Posted 09 November 2017 - 09:48 PM

A seemingly stupid question, but does it mater if I orient H-a filters backwards?  (initially, I think not)  With some initial 1x configurations I've tried, I simply taped a filter to a filter ring that fits my NV objective threads, and it so happened that the filter and filter ring met better if the filter was reversed / backwards.  The meshing of the two really wasn't that bad, and perhaps I should just try the filter oriented correctly next time, but I thought I'd ask anyway.

 

Thanks,

Tim



#99 GlennLeDrew

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Posted 10 November 2017 - 08:26 AM

On the face of it, I should think that installing an interference filter 'backwards' should help where the arriving wavefront has a notable tilt. That's because refraction in the filter's glass after transiting the first surface makes for a ray path made nearer to perpendicular at the point of meeting the deposited layers on the rear surface.

 

It would make for an easy enough experiment to perform where a filter could be flipped and simply taped in place or even be hand held (against a steadying surface, of course.)



#100 Mike Lockwood

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Posted 22 November 2017 - 11:21 AM

I honestly don't know if the coatings are direction dependent.  My instinct tell me that yes, there could definitely be effects, but it depends on how the filter layers are designed.

 

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.

 

I'll report more and post more to my web page/blog when I have another tube to test.  I'll also try turning a 2" filter around backwards for 1X observing.




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