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

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#1 Mike Lockwood

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Posted 17 October 2017 - 07:32 PM

I did some afocal nightvision observing using my 20" f/3.0 and a wide variety of TeleVue eyepieces a few nights ago, and I was able to take some reasonably good photos to show the differences.  I also compared with prime focus.

 

I wrote a short article about it, which you can read here:  http://www.loptics.c...ightvision.html

 

There will be more added soon.....


Edited by Mike Lockwood, 17 October 2017 - 08:56 PM.

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#2 GeezerGazer

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Posted 17 October 2017 - 11:10 PM

Very nice photo comparison Mike.  Thanks to you and Peter for taking the time to test, record and share results.  Looking at the AFOCAL 27mm Panoptic and the Prime focus images, it appears that the prime image was substantially brighter, closer to the overall brightness of the image through the 41 Panoptic.  I am wondering how it would appear if you used a .7x reducer with a prime image (to get closer to the FOV of the 41 Pan) or a .5x reducer with prime (getting very close to the 55 Plossl).  

 

I think your photos clearly show the gradual photon starvation that becomes apparent with increased NVD magnification.  Using reducers and barlows with prime, I have found that I can comfortably use prime at .5x and at 2x and 3x with my 140mm f:7 apo, Mod 3C, WP, filmless combination.  I have not tried higher magnifications because most of my targets with the Mod 3C are either globs or nebulae, and are appropriate for the longer focal length/wider FoV applications.  

 

Your photos really do provide a nice gradation in magnification showing those interested in AFOCAL-NVD observing where they might want to begin.  

 

I look forward to more of your reporting on AFOCAL observing.


Edited by GeezerGazer, 17 October 2017 - 11:19 PM.


#3 pwang99

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Posted 17 October 2017 - 11:50 PM

Thanks for the methodical approach to this, Mike.  You've set up a very nice page and I look forward to seeing shots from the 14" scope.

 

It is interesting to see how the visual character of this nebula changes between unfiltered and H-alpha filtered.  With the 55mm and 41mm afocal shots, you can begin to see the brighter arc of nebulosity behind the Swan itself.  It's just at the edge of the field of the prime focus shot (it seems to be faintly present).  But with in the H-alpha filtered view, not only is it obviously present, but the gradations of brightness in the nebulosity between the Swan and the arc take on an almost three-dimensional "billowy" nature.

 

This "billowing" effect is also marvelous in the Lagoon and in M42.  It's really a marvelous and breathtaking effect to see in person.



#4 The Ardent

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Posted 18 October 2017 - 12:10 AM

Here are a couple of suggestions. These are invisible with a normal eyepiece + OIII in my 18", easily visible with NV+ H-alpha

 

Abell 79 in Lacerta

Sharpless 2-188 in Cassiopeia 



#5 PEterW

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Posted 18 October 2017 - 03:28 AM

Very useful comparison, welcome to the green side. I look forward to your further work.... I can supply a long list of nebulae to have a crack at.... ic443, Horsehead, monkeyhead, Pac-Man, wizard, cocoon, lowers neb, tulip, propellor. The North America and gamma Cygni regions will overflow your field of view. It’s great that the 55mm can still be used at f3.4 to speed the view up.... is this the “best” eyepiece to go for?

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#6 Eddgie

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Posted 18 October 2017 - 10:29 AM

I think that afocal is a great way to allow people that don't have fast scopes and insufficient in-travel to use a reducer to get a brighter view but I too think that in this test, the prime focus image actually presented a view that was brighter than the afocal view with the 27mm Pan and that kind of supports my only experiment with afocal.

 

Take a look at the limiting magnitude.  In particular, look at the prime focus view, and go to the Swan's tail, then go up from the tail to about half way to the top of the frame. Here, you will see a string of one brighter and four fainter stars in a line.  Also, if you look you will see that there is a little star slightly to the left of the brighter star and the first dim star in the chain.

 

Now, look at the same chain in the 27mm afocal picture. Here, the stars are much less sharp and just don't show with the same authority as in the prime picture.  Note also that the little faint star (which should now be to the right of the string) is almost gone.

Another area is the swan's beak area.  The sharpest view (and I think the best view) of this area is the one shown at prime focus. (not counting the 7nm filter view, which of course we don't have anything to compare with).

 

And of course the nebula is simply better in the afocal view vs the 27mm Pan and I think not any worse than the view in the 55mm pan due to the reduced scale in that eyepiece.  Now, I do have to say that the 41mm Pan image is I think the best of the afocal views, but the real star here is the 7nm filter. 

 

I looked over all of these pictures, and I have to say this.  I can see everything in the prime focus image that is shown in any other of the unfiltered images (except of course what does not fit into the field) and I actually think that it is one of the most pleasing views.

 

When I tried afocal, the difference at the eyepiece was just not what I had expected I had expected the afocal view to have brightness  closer to to the view in my f/2.8 scope and it was much closer to the f/4.9 view (though effective was f/3.2)  My target was the Crescent Nebula, and I was just disappointed that the result was not better than it was. It was brighter though, so I am not disputing that afocal works, but the question then is how well does it work.  Brighter is brighter though,and in my experiment, the view was slightly brighter, and because of that, I think afocal has a place in NV astronomy.  I think a 55mm Plossl would have made it worth it, but for me, the 42mm eyepiece simply did not offer a brightness increase I had hoped for.   As with above, I do not want to loose to much image scale because then the views would not be so much larger than in my 6" f/2.8.

 

I love the comparison though, and I do encourage others to do some afocal work as well.  I think this is a great contribution and I am eager to hear/see yours and others work and opinions, and I appreciate your and Peter's efforts to help us better understand afocal result and expectation. 


Edited by Eddgie, 18 October 2017 - 11:38 AM.


#7 Rickster

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Posted 18 October 2017 - 11:30 AM

Excellent work Mike.  We can now refer to this article whenever afocal vs prime questions arise.



#8 Eddgie

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Posted 18 October 2017 - 11:33 AM

Doing some math.   The scope was working at f/3.45 in prime focus with Paracorr so effective focal length of about 1750mm

 

Effective focal ratios:

 

27mm Pan = ~ f/3.4 (Nominal f/stop increase)

 

41mm Pan = ~ f/2.4 (1.04 stops)

 

55mm Pan = ~ f/1.7 (1.41 stops)

 

I did not do the shorter focal length lenses because there is no gain here and like Borlow, the image just gets dimmer an dimmer.

 

For those wanting to compute the effective focal ratio, it is simple to do:  

 

Find the exit pupil for the lens with the telescope you are going to use it in

Divide that number into the focal length of the lens on the camera (or NVD in our case)

 

The resulting number is the effective f/stop presented to the photocatode. 

This would be useful I think for people that have a scope that they are interested in doing afocal with.

 

For example, my 120ED is f/7.5 so if I used a 55mm Plossl, the focal ratio would be reduced to f/3.68 and this would be an effective f/stop reduction of 2 stops and a 41mm Pan, it would be reduced to f/5 so about 1.17 stops. 

 

This means that for scopes that start with relatively slow focal ratios that have no hope of reaching focus with aggressive focal reducers (like SCTs where only the very center of the field is fully illuminated with reductions faster than f/6.3) afocal offers great promise. While my own experiment did not result in the gains I had hoped, I have to say that there is promise here and I intend to try again with a 55mm Plossl.  The GSO 42 was just not a very good eyepiece and perhaps just did not have very good coatings and I called that attempt a fail. 


Edited by Eddgie, 18 October 2017 - 11:49 AM.


#9 Mike Lockwood

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Posted 18 October 2017 - 11:36 AM

Thanks pwang.  Having a 2" H-alpha filter enables some amazing things, and PeterW, I have actually already observed and crudely photographed a lot of the objects that you mentioned with both the 20" f/3.0 and 14.5" f/2.55.  Many filtered images will be seen in the next installment.

 

Eddgie, if you're going to get picky about limiting magnitude, the images themselves need to be of the same quality, and they clearly are not.

 

Keep in mind there is some vignetting in the field of view and the camera and the eyepieces, and though the pictures are pretty fair, there are variations of how the object is placed, how well the camera focuses, and whether or not I flinched during the exposure.  The camera was simply hand held against the NV eyepiece.

 

I am not convinced that the 27mm Panoptic image is as sharp as it could have been for a couple of reasons, and it will probably get re-shot for verification.  Also, different eyepieces respond differently to fast telescopes (even different focal lengths within the same line), and the 27mm Panoptic may not be the best eyepiece to evaluate - there are others around that focal length that may perform better according to my notes on eyepiece performance with fast telescopes (yes I have done that test).

 

Finally, keep in mind that those are merely PICTURES, and though they are a decent representation of what is seen visually, they are not an exact representation.

 

Having actually done this, I can say that the 55mm was FAR brighter than prime focus, and the 41mm was brighter, and I could see far more using those than I could at prime focus.

 

The wide field of view of the 55mm was wonderful to use.  It is all that I have used since, though I will go back to the 41mm at some point.  I also should test a 26mm Nagler and 24mm Panoptic at some point to see how they compare with the 27mm Panoptic.

 

Given the inconvenience of prime focus for use with my 20" (added some notes on that to the article), I'm not going back to that very often.  However, I do hope to someday mate a NV unit with some fast camera lenses that I have.  It's pretty clear that we need a wide variety of image scales to appreciate everything that the sky has to offer.


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#10 PEterW

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Posted 18 October 2017 - 11:49 AM

Speed affects nebula brightness rather than stellar brightness, so more tests required to ensure the most consistent results. I am waiting for somebody advice on which long eyepiece I should look to get.

PEter

#11 pwang99

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Posted 18 October 2017 - 12:01 PM

Having actually done this, I can say that the 55mm was FAR brighter than prime focus, and the 41mm was brighter, and I could see far more using those than I could at prime focus.
The wide field of view of the 55mm was wonderful to use.  It is all that I have used since, though I will go back to the 41mm at some point.  I also should test a 26mm Nagler and 24mm Panoptic at some point to see how they compare with the 27mm Panoptic.


Your test results definitely motivates me to try out the 55mm more.

 

Given the inconvenience of prime focus for use with my 20" (added some notes on that to the article), I'm not going back to that very often.  However, I do hope to someday mate a NV unit with some fast camera lenses that I have.  It's pretty clear that we need a wide variety of image scales to appreciate everything that the sky has to offer.


Yeah, check out this motivational shot from the MDW H-alpha survey: https://www.mdwskysu...=style-j40p1kmn

This is the Northern Hemisphere winter-time work list... Heart & Soul down to the Seagull. Big & fast camera lenses, under dark skies, can reveal structure at much larger image scales than we typically tackle with telescopes.

 



#12 Eddgie

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Posted 18 October 2017 - 12:04 PM

I posted the formula above and I think that as Mike suggests, the question is what your goal and how the target will respond to image scale.  As you can see, the most aggressive reduction produces a brighter image, but the reduced scale may make it hard to now resolve some detail.

 

An example of this was my session with Peter a few weeks ago with Peter using afocal in his beautiful Lockwood dob.

 

Peter was working at f/2.7 and while the images were no brighter than my 6" working at f/2.8, he had double the scale, so this made seeing fine structure in the Veil and Crescent Nebula much easier than it was in my scope which is working at only 17x.  While I could fit most of the nebula in my field while we had to pan the Peter's scope, the much larger scale revealed fine structure that was to  small to see in my scope.

 

On the other side of the coin, the small scale of my 6" is great for seeing vast extension of nebula that simply runs out of the field of the bigger scope.   

When I did the experiment with the 42mm ES with my 12" f/4.9 dob, I did not think the brightness reduction (in this particular case, and I blame the eyepiece more than the fact that I was doing afocal) was worth the loss of detail that came along with the loss of scale.

 

This will be of course easily addressed by having two or more eyepieces like the 41mm Pan and 55mm Plossl.  

 

I can see that Mike likes the benefit of having a variety of scale options and I agree that it is a compelling benefit of afocal.


Edited by Eddgie, 18 October 2017 - 12:04 PM.


#13 Eddgie

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Posted 18 October 2017 - 12:12 PM

 

Eddgie, if you're going to get picky about limiting magnitude, the images themselves need to be of the same quality, and they clearly are not.

 

 

I am sorry if I sounded picky. That was not my intention.

 

I am not the only one to make the observation that the prime focus image looked brighter than some of the others, but I seem to have been singled out for my own comment.

 

GeezerGazer had this to say (emphasis added by me):

 

 

Very nice photo comparison Mike.  Thanks to you and Peter for taking the time to test, record and share results.  Looking at the AFOCAL 27mm Panoptic and the Prime focus images, it appears that the prime image was substantially brighter, closer to the overall brightness of the image through the 41 Panoptic.

 

It was an experiment we were presented with and my observations were just based on the data that was available.  When I viewed the pictures I went under the assumption that the experiment tried to be as equal as possible, and when I looked carefully at the pictures, this was just something that I saw. 

 

I simply described what I saw.

 

I don't know what else I can say to get on the right side of this. If the purpose was to provide us with the data so we could discuss it, then you should be willing to accept that some people will see things that others miss and comment on it. I was not alone in my observation about the brightness though.

 

It kind of leaves me at a loss for what to do the next time you post this kind of comparison though.  If we can't freely say what we like or don't like without feeling like our input has value, then it makes it hard to see these tests as useful for discussion purposes. 

 

Sorry though for seeming picky. I was happy to see the test and thanked you for your efforts, but will refrain from making comments like this in the future.  


Edited by Eddgie, 18 October 2017 - 12:43 PM.


#14 Mike Lockwood

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Posted 18 October 2017 - 12:29 PM

No problem Edggie, and no need to apologize, I just wanted to point that out before others drew more conclusions.  I did expect people to compare, but I tried to express the limitation of my images in the article.  I only drew conclusions about FOV and brightness because I knew the limitations of the images.

 

Though I tried to have everything equal, sometimes I flinched or the camera didn't focus as precisely.  I can see this in the metadata (which indicates focus distance) and the star shapes.  I was also trying to do the testing quickly to avoid having M17 get too low, so that didn't help.  That's why I let the camera auto-focus rather than going through the time-consuming steps of manual focus, though that is possible with the G15.

 

My notes indicate that the 41mm Pan may be better than the 27mm Pan at f/3.0 (f/3.45 w/Paracorr).  The 26mm Nagler also needs to be tested.

 

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.

 

Also I really need to make myself a smaller f/3 telescope......


Edited by Mike Lockwood, 18 October 2017 - 12:50 PM.


#15 Eddgie

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Posted 18 October 2017 - 01:00 PM

The smaller scope compromise is (as you had suggested earlier in another post) one of scale and this has both positive and negative aspects.

 

I use the Boren Simon 6" f/2.8, and for exploring the full extension of nebula under dark skies (well under any sky, but best under dark skies) it is a fantastic tool.  It is less useful for studying the fine scale structure of nebula.  The Crescent for example is clear and bright in my 6", but in my 12", even though it is not very bright at f/4.9, the large scale makes it easy to see structure that is just not big enough in the 6".

 

One of the things that makes the 6"  fantastic is that the field is 100% illuminated to the edge of the imaging circle, and if you build a smaller scope, I encourage you to size the secondary appropriately.

With faint nebula (and the Swan is a great case in that there is nebula pretty much everywhere in my 2.4 degree true field) the "Classic" chart boundaries do not really fully encompass the true extension of the nebula it is possible to see in the field of view.  

 

I have encouraged people to try this experiment.  Go to North Amercian Nebula, then move the scope maybe 3 to 4 degrees east or west.   Now, 'Fast Pan" back towards the nebula.  I think this works like the old trick of bumping a scope to bring out dim galaxies. If you are using a scope with a small illuminated fieldas you fast pan back in from a darker sky area, any brighter lumps of nebulousness will seem to pop out as they move though the center of the field.  If you overshot, you can come back for closer study.  I use this all the time and there is a lot of nebula around some of these showcase objects that shows up by fast panning from an area of the field that you know to be relatively free of nebula.

Now the reason I like the Boren Simon for this is becaues with the 100% field illumination, I do not loose brightness at the edge of the field, so nebula that I have to fast pan to see in the dob (10mm illuminated field) is already as bright as nebula in the center of the field.

 

That would be my only input.  If you build a smaller scope specifically for the task, size the diagonal for 18mm full illumination at the focal plane  with the lowest power eyepiece you plan to use. 



#16 Eddgie

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Posted 18 October 2017 - 01:08 PM

Here is an example I made for someone from the Deep Sky Observing forum.  They were having trouble seeing North American Nebula. 

 

Glenn had commented that he could see it in his binoculars.

 

I explained that I thought one reason Glenn could see it was that his field was big enough so show some very dark sky off to either side, which would let the nebula show with better contrast.  This is because this nebula is not on a black sky at all. 

Here is the picture I used to explain that.   This is from Sky Safari and it shows the H-a overlay.   Now using the 6" under dark skies, I can follow this nebula out to see pretty much everything that is blue and orange in this picture but the well illuminated field makes it possible to pick up a lot of it without fast panning until I get to the very diffuse outer edges.

 

1508268248260 (1).jpg

This is where smaller, faster scopes really come into their own for night vision.   It makes it possible to study this very large scale but very diffuse nebula because bigger scopes just never get enough dark sky in the view to realize that the sky is not dark where they are looking (if that makes any sense).   

As an example to that, the Swan Nebula has an arc that comes off and extends almost all the way up to the Eagle Nebula.  I can follow this easily in the 6" but it is not really hinted at in the 20" because the field is not big enough to encompass it. 

 

I don't know if that makes sense or not, but I highly recommend that if you build a smaller scope, aim for a fully illuminated 18mm image circle.  For this kind of nebula sweeping, it really helps to have the outside of the field have the same brightness as the center. 



#17 Mike Lockwood

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Posted 18 October 2017 - 01:44 PM

It does indeed make sense to use a larger secondary for a small NV scope, and I sweep a bit like you describe above.

 

My 20" f/3.0 was not built with a 55mm eyepiece in mind, and at some point I may move up to a larger secondary because I know the secondary is right on the edge of being too small.  The secondary in my 14.5" f/2.55 is more conservatively sized.



#18 PEterW

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Posted 18 October 2017 - 01:45 PM

That MDW survey is awesome and will probably lead to frustration at things we still can’t see.
If you have image scale problems, just use a bigger scope, for instance I can’t readily see the Horsehead in my 80mm refractor. Mike... smaller f3, yes please we would like one of those too!
Eddgie, I can see plenty more round gamma Cygni off to the left and up to the tulip men, ic5068 above the North America and sh91 near 68cygni, sp my time last weekend gazing round this area, nice.

PeterW

#19 pwang99

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Posted 18 October 2017 - 02:42 PM

Now, 'Fast Pan" back towards the nebula.  I think this works like the old trick of bumping a scope to bring out dim galaxies. If you are using a scope with a small illuminated field, as you fast pan back in from a darker sky area, any brighter lumps of nebulousness will seem to pop out as they move though the center of the field.

 

This is like an astronomy version of saccades! :-) https://en.wikipedia.org/wiki/Saccade



#20 PEterW

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Posted 18 October 2017 - 02:50 PM

... or whacking the side of the scope...

Peter

#21 Eddgie

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Posted 18 October 2017 - 05:30 PM

 

Now, 'Fast Pan" back towards the nebula.  I think this works like the old trick of bumping a scope to bring out dim galaxies. If you are using a scope with a small illuminated field, as you fast pan back in from a darker sky area, any brighter lumps of nebulousness will seem to pop out as they move though the center of the field.

 

This is like an astronomy version of saccades! :-) https://en.wikipedia.org/wiki/Saccade

 

Interesting article.  

 

I know that amateurs have over the decades just tapped the scope to make it jump a bit and this helped spot dim galaxies, and I would guess that the same mechanism is at play.

 

In this particular case, the situation is that the field along the Milky Way even is a small scope is often pretty much filled with nebula so if you bump the scope, you don't move it far enough to be able to see a contrast change.

 

The fast pan is what is working best for me.   

 

This is how I found that M29 was actually sitting in nebula.  I had fast panned in from the direction of the star Gienah, which Sky Safari shows to be in relatively dark sky, and as I panned and picked up nebula when M 29 came into the field I did not recognize it, but when I backed up to see the little finger of nebula that it sits in, I thought "Gee, that looks like M 29" And when I unfiltered, it was obvious that it was M29.   

 

Prior to that, I had not a clue that M29 sat in nebula.  (Of course I was under Bortle 3 skies, and this helped.   Pretty much everything that is blue or orange in the picture above seemed to show in the eyepiece by doing fast pan.  This nebula at its extreme extension is very difficult to see, and it would sometimes take multiple fast pans in to feel confident that I was seeing nebula.  Brain sometimes sees things we want to see even if the eye doesn't. LOL). 

 

This is an interesting read though and It sounds like a good expiation of why the old galaxy finding trick works.  I have used it a lot to find galaxies over the years, but eventually came to think that just finding it by this method and not really then being able to see any detail other than a soooo soooo dim spot in the sky just wasn't my thing.   I could write in my log that I had "Seen" it, but that did not really mean that I saw it well.  



#22 Jeff Morgan

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Posted 19 October 2017 - 10:46 AM

Having actually done this, I can say that the 55mm was FAR brighter than prime focus, and the 41mm was brighter, and I could see far more using those than I could at prime focus.

 

I'm a bit confused at this statement.

 

Using a projection eyepiece of longer focal length than the NV device results in a focal reduction effect. It should be brighter, because the effective focal ratio is reduced by some amount. (About 0.5x based on my current understanding of how this works.)

 

Shouldn't the real brightness comparison between between afocal use of the 55mm eyepiece and a 0.5x focal reducer?

 

Or, prime focus on the Mod 3 vs. 26 Nagler afocal projection? (Assuming the "native" Mod 3 is 26mm. I've also heard the value or 27mm referenced.)

 

I applaud your trail blazing on the afocal front and read your posts with great interest. But I'm puzzled at what you mean here.


Edited by Jeff Morgan, 19 October 2017 - 10:54 AM.


#23 Mike Lockwood

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Posted 19 October 2017 - 12:45 PM

Shouldn't the real brightness comparison between between afocal use of the 55mm eyepiece and a 0.5x focal reducer?

 

Or, prime focus on the Mod 3 vs. 26 Nagler afocal projection? (Assuming the "native" Mod 3 is 26mm. I've also heard the value or 27mm referenced.)

You can't use a 0.5x reducer with a 20" f/3.0 or 14.5" f/2.55.  The eyepiece effectively acts like one.

 

It is not trivial for me to switch between prime focus and afocal with my 20" f/3.0.  It's a pain in the ****, so I only did it on the one night.  Read my latest updates below for more on this.



#24 Mike Lockwood

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Posted 19 October 2017 - 12:48 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.


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#25 PEterW

PEterW

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Posted 19 October 2017 - 03:14 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


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