47 replies to this topic

[longbond]

I live in Chicago where skies are Venusian and outdoor observing comes with “occupational hazards”. In other words, Chicago is a good place for indoor testing. I often give GeezerGazer (Ray) a heads-up on my NV testing and he often asks “What about Afocal?” He does mostly NV Prime, but Afocal’s extreme reduction is very compelling.

 

As they say, “Don’t bury the lead”. This is the lead…learn it, know it, live it 

 

 

Except for the left photo being in color, these two photos are nearly identical. If you’re using Afocal for image reduction, this is what you want. To use a blackjack analogy, this is a 21. In the next photo you’ll see why this analogy is apropos.

 

The above image on the left, which I’ll refer to as “visual”, was taken through a 102mm f/7 APO with a TeleVue 55mm Plössl (TV55) converted to 67mm (TV67) and an iPhone 12 Pro Max (1x lens). The photo on the right uses the same setup, but in Afocal with an Envis/Mod-3 connected via TNVC adapter. Unless otherwise noted, NV photos are cropped to size and the camera tint is removed.

 

The point is that there IS a practical limit to Afocal image reduction and that limit is the “visual” image. This is because of the eyepiece field stop and it’s a hard limit. You can reduce ad infinitum, but you can’t go beyond the field of view allowed by the field stop. Here’s what I mean...

 

 

Above is a montage of raw NV images. Left to right is Afocal with: 15mm TeleVue DeLite, TV55, TV67 and an 85mm Plössl by Russell Optics. Top row is with the 102mm f/7 APO, the bottom row is with a 6-inch f/4 Newtonian.

 

From left to right, images are nicely reduced, as expected. As the image reduces, the true field of view (TFOV) increases to fill the NVD eyepiece’s image circle until we get to the Russell 85. Beyond 67mm, you’re basically busted. The field stop is the reason why.

 

The TV55 (which also applies to the converted TV67) has a 46mm diameter field stop. For a 2” barrel eyepiece, you can’t get much larger. The Russell 85 doesn’t have a true stop, the barrel itself is the stop. Indeed, Russell’s image is more reduced, but there’s no increase in TFOV. Below are the photos scaled to their image circles.

 

 

Above, on the left is the TV67, the right is the Russell 85. Top row 102mm f/7 APO, bottom row 6-inch f/4 Newt. When scaled, we see that the Russell 85 gives basically the same image as the TV67. Without the scaling, the Russell 85 image is just a smaller version of the TV67 image. Just to be clear, the Russell 85 is a nice novelty eyepiece, but not for NV Afocal.

 

So, what’s the big deal about the 67mm focal length? It’s because of two key ratios: Image Plane Reduction and Optical Reduction. If you’re going for perfection, the two ratios should match. Image Plane Reduction is basically the smaller NVD sensor target size compared to the larger 46mm field stop. The clear aperture of my Mod-3 sensor is 18.24mm dia. This ratio is 0.397 (18.24/46). Optical Reduction is the Envis focal length (26.5mm) divided by the 67mm eyepiece focal length is 0.396 (26.5/67).

 

0.397 vs. 0.396 is not a coincidence. TeleVue obviously chose 67mm as the focal length for their “67mm Converter” to match the TV55 field stop, NVD aperture and Envis focal length. They left nothing on the table. What puzzles me is how they knew ahead of time to thread their TV55’s to accept this future retrofit. Very clever, NostraNagler!

 

The icing on the cake is that the TV67 40° apparent field of view is the same as the Envis 40° true field of view. As you probably already know, any eyepiece AFOV over 40° is wasted in Afocal. It’s just spillover outside the NVD sensor.

 

Let’s put this 0.396 reduction in context. Here’s a comparison...

 

 

Left to right is Prime with no reduction, Prime with Ackermann reducer/coma corrector (0.73 reduction) and Afocal with TV67 (0.396 reduction). Top row is with 102mm f/7 APO, bottom row is with 6-inch f/4 Newt. That’s a whole lotta reduction!

 

Now for the downside...Edge of Field drop-off.

 

Below are two photos Ray took of the Pelican Nebula (IC 5067 and IC 5070) in Cygnus. Both were taken with his 8-inch f/4 Newt and a 7nm Optolong H-Alpha filter. The one on the left is NV Prime with a 0.73x Ackermann reducer at f/2.9. The one on the right is NV Afocal with the TV67 at f/1.6.

 

 

Both are great photos, but the Afocal edge of field drop-off in the right photo is considerable. Whether this is a big deal is a matter of taste. Many don’t care about the edge, so long as the center is good. But Ray takes his edges “neat”.

 

Could the edge drop-off be due to field curvature? Below is a series of photos with a Lake Michigan breakwater at different locations on field. Top is the f/7 APO, bottom is the f/4 Newt. Both showed minor pincushioning. Both f-ratios show the same magnitude of pincushioning. No problem here.

 

 

My contention is that the reason for the drop-off is large exit pupils. Below, both photos are Afocal using the TV67. The gain is turned down to reduce image saturation and better show any drop-off. The left photo is with the 102mm f/7 APO, the right photo is with the 6-inch f/4 Newt.

 

 

The f/7 photo on the left has very little drop-off. The f/4 photo on the right has a lot of drop-off, very similar to Ray’s Pelican Nebula photo. Normally, a large exit pupil is a good thing. Indeed NV’s ability to go beyond the 7mm limitation of the human iris is a huge benefit.

 

The formula for exit pupil size is eyepiece focal length divided by objective f-ratio. With the TV67, the f/7 exit pupil is 9.57mm and the f/4 exit pupil is 16.75mm...a big difference! Keep in mind that an exit pupil is not a focal point. In effect, it’s an image of the objective.

 

Here’s a ray trace drawing that might help. It depicts the f/4 Newt with the TV67, Envis and Mod-3.

 

 

To keep things simple, I represent the TV67 and Envis as single lenses. Pay special attention to the dotted lines because they define the exit pupil. Although Envis is big enough for the full exit pupil, the TV67 eyepiece isn’t.

 

So, what can be done about the edge of field drop-off? Not much. TeleVue could conceivably design a dedicated 67mm 40° AFOV eyepiece for fast NV Afocal, but it would be big, heavy and very expensive. Also, drop-off isn’t a problem with slower scopes. All things considered, TeleVue’s TV67 is Afocal done right. Nearly everyone who uses it swears by it, and now you know why.

 

One more thing...Narrowband Filters and Afocal. This is an important topic and probably deserves its own report, but in a nutshell, don’t worry about it. For rear-mounted filters (see my post #69)...

 

https://www.cloudyni...position/page-3

 

Small ultra-fast objectives (most notably camera lenses) are the toughest for narrowband filters because of band shift. If you’re using a telescope that’s f/4 or slower, you’re good in Afocal for any barrel-mounted H-Alpha filter down to 3.0nm.

 

As you’ve seen in the lead photo at the top, the “visual” image is the widest TFOV that your NVD can see. This is regardless of any subsequent Afocal reduction. If you have an f/4 scope running Afocal at f/1.6, you don’t need a special f/2 filter. Light has no memory, so everything that happens after the filter is irrelevant. If your filter is okay “visually”, it’s okay with NV Afocal.

[GeezerGazer]

Congratulations Longbond!  Your post provides a great explanation (written and visual) of why some NV afocal systems show EoF darkening while others do not.  This should help anyone interested in attaining a full, clear, afocal FoV with no EoF darkening (vignetting) to achieve their goal.  Hat's off to you!   

Ray

[Mort H]

Hi longbond, interesting.

 

In addition to the field stop of the 55mm Plossl being maxed out at 50 degrees AFOV, which translates to 40 degrees AFOV once converted to 67mm (hence a great match to NV), I think the other main reason the TV 67 is 67 is that's what gives a 20mm exit pupil (again a great match to most NV devices including mine) when using an f/3 scope (i.e. the fastest scope generally available) with Paracorr (1.15x magnification, so f/3.45 effective).  67 divided by 3.45 being 19.4mm.

 

I didn't notice fall-off in my f/5 Dob, but I haven't logged a ton of time with the TV 67, Ray certainly has plenty of experience, and your testing is eye-opening.

 

Thought-provoking post, I'll pay better attention next time I look through my TV 67!

[Gavster]

As an afocal aficionado, this is a very interesting post to read. I have tried prime this year, but I just prefer the lovely bright detailed nebulae views given by afocal. As a result, I’ve recently sold my prime nv setup and will going forward just focus on using a pvs-14 in afocal mode. The mechanics of a pvs-14 are just so great as well!

I’ve noticed the edge fall off with the 67mm and wondered why it was happening. For visual the 67mm works really nicely but for phonetography the edge fall off is too great for me. Basically I use the 67mm for live visual observing and my 41mm panoptic for taking the phone images. The 41mm delivers great results all the way to the edge of the fov. I’ve noticed the 67mm edge fall off with all my scopes from f4 to f7. I will test it again sometime with my slowest refractor (f7) but I think phonetography brings out any edge fall off which is not obvious visually.

Edited by Gavster, 15 December 2021 - 03:02 PM.

[chemisted]

Longbond,  This is a very nice report.  I do have something to add.  First off, I do not have the 67 adapter for the 55mm so I can't make any comparisons with my equipment.  Instead, I use both the Russell Optics 65 and 85.  The point I would like to make is that, compared to my 55 & 65, the 85mm frees up backfocus for me that allows me to do more aggressive reduction.  I'll use my OGS RC-10 as my prime example (no pun intended).  It is native f/9.38 and with the 27TVPH coupled with either of the first two eyepieces afocally I achieve an excellent view at ~f/3.  With the 85mm, however, I can now screw my CCDT67 reducer to the 2" diagonal and drop down to a phenomenal f/2 with this long focal length telescope.  Stars and nebulosity are great right to the edge.  The FOV is larger of course since the focal length of the telescope has been reduced more with the CCDT67 than the 27TVPH.  I could detect no down sides. Ed

[Mort H]

Haha, threads like this remind me how many annoying aspects there are to the pursuit of phonography that are either not part of -- or not as bothersome for -- visual observation!  Makes me happy to remain a visual observer!

 

But I have to admit I sure do love looking at the very cool images taken by CN heroes GeezerGazer, Gavster, Jeff Morgan and others!  Even Joko has given in to the urge I see.  Maybe I will succumb too in time, though I doubt I would equal what I've seen posted to date!

[longbond]

Thanks, Ray. Your check's in the mail  . Thanks also for bugging me (nicely) to write it!

[longbond]

Be careful with those ginormous exit pupils, Mort! You've got a lot of holes to fit through.

[GeezerGazer]

Haha, threads like this remind me how many annoying aspects there are to the pursuit of phonography that are either not part of -- or not as bothersome for -- visual observation!  Makes me happy to remain a visual observer!

 

But I have to admit I sure do love looking at the very cool images taken by CN heroes GeezerGazer, Gavster, Jeff Morgan and others!  Even Joko has given in to the urge I see.  Maybe I will succumb too in time, though I doubt I would equal what I've seen posted to date!

I think the point is made that both visual observers and NV photographers (phone or full-frame) who want a full FoV without EoF darkening can get it with afocal if they understand the cause and parameters.  The two biggest issues surrounding EoF darkening have been explored in the last month or two.  There is now a formula (thanks to Longbond) that allows calculation of the clear FoV for any H-a filter bandwidth so that band shift EoF darkening can be avoided, and now, an understanding of how/why EoF darkening sometimes, but not always, occurs in afocal.  If EoF darkening does not impact an image enough to be a bother, then it does not matter.  But would anyone actually dislike a full-field image that does not show EoF darkening?  It can now be a choice.  As Gavster noted above, he prefers his 41mm in afocal for imaging... a happy circumstance that he found by trial and error.  Now he knows why it works while the 67mm used in the same optical system shows too much EoF darkening for his taste.  

 

Off topic:  Mort, you forget, I've seen your off-site photo gallery    which is remarkable and clearly demonstrates your skills with a camera.  It's not if but when you start taking NV photos, I'm sure they will not be equal to what's been posted... because they will be better.      I look forward to seeing the results of your skills.  

Ray

Edited by GeezerGazer, 15 December 2021 - 06:21 PM.

[longbond]

Off topic:  Mort, you forget, I've seen your off-site photo gallery    which is remarkable and clearly demonstrates your skills with a camera.  It's not if but when you start taking NV photos, I'm sure they will not be equal to what's been posted... because they will be better.      I look forward to seeing the results of your skills. 

True that, Ray! I've also seen Mort's photo gallery and he'll be on-board soon  Resistance is futile, Mort!

[longbond]

As an afocal aficionado, this is a very interesting post to read. I have tried prime this year, but I just prefer the lovely bright detailed nebulae views given by afocal. As a result, I’ve recently sold my prime nv setup and will going forward just focus on using a pvs-14 in afocal mode. The mechanics of a pvs-14 are just so great as well!

I’ve noticed the edge fall off with the 67mm and wondered why it was happening. For visual the 67mm works really nicely but for phonetography the edge fall off is too great for me. Basically I use the 67mm for live visual observing and my 41mm panoptic for taking the phone images. The 41mm delivers great results all the way to the edge of the fov. I’ve noticed the 67mm edge fall off with all my scopes from f4 to f7. I will test it again sometime with my slowest refractor (f7) but I think phonetography brings out any edge fall off which is not obvious visually.

Thanks, Gavster. I actually had you in mind while writing the report. Ray often raves about the quality of your photos, through an 11" SCT, I believe. Your work showed what can be done with afocal. When I saw how good my f/7 drop-off was compared to the f/4, that partially explained to me why your photos are good to the edge. Full discolosure...I actually subscribed a year to Astronomy Now to read your 5/19 article with Mark Catterall. Well played, sir!

[Mort H]

"Resistance is futile"...sigh...I might as well resign myself to the inevitable.

 

But in the name of preserving my sanity with my already ludicrously busy retirement packed full of hobbies, I resolve to stay on the sidelines as long as I can...!

[longbond]

Longbond,  This is a very nice report.  I do have something to add.  First off, I do not have the 67 adapter for the 55mm so I can't make any comparisons with my equipment.  Instead, I use both the Russell Optics 65 and 85.  The point I would like to make is that, compared to my 55 & 65, the 85mm frees up backfocus for me that allows me to do more aggressive reduction.  I'll use my OGS RC-10 as my prime example (no pun intended).  It is native f/9.38 and with the 27TVPH coupled with either of the first two eyepieces afocally I achieve an excellent view at ~f/3.  With the 85mm, however, I can now screw my CCDT67 reducer to the 2" diagonal and drop down to a phenomenal f/2 with this long focal length telescope.  Stars and nebulosity are great right to the edge.  The FOV is larger of course since the focal length of the telescope has been reduced more with the CCDT67 than the 27TVPH.  I could detect no down sides. Ed

Thanks, Ed. You are a reduction maniac   But it's actually not quite so maniacal if you're starting from f/9.38. As long as your reducer isn't constricting the eyepiece's field stop, you basically get a fast scope upon which you're doing afocal.

 

I agree with you on the Russell eyepieces. They're very nice and I like the Delrin bodies. Besides, where else can you find an eyepiece with a focal length that extends from here to Toledo? Just keep in mind that the AFOV is a teeny-tiny 23.9°.

[chemisted]

I am getting just over 1.5 degrees TFOV which is just perfect for a lot of targets.  This last summer M16 and M17 were phenomenal.  Thanks again for all your work and careful analysis.  I just wish the larger amateur astronomer population could wrap their minds around the extraordinary benefits that this technology provides.

Edited by chemisted, 15 December 2021 - 08:26 PM.

[GeezerGazer]

"Resistance is futile"...sigh...I might as well resign myself to the inevitable.

Indeed!  

[ButterFly]

Finally got a chance to read through it.  Lovely.

 

I find the prime pretty.  I look forward to taking my objective off the day the warranty expires.

 

 

To keep things simple, I represent the TV67 and Envis as single lenses.

There be devils in those details.

 

I have noticed the vignetting at 1x on a PVS14 with only the eyepiece adapter in place.  I never would have without taking it off while looking.  The filter adapter makes it even worse.

 

And then there's the very, very long barrel of the 67PP.  Using a diagonal without extension tubes makes a noticeable difference.  The diagonal body can really cut off the cone without an extension tube on the eyepiece.

 

What I find truly NostraNagler is the serendipitous ability of my screwed on filter wheel to come so close to that needed barrel length.  Every visual scope I stick it in, it comes to focus too.

 

 

 

So, what’s the big deal about the 67mm focal length? It’s because of two key ratios: Image Plane Reduction and Optical Reduction. If you’re going for perfection, the two ratios should match. Image Plane Reduction is basically the smaller NVD sensor target size compared to the larger 46mm field stop. The clear aperture of my Mod-3 sensor is 18.24mm dia. This ratio is 0.397 (18.24/46). Optical Reduction is the Envis focal length (26.5mm) divided by the 67mm eyepiece focal length is 0.396 (26.5/67).

Notice the equivalent way of looking at this.  You have a/b ~ c/d.  This is equivalent to a/c ~ b/d.  "a/c" here is 18.24 / 26.5 - the true field of view the sensor sees in radians!  The 67mm eyepiece just maximizes the true field, given a field stop of 46mm.

 

[Gavster]

Here’s a couple of sets of phone photos (all taken from very light polluted London) that show the considerable fall off of the 67mm in afocal mode compared to the 41mm panoptic. (The 55mm doesn’t show much fall off either but the edge stars go a bit linear particularly in faster scopes). The phone settings were the same in both cases so you can see how the 67mm gave much brighter views which is very useful for visual (but not so needed for phone photos).

 

First up is the rosette with my fsq85.

Second set is the horsehead with my c11 edge and 0.7x reducer.

 

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[longbond]

Notice the equivalent way of looking at this.  You have a/b ~ c/d.  This is equivalent to a/c ~ b/d.  "a/c" here is 18.24 / 26.5 - the true field of view the sensor sees in radians!  The 67mm eyepiece just maximizes the true field, given a field stop of 46mm.

I really like that, Professor Algebra! It does put a new spin on the equation equivalence. 18.24/26.5 = 46/67 =39.34°. Basically, the TV67 Afocal setup is an analogy for the Envis objective  I dig, man!

[longbond]

I am getting just over 1.5 degrees TFOV which is just perfect for a lot of targets.  This last summer M16 and M17 were phenomenal.  Thanks again for all your work and careful analysis.  I just wish the larger amateur astronomer population could wrap their minds around the extraordinary benefits that this technology provides.

Ed, for me it's night vision or NO vision . The only things that save a Chicago star party are planets and Night Vision (maybe EAA if the natives aren't restless ). Faint fuzzies through a regular eyepiece...fugettaboutit!

 

I too wish that the larger astronomer population gave NV a try. A steep price point, my Mod-3 cost twice of what my 16" Dob cost, but eminently worth it!

[longbond]

Here’s a couple of sets of phone photos (all taken from very light polluted London) that show the considerable fall off of the 67mm in afocal mode compared to the 41mm panoptic. (The 55mm doesn’t show much fall off either but the edge stars go a bit linear particularly in faster scopes). The phone settings were the same in both cases so you can see how the 67mm gave much brighter views which is very useful for visual (but not so needed for phone photos).

 

First up is the rosette with my fsq85.

Second set is the horsehead with my c11 edge and 0.7x reducer.

Beautiful photos! The 41mm Pan and its smaller 5.9mm exit pupil (I think) really cleans up well! In my report, mused on what a dedicated NV Afocal eyepiece might look like...big, heavy and expensive. Sound like a 41mm Panoptic? 

[rgk901]

I am getting just over 1.5 degrees TFOV which is just perfect for a lot of targets.  This last summer M16 and M17 were phenomenal.  Thanks again for all your work and careful analysis.  I just wish the larger amateur astronomer population could wrap their minds around the extraordinary benefits that this technology provides.

I am very new and way to poor for this, but am reading about NV but also EAA and AP to supplement my visual... eventually.

 

At this point at  $3-5,000+ I think I'd prefer a AP setup giving me the ability to EAA and do long exposure to hang pretty pictures on my wall. BUT I can see owning both NV for the quick views and AP for the pretty on the wall views.

 

I don't think wrapping one's head around NV is the problem.. for me at least, it's the cost, for now. I'm sure it will fall in price and up in quality as things progress as with anything.

 

For now I am enjoying what I have and look at all the cooler stuff on here

 

Thanks for all the hard work, this will make things much easier for everyone coming into the NV.

[longbond]

I am very new and way to poor for this, but am reading about NV but also EAA and AP to supplement my visual... eventually.

 

At this point at  $3-5,000+ I think I'd prefer a AP setup giving me the ability to EAA and do long exposure to hang pretty pictures on my wall. BUT I can see owning both NV for the quick views and AP for the pretty on the wall views.

 

I don't think wrapping one's head around NV is the problem.. for me at least, it's the cost, for now. I'm sure it will fall in price and up in quality as things progress as with anything.

 

For now I am enjoying what I have and look at all the cooler stuff on here

 

Thanks for all the hard work, this will make things much easier for everyone coming into the NV.

Keep reading, fellow Chicagoan! It might not happen now, but NV hope springs eternal. This is Night Vision territory, but there's a lot to be said about EAA if you've got a camera, laptop and tracking mount.

 

If you're looking into Astrophotography, I can't think of a more inspirational story than Chuck Ayoub's YouTube channel, "Chuck's Astrophotography". He's an average Joe, does great work and his videos show the thrill of victory, as well as the agony of defeat.

 

https://www.youtube....strophotography

 

In just two years, I've watched this guy go from total beginner to someone who does APODs (Astronomy Pictures of the Day) for NASA. All this while living near Detroit Metro Airport!

 

Unfortunately, it might be a long wait until NV prices come down. As economist John Maynard Keynes said, "Markets can stay irrational longer than you can stay solvent." As long as NV devices are in the government domain, prices will likely remain high for a long time. But stay tuned...there are some guys here that have some great advice on how to get into NV on a budget.

[rgk901]

Thanks, all in good time... between the couple days a month of clear weather and all the other things fighting for my attention, a quick visual peek is all I realistically have time for

 

Still fun to learn for future possibilities.

[rgk901]

ut

" As long as NV devices are in the government domain, prices will likely remain high for a long time. But stay tuned...there are some guys here that have some great advice on how to get into NV on a budget.

I will say that I'd much prefer NV to AP, the possibility of just throwing the tube into my backpack next time I'm out at Glacier or something would be incredible.

 

If the tech comes to the the private domain and things become more standardized and more options availability and price lowers the barrier to entry that will give wven EAA a run for its money it could spark a renewed 'visual' obsving Renaissance??
 

At $1,500 for white phosphorus with decent specs .. never be able to keep them in stock?!

 

Maybe the big boy telescope makers would enter the market if they eventually see the potential for more sales.

 

Very interesting developments for sure.

 

Keeping an eye out for sure.

[scoale]

Very interesting.  I have not noticed edge of field drop off, but that is probably due to the fact that I don't (yet?) do phonetography and I haven't yet experienced super high exit pupils.  Using my PVS-14 and Televue 67 with my C11 at F/7.5 (AP .75 reducer) and my 5" refractor at F/7, I think the largest exit pupil I've run is around 9.5.  However, that is about to change.  This week I received my 127 NPis which is native F/5.2, so that will afford me the opportunity to achieve larger exit pupils and brighter images. Using the TV 67 in afocal will put me around 12.9.  I do own a 41 panoptic, so I will definitely compare the views.  Ultimately, as a visual observer, I suspect I will prioritize brightness over full field - to a point.

 

Question - is there a way to calculate at what exit pupil edge of field drop off begins?

 

Thanks for a great post!