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Band Shift and Filter Position

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#1 GeezerGazer

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Posted 10 June 2021 - 08:04 AM

Last summer, Eric (CNer Longbond) and I became acquainted while trying to solve some NV adapter problems and we have been corresponding ever since, mainly concerning H-a filtration with NV.   Last fall, he started using an unmounted, 24mm diameter filter inside his Mod 3C with C-mount lenses.  Thus began a study of how REAR mounted filters, those which are mounted behind the optical system, effect the image that is ultimately presented at the NVD ocular. 

 

Front mounted H-a filtering (also called first surface) is quite rare when used with a telescope.  It is limited by the number of lenses or optics that can accommodate 1.25” and 2” filters without causing significant mechanical vignetting (a smaller filter covering a larger aperture).  The Envis lens, and shorter focal length C-mount or camera lenses and just a few astro lenses like the Askar 180, can accommodate front mounted 2” filters.  Custom filters in larger sizes become exorbitantly expensive. 

 

We know that H-a images we see or photograph through our NVDs are sometimes effected by band shift, the optical aberration which causes a loss of filter performance (H-a contrast) at the EoF.  Band shift is caused when light rays passing through an H-a filter encounter steeper angles of incidence as they pass through steeper angles of glass at the edge of a lens. The faster the optical focal ratio, the steeper the angle of incidence, resulting in the filtering process being pushed out of phase at the EoF.

 

We also know that band shift severity is effected by the pass band width of the filter and the focal ratio of the optical system.  But a question arose in our communications… how does filter placement in the optical train effect band shift and filter performance?  Eric and I both wanted to know how filter performance was effected when the filter was placed after the optics, but in front of the NVD, in what we call a rear-mounted position… at what point do rear mounted filters become most effective in use.  We started testing in January. 

 

Test #1:  This is a comparison photo using a 50mm Nikon camera lens at f:1.4 with a 7nm filter of the Rosette Nebula from my light polluted, red-zone home.  The image on the left was taken with the filter rear-mounted (behind the lens) while the image on the right was taken with the filter front-mounted (in front of the lens).  This photo presents the same filter in both mounting positions with exactly the same camera settings of ISO 100, 1s exp, 10s average in NightCap.  On axis, the front mounted filter is significantly superior to a rear mounted filter, as was expected.  

 

7nm back & front mount on N-50mm.jpg

 

The same thing happened with the 3.5nm filter, taken the same night at ISO 250, 1s exp, 10s ave.  Again, the filter was rear-mounted for the left photo; the right photo was front mounted.  It is obvious that front-mounted filters provide better H-a contrast when the subject is on-axis.

 

3.5nm back & front mount on N-50mm.jpg

 

Now look at both comparison photos.  The front-mounted filter images (on the right in both comparisons) look correct, with the 3.5nm filter showing more H-a contrast than the 7nm.  But the rear-mounted images (on the left) show a tiny bit better contrast with the 7nm filter.  The 3.5nm filter performed worse than the 7nm filter when rear-mounted with this optical system.

 

Test #2:  In the next photo, the 7nm filter was rear-mounted on the 50mm lens at f:1.4 and shows the Rosette centered and at the very edge of field, with no significant loss of contrast from band shift, ISO 250, 1s/10s average.  So although H-a contrast is lower with a rear-mounted filter, there is little or no loss of the H-a subject near the EoF from band shift.  When I performed this test with the 3.5nm rear mounted, results were the same; no EoF drop-off in H-a contrast/brightness. 

 

 

7nm center:edge.jpg

 

Test #3:  The next night I repeated the first two tests but used a 300mm Nikon at f:2.8.  The difference was less EoF darkening (alternately referred to as “spotlight effect,” or “vignetting”) using the slower optic.  But using the 3.5nm filter and the slower optic with the rear-mounted 3.5nm filter, did show the H-a object better, with more contrast.  This photo compares the contrast between the 3.5nm (on the left) and the 7nm filter (right) at f:2.8:

 

3.5:7nm w:N300.jpeg

 

Using the 50mm lens again, I took two photos of the NAN/Gamma Cygni complex using a rear-mounted 3.5nm filter, the first image at f:1.4, ISO 500, 1s exp, 15s average, and the second at f:2.8, ISO 800 to compensate for the slower focal ratio, to show the significant improvement in H-a contrast using the slower focal ratio with the same rear-mounted filter: 

 

IMG_2065.jpeg

 

IMG_2066.jpeg

 

The results of these tests show that both front and rear-mounted narrowband H-a filters present disadvantages.  Front-mounted (first surface) filters show H-a with greater contrast on-axis, but create significant EoF darkening with a substantial loss of contrast when the H-a subject approaches the EoF, where H-a filtering is pushed out of phase with fast optics.  Rear-mounted filters reveal H-a all the way to the EoF, but reveal the H-a subject with less overall contrast.   Concerning this issue, Eric wrote:  “With front-mounted filters, band shift is EXPLICITLY localized because the angle of incidence is dependent on the object’s location relative to the optical axis. However, rear-mounted filters are affected by the angle of incidence from the light cone. Even an object in the center of the field gets MOST of its light from the steep part of the light cone.  If the edge of the light cone becomes too steep, you lose s/n.”  The effects of band shift are still present with rear mounted filters, but instead of a high contrast image at the center FoV, the filter spreads the effects of EoF contrast loss across the entire FoV. 

 

Since most of our NV filtering places the filter behind the optics, you may find that a narrower band filter performs no better than a wider pass band filter, because the optical system may be TOO FAST.  Generally, the shape of the light cone in faster optics creates limits for the application of H-a filters.  The wider the pass band, the less concern there is about phase shift; the narrower the filter pass band, the more you may need to pay attention to the optical focal ratio.  In addition, the relative aperture of the optical system, as compared to the sensor size, seems to have a bearing on filter performance. 

 

Thankfully, most telescope systems are slower and handle rear mounted H-a filtering very well with little or no EoF H-a drop-off.  I am content with the performance of my Newt at f:2.8, with a 7nm filter.  When I use the Newt at f:4, I may choose the 3.5nm filter.  But when using a faster optic in prime focus, like an f:1.4 or f:2 lens, a narrower 3.5nm filter performs no better (or even worse) than a 7nm filter. 

 

This study did not involve afocal testing with a rear-mounted filter, as I do not have a 24mm filter to use inside the C-mount of my Mod 3C.  But my limited experience with afocal has revealed that significant EoF darkening is usually present when a narrow band filter (such as a 3.5nm) is mounted directly to my TV 55/67 eyepiece in my 8" Newt at f:2.8. 

 

Understanding how filter placement in the optical system effects performance may help those who are considering the purchase of a narrower band (~3nm) filter.  If the optical system is too fast, a 3nm filter may not perform well.  On the other hand, if the optical system is too slow, increased scintillation/noise will result.  Between f:2.8 and f:5.6 seems to be a sweet spot for these very narrow filters when used rear-mounted.  When used front-mounted, the fastest optical systems do provide best H-a contrast on-axis, with the caveat that a bigger portion of the FoV will be pushed out of phase than when using a 5-7nm filter.  . 

 

Much thanks to Eric for his help with this study and in editing this material.  This effort was a true collaboration.


Edited by GeezerGazer, 10 June 2021 - 07:06 PM.

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#2 GeezerGazer

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Posted 10 June 2021 - 08:20 AM

Another test that was completed may be more revealing than the above photos.  Using the 50mm Nikon (having a 20° FoV) with rear-mounted 3.5nm filter, I took 5 photos at f:1.4, f:2, f:2.8, f:4 and f:5.6 of Sh2-27, a very large (10°) but dim H-a subject.  All exposures were 1s, averaged for 15s in NightCap.  ISO was increased in incremental steps to compensate for smaller apertures; at 400, 640, 800 and 1600.  Compare the following photos, which show how H-a contrast increased with slower focal ratios when using a 3.5nm filter.  There was very little difference between f:2.8 and f:4.  I saw no difference between f:4 and f:5.6; the images looked identical, so I’ve not posted the f:5.6 photo:  The first photo is at f:1.4:

 

IMG_2064.jpeg

 

below, f:2

IMG_2063.jpeg

 

below, f:2.8

IMG_2062.jpeg

 

below, f:4

IMG_2061.jpeg

 

 


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#3 a__l

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Posted 10 June 2021 - 08:59 AM

Based on your experiments, 75 mm Fujinon f/1.8 (front filter) and ~ 13 degrees is clearly better than 3x f/1.4 (rear filter) for h-Alfa objects.
Actually, this was clear from the theory.



#4 GeezerGazer

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Posted 10 June 2021 - 10:41 AM

Based on your experiments, 75 mm Fujinon f/1.8 (front filter) and ~ 13 degrees is clearly better than 3x f/1.4 (rear filter) for h-Alfa objects.
Actually, this was clear from the theory.

None of these tests involved the Fujinon C-mount lens.  But you are correct; front mounted (first surface) filtering is best at showing the most contrast between the H-a subject and the sky.  The tradeoff is EoF darkening from band shift, which can be onerous with front mounted filters. 



#5 longbond

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Posted 10 June 2021 - 11:47 AM

Excellent report! The photos effectively isolate the important variables and clearly show their effect. The front-mount filter photos are a great visual reference standard for the 50mm Nikon's FOV.

 

The rear-mount photos are pretty devastating at 50mm when compared to the front-mount. However, your stop-down photos nicely show the trade-off between f-ratio and filter performance. If you're willing to stop-down and increase exposure time, you can get a good photo with a 3.5nm filter. It's one thing to state an effect, but it's something else to see it so clearly.

 

You make a very important point, "In addition, the relative aperture of the optical system, as compared to the sensor size, seems to have a bearing on filter performance." This explains why camera and C-mount lenses often have problems with rear-mounted narrowband filters. With telescopes, simple f-ratio angle of incidence formulas can give a good approximation of filter performance. However an NVD 18mm sensor is fairly big compared to a camera lens aperture. This increases the angle of incidence to the point where some ray tracing becomes necessary.

 

Your inclusion of Nikon 300mm f/2.8 (Test #3) is very important because this is the a gateway between camera lenses and actual telescopes. Here, the 300m Nikon f/2.8 has clear aperture 3x the size of the 50mm Nikon f/1.4 and this reduces the effect of the sensor size on angle of incidence.


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#6 GOLGO13

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Posted 10 June 2021 - 01:43 PM

I’m not technical enough to understand everything, but I wonder if something similar to hydrogen alpha solar scopes could help with this issue. They put tilt wheel to adjust the etalons. No idea if that is at all similar to this problem.
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#7 longbond

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Posted 10 June 2021 - 02:57 PM

I’m not technical enough to understand everything, but I wonder if something similar to hydrogen alpha solar scopes could help with this issue. They put tilt wheel to adjust the etalons. No idea if that is at all similar to this problem.

Economics and FOV aside, your suggestion does make sense in principle. I have two tilt-tuned solar scopes and the tilt ring adjusts the center wavelength to get the "sweet spot". Some filter makers address the band shift problem by pre-shifting the center wavelength. One of those is Astronmik's MaxFR, which I'm testing right now with H-Alpha spectrum tubes.


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#8 maxmir

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Posted 11 June 2021 - 08:23 AM

A 12nm Ha might be the way to go for fast rear mounts under F3.

Also Badder has filters tuned to fast optics in the F2 range.



#9 chemisted

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Posted 11 June 2021 - 01:13 PM

A 12nm Ha might be the way to go for fast rear mounts under F3.

I use a 12nm filter pretty much always.  The Astrononik literature says it is good from f/infinity to f/1.6 and this has been substantiated by my use both with camera lenses and telescopes down to f/1.56.



#10 PEterW

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Posted 12 June 2021 - 07:03 AM

The impact depends on the actual design of the filter, though narrower will get hit worse. The greater the off axis angle the greater the shift, so rear mounting (when we want very steep/fast) light cones should be worse. But by playing about we should be able to find combinations that still work well… which others can then use… it’s great having so many people testing stuff out!

Peter
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#11 GeezerGazer

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Posted 12 June 2021 - 10:32 AM

The impact depends on the actual design of the filter, though narrower will get hit worse. The greater the off axis angle the greater the shift, so rear mounting (when we want very steep/fast) light cones should be worse. But by playing about we should be able to find combinations that still work well… which others can then use… it’s great having so many people testing stuff out!

Peter

There may be additional information concerning this subject matter... see the last sentence in post #7.

 

Although it is a topic beyond the subject of this thread, looking at the last two images of post #1 and the images in post #2, it appears that star bloat with H-a filtering, may be associated with fast optical focal ratios combined with very narrow band filters (~3nm) when using rear-mounted filters, but perhaps front-mounted filters as well.  The images in this thread that were made with faster focal ratios show substantially more star bloat than those which were made with a slower focal ratio.  There is a separate thread on this issue, so someone who has time may want to test the basis of my observation and report in that thread.  If true, it might explain why star bloat is sometimes present with a specific lens... thus providing perfect results with a wider pass band filter or slower optical focal ratio.  It seems likely to me that star bloat is not strictly caused by the optics, but by the combination of optics and H-a filter at specific focal ratios.  The issue deserves further evaluation.  I can imagine some pretty simple tests.  

 

We won't know until someone with a 3-3.5nm filter tests the results in a couple of lenses at f:1.4 to f:4 against the same lenses using a wider (6-7nm) filter.  I'd also take a photo of an H-a subject at f:1.4 using a 3nm filter, and then remove the filter and take an image of the same star field to see if star bloat remains constant... this would show if it is the speed of the optic or the combination of the optic/filter, that is causing the bloat.  

Ray


Edited by GeezerGazer, 12 June 2021 - 10:50 AM.


#12 Jeff Morgan

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Posted 13 June 2021 - 12:47 PM

A 12nm Ha might be the way to go for fast rear mounts under F3.

Also Badder has filters tuned to fast optics in the F2 range.

 

For the last five years I have been a big advocate of the 12nm filter. Quite simply, it is never a bad choice.

 

I realize there is a great variance in preference on what comes beyond 12nnm. 7nm has been workable for me, but I got the itch and bought the Antlia 3.0 nm. So far I have been less than pleased with it, though this thread suggests there is somewhat of a learning curve to its best employment. My early results support f/2.8 and slower for this tight of a bandpass. I'll give it a little more time before I divest. I should add that this is front-mounted, I don't have the capability to rear-mount a 2" filter.

 

Another item I want to check on this 3nm filter: transmission. Reading up on narrow band filters it seems that the difference between excellence and low cost is not the bandpass per se, but the percentage transmission at the desired wavelength. A great filter is passing well into the 90's, whereas a cheaply made one can be 80's or even 70's!

 

I am starting in CMOS imaging and went all-in for Chroma filters, including 3nm Ha. They are 36mm unmounted, but I think that a side-by-side test of some sort can be managed between the Antlia and Chroma.

 

WRT filter position, I have found another factor that is limiting for me (in terms of ergonomics): Lens focal length. After a year of using a 300mm f/2.8 I found it to be very awkward. The last nail in the coffin was the release of the Tele Vue 67 conversion lens. It gave smaller refractors a decided advantage - they can now reach "down" to the focal lengths of the large SLR lenses. And refractors are more easily mounted and have a good solution for right-angle viewing.

 

With all that being said, I am using SLR lenses no larger than 135mm f/2.8 (and I might even trade that down to 100mm). All of these lenses can accommodate front-mounted filters with no vignetting using the appropriate step-down ring.


Edited by Jeff Morgan, 13 June 2021 - 12:50 PM.

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#13 GeezerGazer

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Posted 14 June 2021 - 10:50 AM

Jeff,

I agree that camera lenses are more awkward because they limit to straight-through observing, rather than the more comfy view through a diagonal.  And larger lenses are a bit difficult to mount.  I put a 6" Vixen size dovetail on my 300 to solve that issue and divested of the 180mm.  What remains are my 50, 105, 135 and 300mm Nikons.  But I also really like the portability of the lenses for low power observing and the bonus is that the smaller lenses can be used with front OR rear mounted filters.  

 

The tests in post 1 show that front mounted filters do hold the promise of better contrast on-axis and I have found this to be true in my experience with prime focus.  For instance, Sh2-27 simply shows more contrast when using a front mounted filter... making it easier to see or photograph.  The trick then becomes knowing its size so it will fit within the chosen optic's FoV that is not encumbered by EoF darkening from band shift; there is a decent photo of Sh2-27 in my gallery having used a front-mounted filter on a 50mm lens.  The 50mm lens has a ~20° FoV, but loses about 40% of it's FoV to EoF darkening from band shift. Sh2-27 covers ~10° so it fits in the fully filtered FoV.  I feel that using a front mounted filter is preferred whenever it is possible in prime.

 

The tests in post #1 revealed that there are trade-offs when using a rear-mounted filter, which is what most of our optical systems require.  I seldom use an afocal system and my tests did not cover afocal H-a filtering.  But it is clear to me that a rear-mounted filter does reveal less contrast than front mounted filters where parallel rays enter the filter... rather than converging rays.  But the BENEFIT of rear-mounted filters, with the H-a subject being visible to the very edge of FoV, may be of enormous value to us... especially in afocal.  

 

The following is speculation on my part, but my thoughts are based on what I have learned about H-a filtering.  Someone needs to do afocal tests to confirm, but please read on.

 

Although rear-mounted filtration contrast may be subdued compared to a front-mounted filter, with rear-mounted filters, the benefit of seeing the H-a subject right to the EoF is really important.  I know this to be true in my prime phonetography.  In afocal, as Gavster has pointed out in PMs, the reduction of focal ratio from a long FL eyepiece like the 67mm, does not impact H-a filtration.  In afocal, the filter is receiving the light cone directly from the optical system as long as the filter is attached to the bottom of the eyepiece.  So if you are using an f:7 scope with the 67mm eyepiece in afocal, the rear-mounted filter is receiving the f:7 light cone, not the 67's f:2.8 light cone.  I do not know what effect the 67mm eyepiece, with its focal ratio reduction, has on the filtration process. Nor do I know how the system would react with a filter placed inside the C-mount cavity... AFTER the 67mm eyepiece, but before the NVD.  I would guess that inside the C-mount, after the eyepiece, the light cone would be even steeper, causing more EoF darkening.  But that is not where afocal H-a filtering typically takes place in an afocal system.  

 

This means that an afocal system could potentially benefit more from rear-mounted filtering than a prime system... as long as full-sensor illumination is maintained and EoF aberrations are controlled, afocal might perform much better with a 3nm filter.  My tests showed that a 3nm filter performs best when rear-mounted with a slower focal ratio light cone of at least f:2.8, but better at f:4 or f:5.6   That combination fits many of our scopes without using reducers which usually introduce optical aberrations anyway.  Does that make sense? 

 

I'm out of state and won't have access to my gear for a couple of weeks, but anyone should be able to test this issue.  I would be using an ED doublet f:7 refractor in afocal, so nothing special.  

Ray



#14 chemisted

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Posted 14 June 2021 - 06:29 PM

For the last five years I have been a big advocate of the 12nm filter. Quite simply, it is never a bad choice.

 

WRT filter position, I have found another factor that is limiting for me (in terms of ergonomics): Lens focal length. After a year of using a 300mm f/2.8 I found it to be very awkward. The last nail in the coffin was the release of the Tele Vue 67 conversion lens. It gave smaller refractors a decided advantage - they can now reach "down" to the focal lengths of the large SLR lenses. And refractors are more easily mounted and have a good solution for right-angle viewing.

 

With all that being said, I am using SLR lenses no larger than 135mm f/2.8 (and I might even trade that down to 100mm). All of these lenses can accommodate front-mounted filters with no vignetting using the appropriate step-down ring.

 

Jeff,

I agree that camera lenses are more awkward because they limit to straight-through observing, rather than the more comfy view through a diagonal.  And larger lenses are a bit difficult to mount.  I put a 6" Vixen size dovetail on my 300 to solve that issue and divested of the 180mm.  What remains are my 50, 105, 135 and 300mm Nikons.  But I also really like the portability of the lenses for low power observing and the bonus is that the smaller lenses can be used with front OR rear mounted filters.  

I am using a Nikkor 300mm f/4.5 camera lens that is half the weight of the 300mm f/2.8 and rides beautifully on a lightweight tripod.  I am doing this at f/2.7 because I use a Celestron 45mm Plossl afocally with this lens and my NVD Micro (LINK).  Thus far I have used only the 12nm Astonomik filter that screws onto the Plossl and generally suits my dark skies.  Following the theme of this thread I could try my setup with a 7nm Optolong filter instead to report back on the comparison.  I would probably want to do this with some moonlight present to give some semblance of light pollution where the narrower filter should show its stuff.  Let me know if this is worth doing and I will pursue it.

 

Incidentally, I also use this setup with my home built 90 degree diagonal with great success (LINK) so if one is so inclined this is the equivalent of a 180mm focal length refractor with comfortable 90o seated viewing.



#15 Mike Lockwood

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Posted 15 June 2021 - 12:26 PM

The geometry for light coming into the front of a lens is "simpler" than that for the light behind the lens.

 

Every lens is different, so I can't make any definitive statement about what happens between the back of the lens and the sensor, but this slide may give you a little bit of an idea of some lenses, see page 12:

 

http://graphics.stan...ures/camera.pdf

 

So, all of this depends on the 1) lens and its design and geometry, 2) filter design and how it performs for off-axis sources, and 3) to some extent how the lens coatings perform across the spectrum and off-axis.

 

All of this together makes this a difficult issue to make a definitive statement about.

 

For the case of afocal use, as I said in my article/blog some time ago, the best place for the filter is usually on the bottom of the eyepiece because the light cone rarely is faster than f/2.8 or so.


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#16 GeezerGazer

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Posted 15 June 2021 - 09:54 PM

Following the theme of this thread I could try my setup with a 7nm Optolong filter instead to report back on the comparison.  I would probably want to do this with some moonlight present to give some semblance of light pollution where the narrower filter should show its stuff.  Let me know if this is worth doing and I will pursue it.

 

I think you should Ed.  Testing helps us understand limitations.  

 

So, all of this depends on the 1) lens and its design and geometry, 2) filter design and how it performs for off-axis sources, and 3) to some extent how the lens coatings perform across the spectrum and off-axis.

 

All of this together makes this a difficult issue to make a definitive statement about.

 

I'm really glad you chimed in Mike, and I understand your point about different lenses and optical systems.  I did conduct the rear mounted filter tests with my 8" Newt at f:2.8 (using a reducer/corrector) and at native f:4 finding the same results as with the camera and C-mount lenses.  With the filter rear-mounted, there was no loss of H-a subject at the EoF from band shift... even with the 3.5nm filter.  Of course, I cannot conduct a front-mounted filter test on the larger optics, but the rear-mounted filter tests have all been consistent so far.  

 

The 3.5nm filter when rear-mounted, started to show less contrast as the focal ratio became faster than f:4 in the camera lenses, but was still satisfactory to me at f:2.8.  It was at f:2 and f:1.4 where the narrow 3.5nm really became unsatisfactory concerning the loss of H-a contrast.  I don't have larger optics that fast to test my findings.  It would be great if someone does have an f:1.8 or f:2 optical system to test the 3nm or 3.5nm rear-mounted filter.  

Ray


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#17 Jeff Morgan

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Posted 16 June 2021 - 12:20 AM

For those interested, the aperture of the a SLR lens is related to its speed. For two identical focal lengths (say, 135mm) a f/2 lens must have a larger aperture than a f/2.8 lens.

 

I get that some people want to use large lenses. I've done it myself and have glued filter threads into my Canon to C adapter.

 

But if you want to stay with front-mounted lenses, it puts a definite limit on what works with common M48 filters.

 

 

 

 

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#18 AstroCatinfo

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Posted 17 June 2021 - 05:47 PM

I have already recieved my long awaited ASI2600MM, that will be paired with Sigma Art 85mm f/1.8 and Antlia 3nm Ha. I also got an Optlong 7nm Ha from my dealer in order to be tested.

 

I will do some tests and let you know how this fast system works with Antlia 3nm Pro Ha. My intention is to close it at 3.2, but I can test it at native apperture.

 

For broad band I have a full set of Astrodons E series LRGB 36mm.

 

Regards,

 

Aleix


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#19 GeezerGazer

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Posted 17 June 2021 - 10:35 PM

I have already recieved my long awaited ASI2600MM, that will be paired with Sigma Art 85mm f/1.8 and Antlia 3nm Ha. I also got an Optlong 7nm Ha from my dealer in order to be tested.

 

I will do some tests and let you know how this fast system works with Antlia 3nm Pro Ha. My intention is to close it at 3.2, but I can test it at native apperture.

Aleix

Aleix, it would be great if you test both the 3 & 7nm filters with an f:1.8 lens; running the test at f:1.8 and then at a slower focal ratio, perhaps f:2.8 or f:4, with equal exposure settings but adjusting ISO enough to compensate for the slower focal ratio would be of significant value.  


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#20 chemisted

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Posted 26 June 2021 - 04:01 PM

I am using a Nikkor 300mm f/4.5 camera lens that is half the weight of the 300mm f/2.8 and rides beautifully on a lightweight tripod. I am doing this at f/2.7 because I use a Celestron 45mm Plossl afocally with this lens and my NVD Micro (LINK). Thus far I have used only the 12nm Astonomik filter that screws onto the Plossl and generally suits my dark skies. Following the theme of this thread I could try my setup with a 7nm Optolong filter instead to report back on the comparison. I would probably want to do this with some moonlight present to give some semblance of light pollution where the narrower filter should show its stuff. Let me know if this is worth doing and I will pursue it.

Incidentally, I also use this setup with my home built 90 degree diagonal with great success (LINK) so if one is so inclined this is the equivalent of a 180mm focal length refractor with comfortable 90o seated viewing.

I think you should Ed. Testing helps us understand limitations.

Ray

Last night I had a chance to compare the 12nm and 7nm filters with the one-day-past-full moon rising in the east. I focused on the showpiece nebulae in Cygnus and alternated the filters screwed onto the 45mm Plossl. I can say the 7nm showed a bit more detail but there were problems, that for me personally with my viewing standards, caused me to like the 12nm filter better. The 7nm had a falloff of both nebula intensity and star brightness at the very edge of the field of view that did not happen at all with the 12nm. Also, my personal preference is to see as many stars mingled with nebula as possible since many of these structures are star factories. As expected, lots of stars are lost in going from the 12nm to the 7nm. This is all quite subjective and I am sure YMMV but, again for me, the 12nm will remain my go to filter for most all nebula viewing sessions.

I used my diagonal while doing this comparison. This arrangement with the Nikkor 300 f/4.5 is really comfortable and the ease of instantaneous setup is a definite plus. I am totally glad I took the time to develop it.

Edited by chemisted, 26 June 2021 - 04:08 PM.

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#21 joelin

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Posted 04 July 2021 - 11:50 AM

thanks Ray for sharing all the extensive research and findings here!!

 

a few thoughts/questions I had

1) everything here only applies to Ha filters right? so long pass filters wouldn't be affected? what about a long pass filter around 650nm which is close to the Ha line?

2) it seems to get the best contrast and see the faintest possible Ha, having the front mounted is the way to go, this is easy for 1x and for 2x (i.e Computar V5013 50mm F/1.3 C-Mount) its doable with the 46-48mm step up ring and 2" filter

3) now the question is what to do with the 3x night vision magnifier. the only way i know is to mount the Ha filters in the back....and mounting a 2" filter on the front would constrict the aperture....?


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#22 GeezerGazer

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Posted 04 July 2021 - 04:04 PM

Joe, yes, my testing was specific to H-a filtration.  Long Pass filters do not suffer band shift in any measurable way, in my experience.  My 640nm Lumicon Deep Sky filter displays no band shift.  I have used step down rings w/2" H-a filters on both my Computar 50mm and the Nikon 50mm with good results as long as the H-a subject is small enough (angular size) to fit within the clear aperture of the system... where H-a contrast drop-off does not occur.  If the H-a subject is bigger than the clear aperture of the H-a filtered system, then I move to a rear-mounted filter which shows less overall contrast, but has no drop-off at the EoF.  

 

In fairness, I am not fond of the 3x lens and sold mine a couple of years ago.  Some of the 75mm C-mount lenses provide an image equal to the 3x, but to filter these lenses in prime requires an unmounted 24mm H-a filter that fits within the C-mount cavity of the Mod 3C. This method of filtering is useable with all C-mount lenses (except the Envis which has a substantial rear protrusion which could break the small filter), all camera lenses, and all telescopes.  Although it might not be as convenient as a front mounted filter, it is much less expensive than using 2" filters.  I have done no testing with an afocal system to know how filter position effects its performance.

 

Yes, mounting a 2" filter on the front of the 3x lens will cause mechanical vignetting in addition to the potential for band shift with H-a filters, depending on the filter's bandwidth.  This is not to say you cannot get a decent visual image with such a system, but you should expect to see EoF darkening.  

Ray



#23 GeezerGazer

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Posted 04 July 2021 - 04:47 PM

Last night I had a chance to compare the 12nm and 7nm filters with the one-day-past-full moon rising in the east. I focused on the showpiece nebulae in Cygnus and alternated the filters screwed onto the 45mm Plossl. I can say the 7nm showed a bit more detail but there were problems, that for me personally with my viewing standards, caused me to like the 12nm filter better. The 7nm had a falloff of both nebula intensity and star brightness at the very edge of the field of view that did not happen at all with the 12nm. Also, my personal preference is to see as many stars mingled with nebula as possible since many of these structures are star factories. As expected, lots of stars are lost in going from the 12nm to the 7nm. This is all quite subjective and I am sure YMMV but, again for me, the 12nm will remain my go to filter for most all nebula viewing sessions.

Hi Ed.  Thanks for taking time to conduct the comparison and to render your thoughts about your afocal system.  I know you live under pretty dark skies, so your preference does not surprise me.  There are so many variables with sky/observing conditions, equipment, and personal preferences, that it is difficult to find a singular H-a filter system for recommendation.  Having more opinions about preferences does provide insight though.  

 

Long ago I took 4 photos of the Lagoon Nebula, using a 12 & 7nm H-a, a 640 long pass and no filter at all.  I sent these 4 unlabeled images to Gavster and Moshen to see which they preferred.  One preferred the additional H-a revealed by the 7nm, almost making the H-a appear as a solid material; the other preferred the 12nm image which showed the H-a as more of a translucent gas with a better rendition of the stars associated with the nebula.  I understand and appreciate both viewpoints.  

 

Regarding your findings, where the 12nm filter shows no drop-off in H-a contrast at the EoF, but the 7nm does... this is a bit surprising to me.  But again, I have not tested afocal filter placement.  The light cone hitting your 7nm filter is at f:4.5 from the 300mm lens and you see EoF drop-off in both brightness and H-a contrast... where none is visible using the 12nm filter.  This may be the result of the afocal system.   We need more testing.  


Edited by GeezerGazer, 04 July 2021 - 04:53 PM.

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#24 ButterFly

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Posted 04 July 2021 - 06:42 PM

1) everything here only applies to Ha filters right? so long pass filters wouldn't be affected? what about a long pass filter around 650nm which is close to the Ha line?



It's an issue with narrow interference filters. As a simple toy model, interference filters are several stacked layers. Light reflects off all the layers, with the spacing determining whether bounces from the different layers are in phase or out of phase. As the several layers tilt with respect to the incomming light, the spacing "looks wider" to the incomming light. Those "wider looking" layers are tuned to a different wavelength, so the band shifts.

When the band is wide, the huge band still passes the shifted band. The 3nm h-alpha has very noticable vignetting at 1x. The 12nm has essentially none at 1x. The Astronomik 642 is an interference filter with a huge band pass from about 640-850 nm. A band that huge passes huge band shifts also. A 3nm Silicon (SII) filter centered around ~670nm line will work similarly to a 3nm h-alpha centered around the ~650nm line. If you happen to have an SII filter, try it out.
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#25 joelin

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Posted 06 July 2021 - 07:03 AM

A thought I had: for slower optics like f2.8, how would rear vs front mounted compare in terms of contrast and eof?
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