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Afocal Observing

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#1 Eddgie

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Posted 01 October 2017 - 10:54 AM

Peter wanted to take his new (to him) Lockwood Dob out last night to try it out and invited me to join him, which I happily did.

 

I am hoping Peter will do a writeup and I don't want to steal his glory on the new scope so I will not go into great detail but there is something I came away with.

 

Sadly, Peter could not reach focus with his PVS-7 and my Mod 3 was also about 20mm short. This meant we were forced to do all observing afocally, and this turns out to have been a very good thing.   When we looked at the Veil for example, we were using the 41mm Panoptic and I was just floored that even though is scope is f/4, the image was just as bright a I could get using my 6" f/2.8 scope.  

 

I guess that I did not realize it and maybe my logic on this is off, but using the Mod 3 at 1x and peering into the eyepiece gave an image scale that was about .7 times the scale would be if the NVD was used afocally.  This means the energy concentration at the photocatode would then be the same as if we were using a focal reducer (which essentially does the same thing, which is to say that it squeezed the image into a smaller area on the photocathode so more photons striking a given area makes a brighter image).

 

If this is correct, then the 41mm Pan would have given the same image scale as he would get using a .73x focal reducer, and that would result in the same brightness at the photocatode as I get using the Boren Simon f/2.8. Does that sound right?

 

This is what I saw.  I will not steal Peter's thunder by talking about every target we viewed, but when we were viewing the Veil, I was quite surprised  at how much brighter it was than in my f/5 dob.   f/4 Is going to be brighter, but not to the difference we could see last night.   Again, the Veil seemed far brighter than it should be for f/4 and pretty much equally bright as it was in my f/2.8 scope.  The veil was stunning!  There are no other words.  The amount of detail was easily the very best I have ever seen and just like with the Boren Simon, when we pushed off to areas around the primary nebula, there were other significant features to be found. In Arigua, we saw a nebula that did not show in charts (again, much like when using the Boren Simon) much the same way I do with the f/2.8 scope.

I did this comparison on several nebula, and in all cases but one I felt that the afocal view in the f/4 with the 41mm Pan was delivering an image that was about as bright as I was getting at f/2.8.   

 

If this is correct it would mean that while there is little value going afocally once you get an eyepiece that is shorter in focal length than the 27mm eyepeice gives at prime focus, by going to longer focal lengths, you are getting the same function as you would get with a reducer, which is to concentrate the photons into a smaller area on the photocathode.  Once you get shorter than 27mm eyepeice of the device, you now dim the image in the same way that you would if you Barlowed. 

 

As a kind of confirmation to me, we did put in a 22mm Nagler, and here, it was like looking though my f/5 scope.  It had lost enough brightness that I felt we stepped back a notch. 

 

I was quite floored at how bright the images were, and if this really does give the same benefit as a focal reducer, then that would mean that by using a 55mm Plossl and afocal projection, I would get the same image scale in my 12" dob as if I used a .5x reducer and this would suggest that I would get the same brightess as an f/2.5 instrument would.

 

Is my understanding off here?   While I dislike the mechanics of Afocal projection, If I could the same benefit of a focal reducer without having to cut the trusses on the dob, I would be interested in trying.

 

(There was one place where the 16" used afocally did not equal the Boren Simon for brightness and it was an oddity.   We looked at the Crab Nebula, M1.    Peter wanted to see it and since I had been doing comparisons, I slewed the scope over to it.  Now as luck would have it, the previous target was stellar and I had the filter wheel turned to an empty slot.  When the scope stopped, BANG, there was the Crab Nebula.  I have looked for it in the past, but always with an H-alpha filter, and it has been disappointing because I was never able to see it.  Of course not having the filter in place I thought it would be better with the filter, but clicking in the 12nm dimmed it considerably, and the 7nm just about estinguished it. Peter brought the 16" to bear and for this one target, I would say that the 6" showed it as being brighter, but this could have simply been something to do with the very small size which probably must made it seem more intense than it was. Otherwise, when I compared the different nebula in the 16" with the 41 Pan used afocally, I think the image was just as bright as I was getting at f/2.8)

Anyway, if my logic is off here someone will help me understand why, but if not, I would say that afocal has a major role to play if there is no way to get a focal reducer to work with a scope.  For eyepeices shorter than 27mm, this logic (if correct) would suggest that you get much of the same benefit.

I am eager to hear arguments to this because if this is correct, I am going to find a used 55mm Plossl and set up an afocal rig for it.  I would still do most of my observing at prime focus, but I could see myself using this in the 12" dob if it gave the equivalent of a .5x reducer. 

 

Not great conditions, but we still had a wonderful session. 70% Moon and so-so  transparency, but that did not stop us from getting one of the best views of the Veil that I could ever have hoped to have seen. Fantastic.. I envy Peter's new scope but I will let him tell you about that.. LOL. 


Edited by Eddgie, 01 October 2017 - 10:58 AM.

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#2 MikeTahtib

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Posted 01 October 2017 - 11:33 AM

A couple things.

1  I had the opportunity to see the veil through a Lockwood 25" mirror, and it was quite bright and detailed.

2. What is an afocal projection rig, and where do I find out more about it?  Is it used only with EAA gear, or is it a visual observation method?  I have never heard of this before, always eager to learn new things.



#3 Rickster

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Posted 01 October 2017 - 11:49 AM

Interesting report.

 

I think your theory/observations are right.  A 55mm eyepiece should work as a .5x focal reducer.  And not only that, but a high quality eyepiece is likely to give better correction than a focal reducer. 

 

I have often wondered why no one on this board (myself included) has experimented with this option.  In my case I know I was turned off at the thought of adding extra glass to the optical chain, and by the need to produce a holder/adapter that would maintain the right distance (eye relief) and axial alignment for the various eye pieces that I own.

 

I am curious how you held the NVD in alignment with the eyepiece.  Was it as difficult/awkward as I imagine?

 

How about FOV.  Is the FOV expanded if using an eyepiece with a FOV greater than that of the NVD?

 

What about using a NVD to augment short focal length eye pieces, for star splitting, etc?



#4 Eddgie

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Posted 01 October 2017 - 01:07 PM

We just hand held the Mod 2.   As it turns out, the opening in the eye guard of the 41mm Pan is just about the same size as the ENVIS, so you could stick it in and that would hold it laterally, so you only had to worry about tilt and this was pretty easy to acquire the correct alignment and hold it because after all, you are only working at 1x.

 

The apparent field is not altered at all, but just like with a focal reducer, there is some illumination falloff.  People need to remember this about focal reducers.   When you reduce, you compress the fully illuminated field by the same amount as the reduction.  If the scope starts with a 10mm fully illuminated circle and you reduce the field in size by a factor of .7, then you reduce the fully illuminated area to 7mm and this will of course mean that you are seeing a much less well illuminated field at the edge.   Afocal though would be the same as the reducer in that field illumination is fixed to the true field.  An detail .20 arc minutes of a degree from the center of the true field will be exactly as well illuminated as it would be with or witout the reducer.  Field Illumination is fixed in terms of true field by the secondary size.

Your last question is where I was (and still am) in terms of my using afocal for extra magnification.   Once you pass get to focal lengths shorter than the focal length of the eyepiece, I don't think you gain anything over using a Barlow at prime focus other than the ability to get more granularity in magnification.  If that was the goal though, I could indeed see using a Mod 3 afocally with something like the Baader zoom.  

 

I do not know also how the very small exit pupil of a high power eyepiece would play into the equation but we did use the 22mm and had no issues.  Still, if you have ever seen the tiny hole in the top of even an 11mm Plossl, you would know that at some point you are not going to go higher.

 

I myself have no desire to use shorter focal lenght lenses because I love my filter wheel, and I am happy using a Barlow for higher powers.

What this does seem to present  though is an opportunity, for people using scopes that can't reach focus with a focal reducer, to get a similar benefit to a reducer.

 

We did filtering simply by putting the filter in on to the Mod 3 objective.  This was of course possible because we were simply hand holding and not using any kind of adapters to couple the device to the eyepiece.  I actually thought this worked really well because it made changing filters pretty easy as opposed to having to unthread an adapter or pull the eyepiece with Mod-3 connected out of the scope to thread on an adapter.

 

In a nutshell, the 1x lens of the Mod 3 with such a big exit pupil means that it is no more difficult to hold the Mod 3 to the eyepiece than it is to use your naked eye at the eyepiece, and this is the way I intend to approach it.  If this logic is sound, I will turn up a 55mm Plossl and just hold the eyepiece up to it.  It was actually pretty painless to do and I was very impressed with the added brightness. It seemed very much like I we using a focal reducer.  

 

While I am not sold on using Afocal for all observing, I absolutely want to purse this as a way to speed up my 12" dob without having to modify it.   Even if the illumination falloff is pretty large, for galaxies and other smaller, fainter objects, or for exploring faint detail in larger nebula, I could see this being useful. The only eyepeice I currently have that is longer than 27mm is the 31mm Nagler, so this is not a big difference, but I might try it out just to see if I can detect any improvement though I think a 40mm would be better.

 

If the above logic is correct, that would say that a 40mm Plossl would give me the same as a .7x reduction and that would make my Dob f/3.43. That sure seems like it would be worth the $30 to play with.  There really is not any need to use a 41mm Panoptic because you don't come close to using the entire field.  I am very interested if exporing this further though, but once again, this benefit I think is really going to be greatest when you are using focal lengths longer than you get using the 27mm lens on the device.  Except for the ability to be very granular with magnfications, I still think prime focus would be best for other observing.  

 

I intend to explore it more though but one thing is certain, the views we were getting last night with the 41mm Eyepiece were better than the 22mm eyepeice (in my opinion) and seemed to be on par with the 2.8 for brightness.  I was so puzzled why his views at f/4 were as bright as mine, but I think this is probably the reason. 



#5 GlennLeDrew

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Posted 01 October 2017 - 01:56 PM

It all comes down to this simple factor: What is the f/ratio as presented to the detector?

 

Imagine this. Suppose the NV device is fitted with an f/1.4 lens. It will have a particular diameter for its entrance pupil. If you can put a same-size or larger exit pupil at the lens's entrance pupil, you will be operating at f/1.4. If your exit pupil is smaller than the lens's entrance pupil, the working f/ratio is concomitantly slower.

 

Dead simple. wink.gif The working f/ratio equals the NVD lens focal length divided by the telescope exit pupil diameter, or the NVD lens f/ratio if the limiter.

 

As can be seen, a short f.l. lens of fast aperture ratio in conjunction with an eyepiece of long f.l. offers the prospect of a bright image. This can help in achieving more uniform field illumination because a longer eye relief typical of a long focal length ocular, in conjunction with the less 'deep' location of the entrance pupil for a smaller imaging lens, permits a ready coupling of pupils without banging the two parts together.


Edited by GlennLeDrew, 01 October 2017 - 01:59 PM.


#6 Eddgie

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Posted 01 October 2017 - 02:46 PM

Glen, was hoping you would check this and thanks for the simple method of calculating 

 

So if I understand this, this would mean that the effective focal ratio of the 41mm Pan in the 16" f/4 dob would be about f/2.7 (which did indeed match my own visual observation as to how the view compared to the f/2.8 scope).

 

A 22mm eyepiece though would have a 5.4mm exit pupil and in the same scope, this would correspond to about f/4.92 and that indeed seemed to be the case for the 22mm Nagler, which presented an image that was of course much larger than the 41mm Pan view, but did not seem brighter than I see it in my 12"  dob, which is f/4.9.

 

Thank you.  I did not know it would be as simple as that and my own approach was just looking at the linear size of the image on the photocatode, so my thinking was correct, but this is a far more elegant method of calculating not only what the effective focal ratio would be, but at what point your break  even would be in the sense that you would get no added brightness and there is no benefit brightening benefit over prime focus. 

 

I sadly no longer have any long focal length eyepieces, but I may buy a cheap 40mm Plossl to play with it some.

 

Again, thanks for providing such a clear and easy way to calculate this.  I appreciate it. 


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#7 GlennLeDrew

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Posted 01 October 2017 - 04:33 PM

Ed,

This is why a visual telescope cannot possibly deliver higher surface brightness than seen by a smaller aperture--or the eye alone. One's iris presents a certain aperture ratio as seen by the retina. If we assume a 7mm iris and the eye's focal length of 22mm, the eye's operating f/ratio is thus just a tad faster than f/3. An exit pupil equaling (or exceeding) 7mm results in the retina still working at f/3. If the exit pupil is, say, 3.5mm, the eye is effectively working at f/6, and the field mage is dimmer by a factor of 4, or 2 f-stops.

 

The point being that the angular size of the pupil as 'seen' by the sensor directly controls image surface brightness. An afocal camera works optically just like the eye, in that it has an imaging lens of given aperture ratio that fundamentally sets the limit to how bright the image can be. An optical system placed ahead of the device (or eye) and hence used in the afocal configuration, and which presents an exit pupil at least as large as the following entrance pupil, is no different than a direct view as far as image surface brightness is concerned. The sensor 'sees' the very same angular area for the entrance pupil, with image surface brightness therefore remaining the same (ignoring the usual small losses due to obstructions and transmission inefficiency, of course.)

 

In all possible modes and configurations for imaging a scene, image surface brightness is fundamentally controlled by the angular area of the pupil as 'seen' by the detector. The brightest possible image derives from a pupil subtending 2pi steradians, or 180 degrees, with image surface brightness equalling the object surface brightness. We might naively calculate this to be the irrational value of f/0 (because tangential geometry assumes a focal length relative to aperture of zero), but it's actually around f/0.3. (At very fast f/ratios the simple tangential relationship departs from the correct spherical geometry.)



#8 Eddgie

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Posted 01 October 2017 - 05:23 PM

A good explanation.

 

I did not have much interest in using afocal because mostly I just pursued faster telecopes because I do believe that for hunting out the maximum extension of a given nebula is aided by a field that is 100% illuminated to the edge.  The only way you know if you have all of the nebula in the field is to be able to see all of the field at the same brightness.  The Boren Simon has 100% field illumination out to the edge of the photocatode, and many nebula either extend out further than classical boundaries show them, or sit in an area of the sky that contains other nebulosity.

 

For targets where K would  want to study the structure rather than the extension, just like a visual observer, I can be happy with full illumination over a small image circle and for small targets like many galaxies, I have come to believe that added brightess is worth sacrificing some image scale for.

 

I am eager to try a 40mm Plossl in my 12" dob and compare the view of the Crescent Nabula both afocal and at prime focus.  If last night is any indication, I think the image will be meaningfully brighter. It is very bright at f/2.8 but to small to see well.  It is rather dim at f/4.9. Large and not hard to see, but not particularly detailed.  I would love to see if the 40m split the difference.



#9 GlennLeDrew

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Posted 01 October 2017 - 09:17 PM

Remind me, Ed; what's the focal length and f/ratio of the lens on the NV device?



#10 Eddgie

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Posted 02 October 2017 - 07:31 AM

The lenses are about 27mm and the focal ratio is f/1.2



#11 Busguy

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Posted 02 October 2017 - 09:06 AM

Glad to hear it was not all that negative an experience. I used my 55mm Plossl with my scope during an NV session.  I simply drew it out of the tube to achieve the focus I needed.  The Veil was unbelievable to me as well. The unfortunate part was I have no prime focus events for comparison.  Remember also, I had SIPS as added glass in the optical train.  I haven't been disappointed in a thing.



#12 cnoct

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Posted 02 October 2017 - 11:12 AM

Exactly Eddie!

NV astronomy is the rare discipline in which NV systems are able to exploit the attributes o both focal and prime configurations.

I prefer going afocal with short focal length eyepieces preferably in the 8 - 12mm range. I have yet to find better way to get the magnification needed to resolve planetary nebula like the cat's eye.

An NV system must be configurable in both afocal and prime to fully utilize the potential of image intensifier for astronomy. I think you've an excellent case as to why this is waytogo.gif


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#13 Mike Lockwood

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Posted 03 October 2017 - 11:47 AM

I'm waiting for Peter's report, too.

 

We did all afocal observing at the Okie-Tex Star Party with a Mod3 unit, loaned by Peter.  I used the Televue/TNVC adapter for this on a 55mm Plossl and 17mm Delos.  I found some interesting things that I will write up in my Okie-Tex report.  We used the unit in a 22" f/3.3, 24" f/2.75, 30" f/3.3, and a 32" f/3.6.

 

My second talk on Wednesday was an introductory talk on NV, and people seemed very interested.

 

Later in my trip I used it in a 24" f/3.3 at the New Mexico Astronomy Village (NMAV) north of Deming, NM, and saw some very interesting things, including the detailed Elephant's Trunk Nebula and Heart Nebula.  This was through the 24".



#14 pwang99

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Posted 03 October 2017 - 12:56 PM

Mike, are you going to post the slide deck somewhere (maybe on your site)?  I think it would be a fantastic resource for everyone here in the EAA forums.

 

FWIW I'm still working with the Cornell University library system to try to find that one NIST article you were looking for.



#15 Eddgie

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Posted 03 October 2017 - 04:30 PM

 

 

Later in my trip I used it in a 24" f/3.3 at the New Mexico Astronomy Village (NMAV) north of Deming, NM, and saw some very interesting things, including the detailed Elephant's Trunk Nebula and Heart Nebula.  This was through the 24".

Peter and I also viewed the Elephant's Trunk. Even in the 16 dob with the 41mm Pan (effective focal ratio is f.2.7) and even with what I would call somewhat limited transparency (though Elephant Nebula was pretty high and out of the worst of the low level dust) we were able to see the lane of trunk and the little ball on the end with I suppose represents the curl at the end of the trunk.  

 

It was dim and the shape did not jump out.  I reported seeing it to Peter, and he did not catch it immediately, but with a few seconds of study, he was able to identify it and his description of the position angle of the lane and the position of the curl was the same as mine, so very positive ID.  This feature is just too small for me to see in the Boren Simon but I can see a hint of it.   Just not enough angular size though at 420mm. 

 

An afocal eyepiece adapter is on my shopping list. 


Edited by Eddgie, 03 October 2017 - 09:20 PM.


#16 Clutch5150

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Posted 03 October 2017 - 05:30 PM

I'm waiting for Peter's report, too.

 

We did all afocal observing at the Okie-Tex Star Party with a Mod3 unit, loaned by Peter.  I used the Televue/TNVC adapter for this on a 55mm Plossl and 17mm Delos.  I found some interesting things that I will write up in my Okie-Tex report.  We used the unit in a 22" f/3.3, 24" f/2.75, 30" f/3.3, and a 32" f/3.6.

 

My second talk on Wednesday was an introductory talk on NV, and people seemed very interested.

 

Later in my trip I used it in a 24" f/3.3 at the New Mexico Astronomy Village (NMAV) north of Deming, NM, and saw some very interesting things, including the detailed Elephant's Trunk Nebula and Heart Nebula.  This was through the 24".

Wish I could have made the event Mike and listened in. Look forward to your observation reports along with Peter's.

 

Vic


Edited by Clutch5150, 03 October 2017 - 05:30 PM.


#17 Eddgie

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Posted 05 October 2017 - 11:51 AM

Ok, I took the plunge. 

 

I ordered this eyepiece:

 

http://agenaastro.co...w-eyepiece.html

 

This eyepiece should make my 12" f/4.9 dob work at f/3.15 with an equivalent magnification of about 35x.  Now this does not sound like much but it is more than twice the image scale I can get with the Boren Simon 6" and only a bit less than the 40x or so we got with Peter's new 16" dob, so not quite the image scale and not quite the brightness, but better than I think I could have gotten without shortening the truss poles on the dob, and for me, this is an important consideration.  I really have been anxious about doing that because to make it work well, it would have entailed over-sizing the secondary and this remains my primary planetary scope (though I don't do much planetary anymore to be fair) so I was reluctant to compromise it optically. 

 

This eyepiece has a sliding housing so that you can raise or lower the camera (or in my case, the NVD) to position it at the right distance from the eye lens.

 

Here is my primary concern  To make this useful for me, it has to be able to provide a field that is at the very worst case, only mildly vignetted due to the presence of the filter and the distance of the ENVIS from the eyepiece lens.  When we used Peter's setup, we were able to get the ENVIS with the filter very close to the eye lens, but with these adapters in place, I am going to be limited to getting the lens as close as we could by simply shoving the ENVIS in close to the eyepiece.   This is the big unknown.

 

Someone on the eyepiece forum that had the GSO 42mm but not in the same version was kind enough to plug it into his 12" f/5 scope and reported that the field was decently sharp out to about the last 25% of the field, so this gives me some confidence that the center 18mm of the field will be fairly sharp.  If there is aberration, I also have the MPCC and if I suspect that some of it is coma, I can try that, but my guess is that these are Erfle eyepeices and edge sharpness is going to be dominated by astigmatism.  As long as it lies outside of the equivalent of an 18mm field stop though I don't care.  This eyepiece would likely not ever be used visually (though I might try it in the 120ED just to see how it behaves) 

 

The eyepiece apparently does not come with the T2 adatper, so this was a separate cost, and on top of that, there is an adatper that converts T2 to filter threads, so though that one in too, and just for good measure, I ordered an adapter that goes from the threads on the Baader Zoom to T2 so I can try that too, but I worry about vignetting.

 

Agena-Astro has a good return policy and I have bought many things from them over the years, so if this particular thing does not work out, then as much as I dislike to return things that are not defective, in this case I think that if it does not work for the specific purpose I bought it, I think I will feel OK returning it.

 

Ordered it today, so I hope to have it early next week.. I will start a new thread at that time to go into the mechanical connections and hopefully the first use. 

 

Again, thanks to Glenn for the formulas and explanations, and thanks to Peter for the invite to observe with him using his new dob, which provided me to the exposure and benefits of using long focal length eyepieces with these fast scopes  to even further increase their effective focal ratio. 


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#18 PEterW

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Posted 05 October 2017 - 05:33 PM

Sounds a cheap way to get what we are after. Another genius suggestion! Maybe another excuse to design a 3dprinted adapter to hold things aligned, save swapping the c-1.25" adapter on and off which is a pain.
I guess if you used a really long eyepiece on a really fast scope you could end up with an exit pupil larger than the NV objective and lose light, but looks like you'd need a very long or very fast scope to hit this. 55mm on an f3 scope...?! Recommendations on a good cheap eyepiece of 40+mm focal length appreciated, seems plossl don't like optics faster than f6... can we get away with this as we are only using the middle of then field of view?

PeterW

Edited by PEterW, 05 October 2017 - 05:42 PM.


#19 Eddgie

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Posted 05 October 2017 - 07:51 PM

I have ordered the GSO 42mm.  It is five elements, so either a modified Plossl or Erfle. They don't say.  At f/4.9 I am sure the eyepiece is not sharp at the edge, but I will only be using about the center 45% of the field.

 

I have an MPCC, so if I see aberration, I can eliminate coma as  the cause.   Will see. 



#20 chemisted

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Posted 05 October 2017 - 08:06 PM

I am in the afocal game.  My TV adapter came today so I attached it to a 55mm Plossl and popped the eyepiece into the focuser of my Takahashi Sky 90 (native f/5.6).  Since I do not have a 2" 12 nm filter I just held my NVD Micro (with 12 nm filter attached) to the flat surface of the adapter while viewing the North American nebula and Pelican complex.  Even though the moon is up and full the view was just incredible.  This setup yields a 4.4 degree true FOV at f/2.7 and perfectly frames the two nebulae together. The stars throughout the field were round and sharp with maybe some softening at the very edge. I can't wait to use this setup on a moonless night.



#21 Eddgie

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Posted 05 October 2017 - 09:51 PM

I am in the afocal game.  My TV adapter came today so I attached it to a 55mm Plossl and popped the eyepiece into the focuser of my Takahashi Sky 90 (native f/5.6).  Since I do not have a 2" 12 nm filter I just held my NVD Micro (with 12 nm filter attached) to the flat surface of the adapter while viewing the North American nebula and Pelican complex.  Even though the moon is up and full the view was just incredible.  This setup yields a 4.4 degree true FOV at f/2.7 and perfectly frames the two nebulae together. The stars throughout the field were round and sharp with maybe some softening at the very edge. I can't wait to use this setup on a moonless night.

Good to hear. 

 

Conditions were poor here, but I went out with 3 eyeypieces, a 32mm Plossl, a 31mm Nagler, and the Baader zoom.

 

Now to test, I used a 1.25" H-a eyepeice screwed on to the ENVIS and this is where my concern starts.  For the configuration I am contemplating, the eyepiece would have to be mounted on top of the afocal setup and an adapter would be used to couple the T2 mount on the eyepeice to the 1.25" filter threads.  This adapter and the light path of the filter housing would push the ENVIS Back from the eyepiece by 12mm or more.   Now this filter has a rather long cell (as many new filters do) so perhaps the very slim cells on my 12nm filters would reduce this by 5mm.

 

Anyway, here is what I found. 

 

First up was the 32mm Plossl.   I removed the eye guard and with the filter pushed right up to the top of the housing.  The view was not sharp at the edge, but the field was otherwise bright and clear.  Now this was just a test because this eyepiece is too close to the focal length of the ENVIS to offer much more than a tiny effective focal length increase, but my goal here was to see what happened when I increased the distance from the filter to the top of the eyepiece the way that would occur when I have to use adatpers to mount the ENVIS.  And here is what happened. 

 

When I pullled the Mod 3 back a few millimeters, it was fine, but much more than this I could see stars disappearing at the edge of the field.  I would say that at 5mm or 6mm of space it became quite noticeable and the apparent field where stars were visible was maybe 35 degrees.  Movement further back resulted in a corresponding decrease in the size of the apparent field that had stars present.   I did not see vignetting in the classic sense, but I did not have have a way to block ambient light so my bet is that as I drew back, the vignetting was just being covered by the glow of ambient light.  Now again, the adapter will have some thickness and the cell on this filter (650nm) has some length, and some light path length is unavoidable, but the problem here is that the filter cell on this filter is stupidly long.  Baader, Optilong, and Astronomik keep their filter cells very short, so maybe with one of those filters it would be less of an issue.

 

Next, the 31mm Nagler.  Here the vignetting started a little closer.  Stars were not that sharp at the edge, but I stuffed the MPCC on to the front of the 31mm Nagler and hey, it looked much better, so the 32mm Plossl was probably seeing some small amount of coma. 

 

Last was the Hyperion Zoom.  Here it seemed like I could back out a bit more. The Baader has kind of a very flat top which pushes the eye lens up quite a bit, so it starts closer to the ENVIS objective than the 32mm Plossl, which has the lens set maybe 5mm or 6mm below the top of the eye guard mounting flange, and the Nagler, which uses a deeply curved eye lens. 

 

For the zoom, the ENVIS would actually rotate with the zoom function but I could not do this so I just rotated the zoom while I tried to hold the Mod 3 steady.  I had expected to have problems holding it on axis, but for some reason the Hyperion did not seem as sensitive to position.   It actually worked well enough, but the view of course got progressively dimmer as I zoomed, but it does when you barlow up too, so that is just the price you would pay.

 

The GSO 42mm has been designed for this use, so hopefully the T2 adatper results in a very short light path.  Also, I do not know the effect of eye relief on the of axis vignetting, and I suspect that longer eye relief is better.  If the 42mm is an Erfle or modified Plossl, then the eye relief should be much longer than the 32mm and 31mm eyepieces.  The Hyperion seemed to do better than the 31mm and 32mm eyepeices.  The Hyperion Zoom has 16mm to 19mm of eye relief vs the 12mm of the 31mm Nagler but the 32mm Plossl has 20mm.   The difference is probably in the fact that the 32mm Plossl has the eye lens rather deeply recessed, so if I could have gotten the filter closer by the depth of the recess, it probably would have behaved similarly to the Hyperion Zoom ,which was the most tolerant of spacing. 

 

 

 

 

I do not want to have to suck up the price of a 2" narrow band filter I am crossing my fingers that the optimized design of the GSO gets the ENVIS with a filter close enough to avoid apparent field loss. If the eye relief is 20mm and the adatpters have less than 10mm of light path, I think it will work out.  My 12nm is in a thin cell, and if it works, but the 7nm does not because of the stupidly long cell, then buying a cheap Baader filter with a thin cell and transferring the H-a filter glass from the stupidly long filter cell to the thinner cell might be an option. 

 

Sorry for the long post.   The benefit of afocal using very long focal lenght eyepieces is frankly better than I realized it would be and I am sharing the potential issues I see.

 

Eyepiece will probably arrive on Monday.  Then we will know more. 



#22 The Ardent

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Posted 05 October 2017 - 10:40 PM

If only this worked with the PVS-7 



#23 Eddgie

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Posted 05 October 2017 - 11:40 PM

If only this worked with the PVS-7 

Well, in theory it would work, but the concern would be the mechanical strength of the RAF 1.25" filter ring.

 

The Televue unit in all likelyhood used the native thread on the ENVIS and PVS-7 and since this was designed to hold the afocal lens (which is not all that heavy but is a long moment if you are running and bumping into things) so this is almost assuredly a more rugged mount than the RAF filter ring but it would be a very long and perhaps easy to bump and break extension.   I even have a concern about using the Mod 3.

 

I have to repeat this. Peter and I got excellent result just sticking the Mod 3 into the top of the Panoptic and there is no reason why you could not do this with the PVS-7.  Since the PVS-7 and Mod 3 are both 1x when used this way, holding it steady, especially with the big eye guard of the Pan was pretty easy.

 

Also, the GSO has the sliding shroud and this too would add extra stability to someone that was just hand holding.

 

In fact, this has been my backup plan for the GSO 42mm. Even if it does not have the ability to fully illuminate the field with the adapters, I was just going to use it exactly they same way Peter and I used it, which is to just stick the lens into the mount, and on the GSO, the shroud with T mount could be extended up as a guide and the Complete nose of the PVS-7 could be just poked in.   In some ways this is actually more appealing to me because threading and undtreading things to change filters is much busier if you have to untread the device, then unthred the filter, thread on the new filter and thread on the device.  

 

In fact, if spacing is an issue, I  have considered having Rafael at RAF make me a simple tube that has T2 female on one end and is just a tube that is has an inner diameter just big enough to take the ENVIS (and PVS-7) that I could just slide the device into and lock in place with a thumb screw, and then use the shroud on the eyepiece to move it in until it can get close enough not to vignette the field. .   No reason that would not work as well with PVS-7 as with Mod 3. Then if you wanted to change filter, just pull the device and swap the filter, then plug it in again. 

 

That may be an option for PVS-7 owners (and maybe Mod-3 owners too).  I would bet that Rafael would be able to turn one of these out for $50 (prototype price for something like this I would guess). 

 

 

custom adapter.jpg

 

Again, the GSO already has the shroud, so it would need T2 adatper, and the custom piece would be just a tube with the right inner diameter for  the ENVIS/PVS-7 and T2 female threads on one end. 

 

In some ways, this is actually more attractive to me than using the T2 to 1.25" filter thread.  Heck, maybe I will do this anyway because I do still use my PVS-7 a lot.   


Edited by Eddgie, 05 October 2017 - 11:43 PM.


#24 Eddgie

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Posted 05 October 2017 - 11:52 PM

And of course the above is specific to this one eyepiece.  It has the moveable shroud on it already.

 

The Televue Adatper would work with most Televue eyepeices so for someone that was going to use a variety of focal lengths, that might be the way to go.  This is probably the only afocal eyepiece I would use an this would primarily be for galaxies and nebula. I like the idea of faster, but much faster than this would cost more image scale than I would want to loose for my specific situation.   The Boren Simon is bright, but fine structure in nebula is to small to see.  If I went to long with the focal length of the eyepiece on the dob, I would run into the same problem.  For me, the ES 42 would give f/3.15 and this is probably a good balance between brightness and scale.  That will be different for everyone. 

 

If you wanted to use a PVS-7 though, holding it up to the eyepiece would work, but something like the above would probably work better just because you would not struggle to keep it aligned. 

 

Let me see how it works out first, and if you are interested in make the above adapter based on my result, I might get one made too.



#25 Jeff Morgan

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Posted 05 October 2017 - 11:52 PM

Last was the Hyperion Zoom.  Here it seemed like I could back out a bit more. The Baader has kind of a very flat top which pushes the eye lens up quite a bit, so it starts closer to the ENVIS objective than the 32mm Plossl, which has the lens set maybe 5mm or 6mm below the top of the eye guard mounting flange, and the Nagler, which uses a deeply curved eye lens. 

 

 

If you decide to use the Baader zoom outside of the test bed, there is an adapter for it that will make life easier as detailed in this thread:

 

https://www.cloudyni...t-vision-setup/


Edited by Jeff Morgan, 05 October 2017 - 11:52 PM.



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