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Testing preshifted H-alpha filters for front mounting on handheld night vision devices showing large true fields

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#1 C.Hay

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Posted 11 March 2023 - 02:57 PM

Thanks to a generous loan, I have the opportunity to test Baader's entire current range of preshifted and "normal", non-preshifted H-alpha filters. In the new CMOS-optimised series which has superceded the former CCD versions, the range comprises:

 

6.5nm without preshift

6.5nm preshifted f/2

3.5nm without preshift

3.5nm preshifted f/3

3.5nm preshifted f/2

 

Baader developed the preshifted versions because of the bandshift issues that arise when a filter is positioned between a very fast objective lens and the camera sensor. This is explained briefly here https://www.baader-p...r-cmos-filters/ and is presented in depth, together with all the transmission curves, in Baader's 2022 White Paper on the use of narrowband filters on astronomical telescopes: https://www.baader-p..._telescopes.pdf

 

In night vision astronomy, the use case for which these filters were made arises when a very fast lens is used and the filter is placed between lens and night vision device (NVD).

 

However, there is also a second use case when the filter is placed in front of the smaller lens of a handheld NVD. It is then no longer the focal ratio of the lens that determines bandshift. Instead, it is the angle between the central axis of the field of view and the position of a celestial object in the field - the further out from the centre, the more bandshift. In other words, the larger the field of view (FOV) becomes, the more H-alpha signal will be lost, to the point of total loss at a certain angular distance from the centre of the FOV.

 

The FOV of a handheld NVD depends upon the focal length of the lens. Lenses around 25mm focal length show fields in the sky that are around 40° in diameter. With a "normal" 7nm or 6.5nm H-alpha filter, two thirds of that field are compromised due to bandshift! This is where preshifted filters come in. To determine precisely what they can do, and what any downsides there may be, I used a clear night around the last New Moon to carry out systematic tests in Orion and Monoceros.

 

This is the hardware used:

Cosmicar 25mm and 50mm f/1.4 on Bushnell Wolf 3S
Bushnell Wolf 3S night vision device with Gen 2+ tube; 25mm and 50mm Cosmicar C-mount lenses, both f/1.4; Baader 3.5nm f/3 in 2-inch filter cell is shown on the 25mm lens, Baader 3.5nm without preshift is shown on the 50mm lens.
 
Observations with 25mm lens giving approx. 1x (estimated) and 41° FOV (measured in the sky), filters front mounted in 2-inch filter cells. Suburban site, NELM ca. 5m0, good transparency of sky.
"Good field", "deterioration", "loss" refer to galactic nebulae. Stars are generally less affected by bandshift and loss towards edge of field. M42, a very bright and compact nebula at 1x, proves useful to determine the point of complete nebula loss. The Rosette Nebula is useful to determine where loss sets in, as it is more vulnerable to deterioration than M42.
 
Unfiltered: Stars that are at limit of perception at centre of field remain visible until 90% out towards edge of field. They then fall prey to gradual softening plus edge darkening that begins 95% out. Good field of unfiltered lens is hence 37°.
 
6.5nm without preshift: I notice a central brightening affecting 10% of the field (4°). I put the filter on the 50mm lens to see whether the problem arises there; no, it doesn't. Then I check with an Astronomik CCD 6nm H-alpha filter on the 25mm lens; in this configuration the central brightening affects 25% (10°). So it seems to be a systemic issue and the Baader 6.5nm does a better job of suppressing it.
Field is otherwise good over 35% (14°). It then deteriorates gradually until major but not complete loss at 75% (31°).
 
6.5nm preshifted f/2: The preshifted filter is free of the central brightening seen with with the non-preshifted 6.5nm filter. Field is good to 65% (27°). Then loss of galactic nebulae sets in, becoming complete at 75% (31°).
 
3.5nm without preshift: No central brightening issues. Nebulae are good to 35% out (14°) then deteriorate gradually until complete loss at 60% (24°).
The good central 14° of the field is enough to frame the whole of Barnard's Loop. All objects in the good part of the field are seen much more clearly with the 3.5nm filter than with both 6.5nm filters in their good parts of the field: While with the 6.5nm filters Barnard's Loop peters out shortly after Saiph (Orion's right foot), with the 3.5nm filter the Loop continues a good part of the way towards Rigel (Orion's left foot). Similarly, the Cone Nebula complex is seen larger and clearer.
 
3.5nm preshifted f/3: There is a central patch out to 15% (6°) in which there is loss (but not complete loss) of nebulae. From then on nebulae are good to 65% (27°). Then loss sets in, becoming complete at 75% (31°).
All objects in the good zone are seen much better than with the 6.5nm f/2 filter. The overall visual effect is very pleasing. I only need to remember that the central 6° are diminished and avoid putting an object of interest at the centre of field. I notice during the observing session that this quickly becomes automatic behaviour that I don't find annoying at all. Putting the centre of field roughly 2° due south of Betelgeuse, I can frame Barnard's Loop, the Angelfish Nebula Sh 2-264 at Orion's head and the Rosette Nebula in the good part of field, all these objects seen distinctly. This is the most memorable sight of the night.
It is so pleasing that I start looking for other groupings. Two are quickly found: The Rosette Nebula in Monoceros plus the Seagull Nebula at the Monoceros-CMa border, almost 20° apart, are framed well together, both seen clearly and much better than with the 6.5nm filters. So are the California Nebula in Perseus plus the Flaming Star / Tadpole Nebula complex in Auriga.
 
3.5nm preshifted f/2: With this filter, the central patch with (not complete) loss of galactic nebulae covers the central 50% (20°) of the field. From then on nebulae are good to 70% (29°). Then loss of nebulae sets in again, deteriorating quickly to complete loss at 75% (31°). In other words, the good zone for nebulae is an annular ring of 9° angular width (20° to 29°).
 
Conclusions
 
At 1x (25mm lens) all celestial objects suffer loss of visibility and/or detail compared to 2x (50mm lens). So the point of going to 1x is to frame within one FOV objects that are distant from each other on the celestial plane. How do the five filters examined here fare in this regard?
 
Within the 6.5nm pair of filters, the preshifted f/2 variant is the big winner. At 1x it delivers a good field for galactic nebulae from the centre of FOV all the way out to 27°. If I were under a pristine mountain sky, I might develop a preference for the 6.5nm f/2.
 
However, under my suburban NELM 5 sky the 3.5nm f/3 also delivers a good field to 27°, with the caveat that the central 6° are diminished (yet not lost entirely). At 1x with an f/1.4 lens, I see all nebulae in the 27° zone, with the mild exception of the 6° central zone, much better than with the 6.5nm filters.
 
So under the home conditions in which I spend 99% of my observing time, the 3.5nm f/3 is a great gain and absolutely a keeper at 1x. I only need to remember that the central 6° are diminished. I found during further observing sessions that it quickly became second nature to frame several objects - the whole purpose of 1x viewing in a 41° field! - automatically in such a way that none of them are at the centre of field.
 
The question now is: how do these filters shape out on a 50mm f/1.4 lens giving 2x and 21° FOV? I'll be reporting my findings here shortly - stay tuned!
 
Christopher

Edited by C.Hay, 11 March 2023 - 05:41 PM.

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#2 bbasiaga

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Posted 11 March 2023 - 09:34 PM

Thanks for this.  I have been thinking of getting a 3.5nm f/3 for 1x on my PVS14.  Pricey thing, but I'll put it on the 'wish' list. 

 

-Brian


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#3 a__l

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Posted 11 March 2023 - 11:50 PM

bbasiaga, how often do you use 1x with H-Alfa?

 

https://www.cloudyni...ars/?p=12556159

I understand OP where he writes:

"I agree wholeheartedly! 5x and 8° FOV seems to me to be a kind of optimum."

 

I have both 3x and 5x and if I observe with H-alfa, it's 3x.
5x is little heavy for me to use handheld.

I see the use of 1x for meteor observation, but H-alfa is not needed for this.

 

Although, when you have the opportunity to compare of several H-Alfa, it is certainly interesting.


Edited by a__l, 12 March 2023 - 12:01 AM.


#4 bbasiaga

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Posted 12 March 2023 - 08:49 AM

I would say my handheld use is about 50/50% 1x and 3x. I really like to just look up and see the stars as if I were at a dark site.

I think a non shifted 3nm is still going to band shift even at 3x.

Brian

#5 a__l

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Posted 12 March 2023 - 06:47 PM

I really like to just look up and see the stars as if I were at a dark site.
 

3nm filter will extinguish all the stars. Maybe I don't understand something....

 

 

I think a non shifted 3nm is still going to band shift even at 3x.
 

 

 

Conclusions
 
At 1x (25mm lens) all celestial objects suffer loss of visibility and/or detail compared to 2x (50mm lens). So the point of going to 1x is to frame within one FOV objects that are distant from each other on the celestial plane. How do the five filters examined here fare in this regard?
 
 

3x will be clearly better than 2x. In the sense of the filter bandwidth shift discussed here.


Edited by a__l, 12 March 2023 - 06:50 PM.


#6 bbasiaga

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Posted 12 March 2023 - 08:39 PM

From the other filter thread, I think the 3nm non-shifted filters still shift out before the end of the 3x field of view.  I could be mis-remembering. 

 

As to the question on 3nm extinguishing stars, you didn't ask how much I use an Ha filter, you asked how often I use 1x.  A nit-pick I know, but the point is I do use 1x a lot and throw different filters on as I go. 3nm Ha would be for nebula, obviously. 

 

-Brian



#7 a__l

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Posted 12 March 2023 - 11:01 PM

OK. Of course, you decide what equipment to buy.

As for the stars, yes I like the picture that NV 1x gives in the site with some light pollution. No filters. In a dark place, I don't have time for this. All time I use with my telescopes. Incomparably more objects which are visible in much greater detail.


Edited by a__l, 12 March 2023 - 11:16 PM.


#8 C.Hay

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Posted 13 March 2023 - 05:21 PM

I did not start this thread to discuss the relative merits or drawbacks of 1x or any other magnification. My comment made elsewhere that I view 5x and 8° FOV as a kind of optimum does not detract from the special joys of 1x or 2x widefield observing. What I meant by that comment was that within the realm of handheld (as opposed to tripod-mounted) night vision H-alpha observing and when concentrating on individual objects (as opposed to framing several objects in very wide fields) I feel that 5x and 8° FOV is a kind of optimum because it combines a steady, bright image with appreciable nebular detail within objects.

 

My purpose in starting the present thread is to present, in a manner systematic and concise enough to be a useful reference for others who also enjoy widefield observing, the benefits of preshifted filters when the field of view is so large that galactic nebulae shift out before the edge of the field is reached. I therefore beg anyone whose main concern is to extoll the benefits of narrowfield telescopic observation to do so elswhere and refrain from watering this thread down with immaterial comments.

 

I will post my findings for a 21° FOV (2x) over the next weekend. The third step will be to examine 14° FOV (3x), to which I will devote the next good night.

 

CS, Christopher


Edited by C.Hay, 13 March 2023 - 05:49 PM.

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#9 a__l

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Posted 13 March 2023 - 06:05 PM

My purpose in starting the present thread is to present, in a manner systematic and concise enough to be a useful reference for others who also enjoy widefield observing, the benefits of preshifted filters when the field of view is so large that galactic nebulae shift out before the edge of the field is reached. I therefore beg anyone whose main concern is to extoll the benefits of narrowfield telescopic observation to do so elswhere and refrain from watering this thread down with immaterial comments.

 

 

It would be nice to add a photo if possible. Let it be only one thing, it will be handmade and without any processing. In this case, the quality of the image of stars, etc. is not so important. It's really interesting. Few people have so many filters for a one-time comparison in approximately the same conditions.

For example I only have one 7 nm Baader. As long as that was enough for me.


Edited by a__l, 13 March 2023 - 06:26 PM.


#10 bbasiaga

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Posted 13 March 2023 - 06:52 PM

I did not start this thread to discuss the relative merits or drawbacks of 1x or any other magnification. My comment made elsewhere that I view 5x and 8° FOV as a kind of optimum does not detract from the special joys of 1x or 2x widefield observing. What I meant by that comment was that within the realm of handheld (as opposed to tripod-mounted) night vision H-alpha observing and when concentrating on individual objects (as opposed to framing several objects in very wide fields) I feel that 5x and 8° FOV is a kind of optimum because it combines a steady, bright image with appreciable nebular detail within objects.

My purpose in starting the present thread is to present, in a manner systematic and concise enough to be a useful reference for others who also enjoy widefield observing, the benefits of preshifted filters when the field of view is so large that galactic nebulae shift out before the edge of the field is reached. I therefore beg anyone whose main concern is to extoll the benefits of narrowfield telescopic observation to do so elswhere and refrain from watering this thread down with immaterial comments.

I will post my findings for a 21° FOV (2x) over the next weekend. The third step will be to examine 14° FOV (3x), to which I will devote the next good night.

CS, Christopher


Looking forward to the 3x review.

#11 C.Hay

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Posted 19 March 2023 - 03:13 PM

Test No. 2: 2x and 21° FOV
 
In my initial post #1 I set out the logic behind the tests reported here and showed a photo of the hardware. Here comes Test No. 2:
 
Observations with 50mm lens giving approx. 2x (estimated) and 21° FOV (measured in the sky), filters front mounted in 2-inch filter cells. Suburban site, NELM ca. 5m0, good transparency of sky.
"Good field", "deterioration", "loss" refer to galactic nebulae. Stars are generally less affected by bandshift and loss towards edge of field. Rosette Nebula used to determine where loss, moving from centre of field towards edge of field, sets in.
 
Unfiltered: Stars that are at limit of perception at centre of field remain visible until 80% out towards edge of field. At that point softening of image causing loss of stars begins, plus edge darkening that begins 90% out. Good field of unfiltered lens is hence 16°.
 
6.5nm without preshift: Field is good from centre to 50% out (10°). Then loss of galactic nebulae sets in, becoming major but not complete at 75% (16°).
 
6.5nm preshifted f/2: Field is good from centre to 65% out (14°). Then loss of galactic nebulae sets in, rapidly becoming major at 75% (16°).
 
3.5nm without preshift: Nebulae are good to 35% out (7°). Then loss of galactic nebulae sets in, becoming major but not complete at 75% (16°).
The good central 7° of the field is enough to frame most objects. All objects in the good part of the field are seen much more clearly with the 3.5nm filter than with both 6.5nm filters in their good parts of the field. This is noticed particularly in the visibility of the following objects:
- The Angelfish Nebula (Lambda-Ori Nebula at Orion's head) is visible across its full extent, a very striking gain compared to the 6.5nm filters and a highlight of the night.
- The southeast extension of the California Nebula, which makes it 4° large in contrast to the 2° of the brightest western portion shown on star maps, becomes apparent (not really seen with the 6.5nm filters).
- Flaming Star Nebula in Auriga gains length towards 14 Aur.
- Sh 2-232 in Auriga seen (not seen with either of the 6.5nm filters).
- Jellyfish Nebula IC 443 in Gemini seen distinctly (tenuous with the 6.5nm filters).
- Cone Nebula Sh 2-273 in Monoceros distinct with both the northern and south-western wings, these extending overall 5° from tip to tip (with the 6.5nm filters the northern wing was tenuous and less internal detail at the centre of the nebula was seen).
- LBN 1036 (mapped in Charles Bracken's Astrophotography Sky Atlas) directly east of the Seagull Nebula seen clearly (very uncertain with the 6.5nm filters). A further highlight.
- Southern half of Barnard's Loop seen (not seen this night with the 6.5nm filters), and framed satisfactorily in the sufficiently large good zone of the field. Northern half seen with more contrast and internal detail than with the 6.5nm filters, and again framed well in good zone.
 
3.5nm preshifted f/3: Major loss at centre of field compared to non-preshifted 3.5nm and both 6.5nm filters. At 35% out from centre (7°) image picks up and becomes as good as the non-preshifted 3.5nm at centre. At 50% (10°) gradual loss sets in, becoming major at edge of field.
I look for an application of the annular good zone of the 3.5nm f/3 by placing objects in this zone that are more distant from each other than the good central 7° of the non-preshifted 3.5nm allows. This succeeds with the Flaming Star Nebula and Sh 2-232 in Auriga. The 3.5nm f/3 is the only filter among the five tested in this night that allows me to see these two objects at the same time.
 
3.5nm preshifted f/2: At first it appears as if all nebulae are gone. However, moving the Rosette Nebula slowly towards the edge of the field I see how it suddenly flares up 75% out, only to be lost again 90% out. This is a fascinating effect repeated with various more compact, bright nebulae such as the Monkeyhead.
I explore whether I can use this effect in a manner similar to blinking an OIII filter in classic glass observation of planetary nebulae, taking the weaker nebulae Sh 2-282 and Sh 2-284 a couple of degrees south of the Rosette as test objects. Alas, no, it doesn't really work.
 
Conclusions
 
The non-preshifted 3.5nm filter shows much more on all nebulae than the 6.5nm filters do under my suburban NELM 5 sky at 2x and 21° FOV.
 
The 3.5nm f/3 filter can be useful occasionally at 2x to frame distant objects together. I wouldn't buy it for that niche application alone. But it was the absolute winner at 1x (see Post #1 above), so I want to have it anyhow.
 
The 6.5nm f/2 filter has the definite advantage of delivering the by far largest good zone for galactic nebulae at 2x, from the centre of field out to 14°. Then again, the non-preshifted 3.5nm shows more on each individual nebula within its 7° good field at 2x. I find myself torn between the two.
 
Test No. 3 will be on a 75mm lens giving 3x and 14° FOV. If the weather gods grant just one good night before the Winter Milky Way falls into April oblivion, I'll be reporting here soon. If they don't, I may have to wait until the Summer Milky Way rises.
 
CS, Christopher

Edited by C.Hay, 20 March 2023 - 04:41 AM.

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#12 bbasiaga

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Posted 19 March 2023 - 03:41 PM

Very interesting.  The plot thickens......as it always seems to do with filters for this part of our hobby.

 

-Brian


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#13 a__l

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Posted 19 March 2023 - 05:21 PM

 

Test No. 2: 2x and 21° FOV
 
Unfiltered: Stars that are at limit of perception at centre of field remain visible until 80% out towards edge of field. At that point softening of image causing loss of stars begins, plus edge darkening that begins 90% out. Good field of unfiltered lens is hence 16°.
 
 
Test No. 3 will be on a 75mm lens giving 3x and 14° FOV. 
 

the filter thread of your 2x is 46mm.
For a 75mm objective, this is already becoming a significant factor. What is your 3x objective?



#14 sixela

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Posted 20 March 2023 - 03:56 AM

If you screw in 2” filters these 75mm objectives simply can’t become faster than f/1.6, but that’s (in my experience) fast enough to enjoy them.

#15 bbasiaga

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Posted 20 March 2023 - 08:22 AM

I've used 2" filters on my 3x afocal lens for the PVS14, and it does clip the FOV a bit, and it does act as a bit of an aperture mask, slowing the lens down some.  I haven't calculated how much, but I can still pull great views out of my light polluted skies.  So functionally it seems not to be an issue.  And saves a lot of money over getting custom larger filters. 

 

-Brian



#16 C.Hay

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Posted 20 March 2023 - 10:20 AM

I'm going to be exploring the issue of aperture masking when using 2" filters on the 75mm f/1.4 lens. The lens has a front aperture of 53mm and a 58mm filter thread. I have at the ready a 58-48mm step-down ring plus an empty 2" Baader filter cell screwed into the ring, with which I can first examine the effect of unfiltered masking.

 

Beside definitely slowing the lens down and potentially clipping the FOV, both of which I can live with, I want to find out whether there are other effects. In particular, it seems possible that aperture masking causes edge-of-field darkening to set in long before the edge of the FOV. If the sum of aperture masking effects is so substantial that it becomes difficult to differentiate it from filter bandshift effects - the latter being what these tests are about - I see two options:

 

Firstly, procuring a 75mm f/1.9 lens. This should match 2" filters well without masking.

 

Secondly, using Baader's unmounted 65x65mm filters in a custom-made adapter. All of Baader's H-alpha filters are available in this size. I've assembled a few of these for use on my Zeiss 125mm f/2 lens, which I really don't want to mask down. Fitted to the 75mm f/1.4 lens, a 65x65 filter looks like this:

Bushnell Wolf 3S with three Cosmicar f/1.4 lenses and the Zeiss 125 f/2
Far left: Zeiss 125mm f/2 T* lens. Second from left: Baader 65x65 filter on Cosmicar 75mm f/1.4 lens.
 
The Zeiss 125mm f/2 T* lens was made by that company specifically for its Zeiss Victory NV 5.6x62 device (Gen2+, to my knowledge) and has a front aperture of 62mm. The combination of this with the 65x65mm filter is a match made in heaven.
 
To right: Cosmicar 50mm and 25mm f/1.4 lenses fitted with 2" filters. All lenses are C-mount and are used on the Wolf 3S (Gen2+) night vision device shown at right. Despite having no gain control, I prefer the Wolf 3S over the Zeiss NV device because it is far more compact and rests much better in the hand. Ergonomics are just as important in this handheld application as they are when using normal binoculars. Note that the 125mm f/2 lens is not much bigger than the 75mm f/1.4, nor is it heavier.
 
Christopher

Edited by C.Hay, 20 March 2023 - 12:44 PM.


#17 jesse 3

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Posted 20 March 2023 - 10:29 AM

If you stack a standard filter with a pre-shifted filter, you will have a narrower bandpass. A little darker, but you can adjust the gain to compensate. I got good contrast improvement with 6nm filters at center. Not as good as my chroma 3nm, but it is better than a single 6nm.
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#18 sixela

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Posted 20 March 2023 - 12:01 PM

I've used 2" filters on my 3x afocal lens for the PVS14, and it does clip the FOV a bit, and it does act as a bit of an aperture mask, slowing the lens down some.  I haven't calculated how much, but I can still pull great views out of my light polluted skies.  So functionally it seems not to be an issue.  And saves a lot of money over getting custom larger filters. 

 

-Brian

I haven't seen it clip the FOV one bit on my Cosmicar: the FOV is just as large but the f/ratio is definitely a bit "slower".


Edited by sixela, 20 March 2023 - 12:01 PM.


#19 bbasiaga

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Posted 20 March 2023 - 01:17 PM

I haven't seen it clip the FOV one bit on my Cosmicar: the FOV is just as large but the f/ratio is definitely a bit "slower".

What is the objective size on your cosmicar?  I think the one I have is 53 or 58mm.  I 3D printed a reducer so I could thread a 48mm filter in.  And just judging from looking at cabinets in my house It seems to clip off the corners a tad.  I suppose for viewing at infinity it may not have as pronounced an effect.  I haven't scientifically tested it as it is not obvious in use or distracting any way. 

 

-Brian



#20 sixela

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Posted 20 March 2023 - 02:34 PM

That Cosmicar has got a 58mm filter thread.


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#21 C.Hay

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Posted 12 April 2023 - 03:30 PM

Test No. 3: 3x and 14° FOV

In my initial post #1 I set out the logic behind the tests and reported the findings for a 25mm C-mount lens giving 1x and 41°. Test No. 2 in post #11 reported on outcomes with a 50mm lens giving 2x and 21° FOV.

 

In preparation for the third test in this series with a 75mm f/1.4 lens there was some discussion about the possible effects of using 2-inch filter cells in front of the lens, given that this would mask the aperture down from 53mm to 45mm. I have tried this out in the meantime and found that aperture masking, while tightening star images and thus almost appearing positive at first sight, does indeed cause a degree of edge-of-field darkening that would stand in the way of meaningful preshifted filter tests.

 

Using Baader's 65x65mm square filters, pictured in my post #16 above, allows use of the lens' full native aperture and speed. However, I don't have the 65x65mm filters in all preshifted versions. So I've procured a Cosmicar 75mm f/1.9 lens. This has a front aperture of 40mm and no aperture clipping issues with 2-inch front filters.

 
Observations with 75mm lens giving approx. 3x (estimated) and 14° FOV (measured in the sky), H-alpha filters front mounted in 2-inch filter cells. Filters all from the same manufacturer - Baader - and procured simultaneously from the same series - CMOS - of that manufacturer.
Suburban site, NELM ca. 5m0, good transparency of sky.
"Good field", "deterioration", "loss" refer to galactic nebulae. Stars are generally less affected by bandshift and loss towards edge of field. Monkeyhead Nebula Sh 2-252 (aka NGC 2174/2175) used to determine where loss, moving from centre of field towards edge of field, sets in.

 

Unfiltered: Stars that are at limit of perception at centre of field remain visible until 80% out towards edge of field. At that point softening of image causing loss of stars begins, plus edge darkening that begins 85% out. Good field of unfiltered lens is hence 11°.

6.5nm without preshift: Field is good from centre to 50% out (7°). Then loss of galactic nebulae sets in, becoming gradually worse towards edge but not complete.

Monkeyhead Nebula is strong at centre and still present at edge of field. Lower's Nebula Sh 2-261 is faint at centre and no longer present at edge.

6.5nm preshifted f/2: Field is good from centre to 65% out (9°). Then loss of galactic nebulae sets in, becoming gradually worse towards edge but not complete.

Monkeyhead Nebula is strong at centre and still present at edge of field, and stands out in greater contrast there than it did with the non-preshifted 6.5nm filter. Lower's Nebula Sh 2-261 is faint at centre and no longer present at edge, but I get the impression this may be due to the edge losses the lens has in any case, filtered or not.

3.5nm without preshift: Nebulae are good to 35% out (5°). Then loss of galactic nebulae sets in, but only proceeds very gradually. Monkeyhead still present at edge of field.

Within the 5° good zone all nebulae are more distinct than with the 6.5nm filters. Jellyfish Nebula IC 443 was not really seen with the 6.5nm filters, but now it emerges. Cone Nebula complex Sh 2-273 in Monoceros only displayed the brightest central zone a little under 2° large with the 6.5nm filters, but now the northern extension towards Xi Gem and the south-eastern extension towards 13 Mon emerge, making it 5° large (which stills fits nicely within the good zone).

 

3.5nm preshifted f/3: Immediately noticeable gain over the non-preshifted 3.5nm filter - not only for galactic nebulae but also for stars. Nebulae are good to 75% out (10.5°), which is close to the point at which edge softening of the unfiltered image starts in any case with this lens.

5° large Cone Nebula complex and the Rosette Nebula can be viewed well together, which was not possible with any of the other filters. Suprisingly, the image at centre of field is also improved over the non-preshifted 3.5nm filter. This is noticed particularly with a clearer sighting of the Jellyfish Nebula.

 

3.5nm preshifted f/2: All nebulae are reduced compared to the other two 3.5nm filters. Indeed, only the brightest - Monkeyhead and Rosette - are still seen, throughout the entire field but very weak everywhere, with minor improvement at a point 50% from centre.

 

Conclusions
 

Within the 6.5nm filter pair, the f/2 preshifted filter delivers substantial gain. The image is immediately more engaging than with non-preshifted, the good field larger and the presentation of galactic nebulae better in the rest of the field.

 

The non-preshifted and f/3 3.5nm filters make every nebula stand out more clearly than the 6.5nm filters do under my suburban NELM 5 sky at 3x and 14° FOV. Moreover, where the 6.5nm filters fail to show faint outer regions of larger nebulae, these emerge with the two 3.5nm filters.

 

With the non-preshifted 3.5nm filter, loss of nebulae starts fairly early at 35% out from centre of field. However, loss progresses very slowly from there on, reaching at the edge of field the same level as the non-preshifted 6.5nm filter there. As a result, the non-preshifted 3.5nm filter remains pleasant to use despite the relatively early onset of nebula loss.

 

The 3.5nm f/3 filter is the resounding winner of this test at 3x with a 75mm f/1.9 lens. It shows nebulae with all the benefits of the tighter bandwidth over almost all of the field, with nebula loss only starting just before the inherent edge-of-field losses of the lens itself set in. This makes it easier to detect faintest nebulae. It also makes it possible to frame several objects at once across a good field of 10° in a way none of the other filters tested in this night did.

 

CS, Christopher


Edited by C.Hay, 13 April 2023 - 04:38 AM.

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#22 a__l

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Posted 12 April 2023 - 08:10 PM

So I've procured a Cosmicar 75mm f/1.9 lens. This has a front aperture of 40mm and no aperture clipping issues with 2-inch front filters.

 
 

 

Unfiltered: Stars that are at limit of perception at centre of field remain visible until 80% out towards edge of field. At that point softening of image causing loss of stars begins, plus edge darkening that begins 85% out. Good field of unfiltered lens is hence 11°.
 

I think in this situation it's better to go for the good 75mm lens and use one good 7nm filter without preshift. This is likely to be cheaper than experimenting with filters from different manufacturers, which may have different properties, even if they are preshift the same.


Edited by a__l, 12 April 2023 - 11:45 PM.


#23 C.Hay

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Posted 13 April 2023 - 04:40 AM

Both lenses are good. The unfiltered edge-of-field loss seems to be due in good part to the night vision device itself, as it occurs in similar form with all C-mount lenses. Instead of "Good field of unfiltered lens is hence 11°" it would have been more precise to write "Good field of unfiltered lens+device system is hence 11°".

 

The H-alpha filters are all from the same manufacturer, Baader, and the same series of that manufacturer, CMOS. I stated that in my initial post. To judge by a_I's comments, I should have re-stated it in my test report No. 3. I've edited the report to include that information now.

 

Without claiming to be scientific, I've gone to some lengths to carry out these tests in the most stringent manner possible. For instance, the filters were all procured simultaneously to achieve best-possible consistency within my means. I also took care to carry the tests out only when sky conditions were very similar (I hesitate to say identical, as that would require meteorological measurements; but I know my observing site very well and can gauge this well enough for the purposes of us amateurs).

 

Christopher


Edited by C.Hay, 13 April 2023 - 04:57 AM.

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#24 a__l

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Posted 13 April 2023 - 06:10 AM

Both lenses are good. The unfiltered edge-of-field loss seems to be due in good part to the night vision device itself, as it occurs in similar form with all C-mount lenses. Instead of "Good field of unfiltered lens is hence 11°" it would have been more precise to write "Good field of unfiltered lens+device system is hence 11°".

 

 

It made sense to take a closer look at this. 20 percent of the field is quite significant.



#25 chemisted

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Posted 13 April 2023 - 06:43 AM

Test No. 3: 3x and 14° FOV
 

The 3.5nm f/3 filter is the resounding winner of this test at 3x with a 75mm f/1.9 lens. It shows nebulae with all the benefits of the tighter bandwidth over almost all of the field, with nebula loss only starting just before the inherent edge-of-field losses of the lens itself set in. This makes it easier to detect faintest nebulae. It also makes it possible to frame several objects at once across a good field of 10° in a way none of the other filters tested in this night did.

 

CS, Christopher

Thank you for the excellent report.  Will you be trying this filter as the 65X65 version on the 75mm f/1.4 lens?

 

Incidentally, I read this post last night and it prompted me to try a new setup (for me).  I had recently removed the filter element from my Baader 6.5nm HS and placed it in the adapter that allows my Collins I3 device to use C-mount lenses.  Since I have the Cosmicar 75mm f/1.9 lens I put it on and went out at dark.  The views were very much like those that you describe in the quote above.  The nebulae remained unchanged to the edge while stars only deformed in the last few percent of the FOV.

 

I had never tried this before because my go to lens is the Nikkor 105mm f/1.8 which, with its larger aperture, shows more and better everything.  That's part of the reason that I am wondering if you will try the other 75mm lens with the full aperture, front-mounted filter.




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