Don Pensack

40 minutes ago, John said:

Guess what my first target is going to be when I next have the 12 inch dob out ? - the Cats Eye with the Lumicon O-III filter

Just be aware that if you use magnifications above about 15x/inch of aperture, your best view will be without the filter.

The contrast enhancement at high powers is very minor, and the dimming of the nebula more important when the magnification is large.

My best view of the Catseye has been at 493x in the 12.5" but without a filter.

It's been done for over a century.  However, rarely on a scope > 6" in size.

I see you used the term "back focus" to indicate "out focus".

Back focus is a photography term and it means gaining additional focus room outside the focuser, i.e. back focus.

It implies in focus of the focuser.  Sorry for my confusion.

In that case, there are a couple solutions:

1) add an extension tube to the focuser.  These pull the eyepiece out of the focuser by at least the length of the eyepiece's barrel, so the focuser would need to move in at least that far.

2) use only the 2" adapter for the focuser (the tall one provided by the manufacturer) and add a standard 2" to 1.25" adapter to that and do not use the 1.25" adapter that came with the scope again.

This would raise the eyepiece and cause you to lower the focuser.

However, to the point of the large extraneous spike in the image.  It is obviously not caused by the focuser, so what else protrudes into the inside diameter of the tube?

4 hours ago, bomberbaz said:

Sorry @Don Pensack surely if you add height to the focuser you are then having to wind the focuser in further to compensate or did I misunderstand what you meant?

Would pushing pushing your primary mirror up 5mm not be another option assuming that is there is enough travel!

Steve

The new 2" to 1.25" adapter would be SHORTER than the one currently used.   You are shortening the focuser thereby, not adding height.

So you misunderstood what I meant.

Pushing the mirror up 5mm wouldn't be enough unless the focuser only protruded 5mm inside the UTA, and the spike says it sticks in more than that.

My adapter switch would likely make a 15mm different or more.

On 09/06/2020 at 10:35, Barry-W-Fenner said:

I do have the 25mm version, I use it quite a lot to view almost the entire area of M45 & The double cluster. It is also useful for using to locate your targets before changing to higher power. I did notice an improvement over the stock 25mm, The image is definitely sharper, However I have noted that the 25mm isnt clear all the way to the edge of the field, I would say around 70% with the outer edge slightly degraded. I dont know if that is the eye piece and 200p combination doing this or just the eye piece.

Cheers

Baz

It's how the eyepiece handles the light cone from the primary.  This eyepiece works fantastic in an f/15 Maksutov.

17 hours ago, Louis D said:

I read about another person allowing the Dob's mirror to dry in the sun after washing and rinsing.  The angle was such that the sun was focused on the eaves and started to scorch it.

As for the OP, this seems to point to using a Newt to project a solar image which is not a good idea.

Using a newtonian for projection of the solar disc works OK with simple eyepieces like Ramsdens or Huygens as long as they are glass and the housings metal.

Good luck finding such an eyepiece.  They were common in the '60s when I got started, but the ones out there now are usually plastic.

5 hours ago, Barry-W-Fenner said:

Morning Andy,

For reference this is what Arcturus & Vega look like at focus for me.

The 4 bright short diffraction spikes are from the spider holding the secondary mirror, and are normal.

The other long spike is of more concern, however, and you gave a clue of its cause--the focuser.

You mentioned the focuser was nearly all the way in when focus was achieved, and it seems logical to conclude it is sticking into the light path to the mirror and causing a massive spike.

If the focuser is at approximately that angle relative to the spider vanes, that would cinch the identification as caused by the focuser.

So how to get it out of the light path to remove the diffraction?  Well, it would focus farther out if it were shortened, and I think I have a way:

--get rid of the 1.25" adapter that you now use and instead use a tall 1.25" adapter like the Baader Click-Lock, or an Antares Twist-Lock, or something with at least 10mm of height above the focuser.

This would move the eyepiece in quite a bit, and require the focuser be moved out to compensate.  That might be enough to get the focuser out of the light path.

You can check to see how much it intrudes into the tube when you're in focus.  You want the focuser to not be inside the tube at all when the eyepiece is focused.

Chances are there is not a lot of clearance between the mirror and the tube.  If the inside diameter of your upper tube assembly is less than, say, 40mm larger than the mirror, the tight clearance

could easily lead to having the focuser intrude into the light path.

You also have the possibility of simply dropping the UTA toward the primary by 10-20mm to experiment with seeing whether the focuser is at fault.  That would require moving the focuser out by the same amount.  If the long extra diffraction spike disappears, then you know what causes it and what you have to do to eliminate it.

As for the sun, one poster described leaving his big dob on the porch when he went to bed.  The morning sunlight reflected off the mirror on the eave of his house and set the house on fire.

Fortunately, very little damage was done.  But if you ask if sunlight can melt a Barlow or eyepiece?  Yes, it can.

9 hours ago, Ricochet said:

That is what the graph suggests, but people using the NBP for example, report seeing some red in or around stars. This suggests to me that those objects must be bright enough to be activating the red cone cells. Likewise, if you see green in a star or nebula, it must be activating the green cones. Once the object becomes so dim that only the rods are activated then it will simply be grey. If this is true then I think I can make some predictions on the use of the DGM NBP vs the TV Bandmate II:

1. The DGM will be better at low magnifications on bright nebulae which have significant red emission.
2. As magnification is increased, and exit pupil reduced, there will come a point where the nebula loses colour, and the TV will become slightly better on those objects as it has a slightly higher transmission in the B-beta/OIII region
3. On dim nebulae, or those lacking a significant red component, the TV will be slightly better for the same reason as in (2).

Perhaps @Don Pensack can confirm whether this tallies up with his experiences using both types on different nebulae. Hypothesis 2 is probably the most interesting to me.

There is one "fly in the ointment" with the hypothesis that we can see colors under low light.

Psychology lab tests show that when 2 identical grey squares are presented side-by-side to the subject, and both are illuminated so low that the only way to see the squares is with scotopic vision,

and one square is illuminated just a bit more than the other, the mind fills in the color by seeing the slightly brighter one as greenish grey and the dimmer one as reddish-grey.

So if you see red and green in a nebula, which, unfortunately, are the predominant colors in nebulae, it is impossible to tell if the colors are spurious if the object has a surface brightness of 18 mpsas or dimmer.

This shouldn't apply to the Orion Nebula, which has a high enough brightness to damage night vision in a scope, but definitely applies to, say, M27, where the people who report seeing color see the "apple core" as greenish,

and the long wings of the oval as reddish, when color images show the opposite to be true.

A few posts back, I explained why the DGM filter is believed to work for red colors and the size of large bright nebulae.  It works quite well even if color is not seen, but I believe this is likely related to its quite-narrow bandwidth in the blue-green.

Barry,

There will be coma visible in this instrument as long as you don't use a coma corrector, so NO eyepiece will be perfect to the edge.

I assume this is a SkyWatcher Flex-Tube dob with a 1500mm focal length, as such, f/5.

Normally, a complete set of eyepieces for the scope might be 60x/120x/180x/240x/300x

That would be focal lengths of 25mm, 12.5mm, 8mm, 6mm, and 5mm.  Your current set lacks only a 6mm, so the jump from 8mm to 5mm is a big one.

That is not set in stone.  A more minimalist set might be 21mm, 11mm, 7mm, 5mm.  A more maximalist set 30mm, 25mm, 20mm, 15mm, 12.5mm, 10mm, 7.5mm, 5mm, 3.5mm

You have all the eyepieces you need, though there are eyepieces with wider fields and sharper image quality in the outer field (taking into account that coma will make stars in the outer field distorted

so no eyepiece will be perfect).

With that aperture, it is unlikely you would want or need more than one eyepiece to yield a magnification under 100x.  Likewise, if you need eyepieces above 300x, it's just as desirable to 

get there by adding a Barlow lens to a lower power.

Any eyepiece shorter than 7-8mm will be limited by the seeing conditions, so whether you get sharp stars won't likely be determined by the eyepiece.

Any eyepiece longer than 15mm is also less likely to be used except for the largest of objects, and, even then, you'll likely go higher to investigate details more.

"Planetary" eyepieces will be 7-8mm and shorter.

Highest visual acuity and best overall views will be eyepieces from 10mm to 15mm (2-3mm exit pupils).

Low powers will be 12.5mm and longer eyepieces.

It looks like you don't need long eye relief for glasses, so just about everything will work.

I have a 12.5"(31.8cm), but the focal length is 22% longer than yours, so my eyepieces are a little different.

I have a "maximalist" set of 30, 22, 17.5, 14, 12.5, 11, 10, 8, 6, 4.7, 3.7mm focal lengths.  I would note that the seeing conditions at my normal sites are very good, so 300x is always usable and sharp.

Note also that some of the magnifications are a bit close together, so I tend to break the focal lengths down into "sets"--one for poor seeing, and one for good seeing.  So it's really more like a set of 5 and a set of 6.

And, truth be known, I sometimes spend the entire night using only 2 focal lengths--especially if all my targets are small and faint.  So it's normal you might have a night when you use only the medium powers (120-240x on your scope),

and another night mostly lower powers (50-120x).  It will depend on the seeing, and the targets, as to which focal length is best.

P.S. The Catseye nebula will be incredible in your scope with > 200x, or a shorter focal length than 7.5mm.  The amount of detail will be leaps and bounds above what can be seen at 100x.

If looking at the Morpheus, I recommend the 12.5mm for starters.  I think you will use it a lot and it hits a point of high visual acuity (2.5mm exit pupil)

5 hours ago, bomberbaz said:

I have no doubt that cones do work to some extent, otherwise you wouldn't see your red head torch  😆

Seriously though, it's either TV got lazy and/or thought it more cost effective to ignore HA in their bandpass filters or the effect, or not as the case may be of HA in these filters isn't worth having. That is what I would like to experiment to find out.

It's a difference in philosophy.  I'll explain.

Generally, most people have felt through the years that having any red transmission only seemed to create funny looking star images, so the preference was for filters that had no red transmission, like the TeleVue BandMate II, the current Lumicon filters, or the Orion Ultrablock (as an aside, Orion filters come directly from Korea and are not Synta products sold under the Orion label).

But the DGM NPB filter, which is almost like a broadband in its red transmission, despite a narrow blue-green bandwidth, challenged that idea.  Here is why:

Any nebula's light is a combination of light from the sky PLUS the light from the object.  Any DSO is brighter on the Earth's surface than it would be in space.  But, so is the sky background brighter on Earth, which reduces contrast, and the human eye is a contrast sensor.  Otherwise, using filters would have no value, because every DSO is brighter without a filter.  So how do you increase contrast without dimming the object?  Let through the wavelengths emitted by the nebula, but block all other wavelengths.  It works, and we have had good nebula filters for 40 years.

But what if the wavelengths to which the eye is relatively insensitive were allowed to pass unimpeded.  The nebula would be bright at those wavelengths because its brightness would be that of the sky and object.  And if those wavelengths happened to be strong emission lines, like S-II, N-II, and H-α, would that have an effect?  So a fellow named Dan McShane (of DGM) experimented with filters when he worked for a company making filters for industry.

And he found that passing a lot of red made the nebulae appear brighter and of larger extent than not passing the red, and at the sacrifice of only a small amount of contrast.  To enhance the nebula in the center of our visual peak sensitivity, he narrowed the bandwidth more than in other narrowband filters.  Most of the high-end UHC-type filters have a 26-27nm bandwidth these days.  My DGM sample measures 21nm.  That requires EXACT placement of the bandwidth, which is often the Achilles heel of filter making (and DGM's too), but the result is incredible--superb contrast in H-ß and O-III wavelengths, but a larger extent seen in nearly every large hydrogen-emission nebula.  My lifetime-best view of M17 was with a DGM NPB, where the "Swan" shape only appeared to be about 10% of the visible nebula and nebula could be traced, visually, all the way to M16, the other bright point on the same large nebula.

It comes at a cost, however--all the stars are red.  This is obviously because the bandwidth in the red end of the spectrum is so much wider than the one in the blue-green.  Some observers find that terrible.

It's a fairly unique approach to making filters, and one that is not favored by Lumicon or TeleVue.  The Astronomik basically has a sharp spike in transmission at the H-α wavelength for photographic purposes because the red transmission isn't sufficient to have the effects of the DGM NPB (narrow pass band) filter.  And there are a lot of nebulae where having the red transmission doesn't help, pointing to the design philosophy of TeleVue and Lumicon as being better in application due to increased contrast for visual use.

Still, I've found it effective to have both designs on hand, and to compare nebulae with them.  I've found nebulae that responded better to the DGM philosophy and others that responded better to the Lumicon and TeleVue philosophy.

Filters that take more out of the spectrum cost more, due to more coating layers (called cavities) being required, so the advantage to DGM's approach is that the filters ended up less expensive to make.

So if you experiment, I'd try one of the DGM philosophy and one of the Lumicon/TeleVue/Orion philosophy and make your own conclusions.  For me, I'll keep both.

6 hours ago, Stardaze said:

I do know a little bit about this subject so it's making some sense. I do know that it's only been quite recently that we now believe that the cones also play a part at night, it was believed that they didn't at all. Mesopic is that twilight period, or the 'crossover'. Surely though, when you are fully dark adapted and your rods are the majority of your vision, you haven't the ability to see anything over 625nm, according to the above graph?? 

Yes, but that depends on the light intensity.  We see a red flashlight because it is so bright.  Bright stars, planets, Moon, even some brighter DSOs, turn vision "mesopic", where cones are partially activated.  It's why we see colors in stars and planets.

I've noticed M42 can actually reduce my night vision, so it's no wonder I often see colors in M42.

8 hours ago, Stardaze said:

Who are Orion? There's a couple of obscure vendors here in the UK, but none of the mainstream suppliers seem to distribute? Are they not as they once were?

The other thing that I'm not finding an understanding with is the H-Alpha line as, from the theory I knew, it falls outside of the human eye's sensitivity range. Just looking at the Astronomik UHC filter the second emission line starts around 630nm and extends well beyond 750nm?

Orion in filters means Orion in the US, a California company.

The red transmission in the Astronomik filter can be seen as a red character to the star images, but it has almost no effect on nebulae visually.  it's primarily there for photography.

4 hours ago, Philip R said:

I don't normally use filters, except for a variable polarising and the Baader Neodymium.

Sometime last year [2019] another SGL'er was selling the Explore Scientific UHC & O-lll and so I decided to take the plunge/bite the bullet, so I purchased them. So far I have had no qualms about them and they do seem to make a difference in teasing out detail. I did read here on SGL in another post/topic and other forums, online vendors, etc., that cheaper UHC's and O-III's are/were best avoided. 

Personally, I think the Explore Scientific CLS is slightly better than the Baader Contrast Booster.

My sample of the Baader UHC-s has a 62nm bandwidth in the blue-green.

Comparing with my ES CLS:

FWHM        H-ß      O-III   O-III   H-α    low         High

75               92.1      95.2   94.0    89.9   451         526

Both are decent broadband filters.

The Baader UHC-S has the edge for visual contrast on nebulae simply due to a narrower bandwidth.

The CLS and Baader differ in their long wavelength cutoffs, though.

The Baader cuts off at 675nm, with almost no output at all up to 1000nm., while the ES CLS cuts off at 756nm, with a broad bump centered on 950nm.

I'd say that the ES CLS would be better for imaging because of that.

14 hours ago, Stardaze said:

That’s a great link, thanks. Also backs up caution when buying old filters. How common is oxidisation/degradation of the coatings over time in your opinion @Don Pensack?

Uncommon on filters made since 2005.

2 hours ago, Stardaze said:

Probably best I don't... I've only just remortgaged 

Always the way, huh! Good to have things to aspire towards. I'm going to order the APM 13 next month and the 20 when it's back in stock. I'll buy a bandmate O-III too but want some longer binoculars for hopefully getting away still at the end of Aug if we can. I think the trick is really the secondhand market, but being prepared to wait for the right items. This coming year sadly, might see a bit coming up.

A couple points:

1) If buying used, avoid the TeleVue filters called BandMate.  You want the BandMate II, the later ones made by Astronomik.  Much better bandwidths and performance.

2) if you use a Paracorr coma corrector in a dob, the lowest possible setting is still not optimum for the Nikon HW 17.  They left a long length of filter threads on that one, and it simply cannot get close enough to the lens for optimum coma correction.  It's pretty close, though, and certainly no worse than the 31mm Nagler in the original Paracorr, which also could not achieve an optimized position.

3) APM is back in stock on the 20mm XWA HDC eyepieces.

12 hours ago, bomberbaz said:

Great website that Don. 

I notice the astronomik changing versions of UHC but when compared the bandwidth remains pretty much the same.

However the same comparison with the already tighter tv nebustar shows the the v2 one tightens up even further over the v1. Not reading anything into that, just curious. 

Regarding the ES O-III and HB they appear to have some bandwidth drift which in the case of the O-III is quite significant on the face of the lab tests. It would be very interesting to run a comparison on relevant bandwidth appropriate DSO's. 

Original Nebustar was made by a different company than the Nebustar-II.  The original was a lot wider.

The results on the ES filters shows the effect of making things very cheaply without any QC.

The #1 problem with the inexpensive filters isn't the too-wide bandwidth, it's inconsistency from filter to filter.

Unless you test it, you don't know what you have.

This thread should be mandatory reading for those who want to know about filters:

https://www.cloudynights.com/topic/527199-spectroscopic-analysis-comparison-of-nebula-filters/?hl=%2Bspectroscopic

2 hours ago, bomberbaz said:

Hello Don, love your contributions to these threads, you bring a lot of science and knowledge to them.

I have a UHC-S dating back to 2015 I think although I am not sure. I note from your testing you stated UHC-S post 2017.  Regarding my UHC-S I don't find the results subtle at all, to me the differences were quite noticeable  with good contrast.

I have also used it on Jupiter as it really helps the whole planet and especially the  GRS stand out.

Do you know if the post 2016 models of a different design by any chance. 

 

I don't know if they changed it, and I regard it as one of the best, if not THE best broadband filter.

I think the tune up of contrast is there, and it is noticeable, but I always have to add the caveat that it is subtle because so many are led to believe nebula filters provide a huge improvement,

and they will be disappointed if using a filter this wide.  Additionally, in high light pollution, often the case with city dwellers, the broader the bandwidth, the more the internal scattered light in the filter.

Using an Astronomik CLS filter (99nm bandwidth in the blue-green and wider in the red) here in Los Angeles made the view WORSE than the view without the filter in place.  Light scatter was horrendous.

So I think that a broadband filter has to have certain caveats attached to it.  I think they work their best at sites where the sky is already quite dark and many/most objects need no filter at all, and the presence of the broadband 

just turns up the contrast a bit, making things a bit more visible than without the filter, but not dimming the stars quite so much as in the narrowband filters.

I also cannot know how sensitive an observer is to small changes in contrast.  A narrowband drops the brightness of the background sky by around 2.3-2.5 magnitudes and an O-III filter around 3 magnitudes.  That is not subtle.

Here is a simulation of various nebula filters:

https://www.cloudynights.com/topic/385867-filter-simulator/?p=4939179

I wouldn't call the broadband filter improvement subtle, but I think most would compared to the narrowband.

[this nebulae doesn't respond well to H-ß, which explains that simulation.]

There are objects where the broadband shines, though, like NGC7023, IC405, or Sh2-155.

And, quite certainly, if used in appropriate circumstances, it will be much better than no filter at all.  

Just had a long conversation with Al Nagler of TeleVue who confirmed some thoughts of mine:

There are 3 different ways chromatic aberration expresses itself in an eyepiece near the edge of the field:

1) Chromatic aberration of the exit pupil, as found in the 31 and 26mm Nagler and 30mm Explore Scientific (and others).  It is a ring of color extending well into the field.

This is what is termed "the Ring of Fire".  It will be different in different f/ratios of scope since the exit pupil will differ.  He suggested experimenting with aperture stops to see the difference.

2) lateral chromatic aberration that yields a thin ring of color right at the field stop.  It is produced by the upper section of the eyepiece and it is caused by chromatic aberration of the system, which is why it is seen more in wider eyepieces than narrow ones.  I asked if it would be covered completely if the field stop were made narrower, and he said the ring would grow thinner, but unless the field stop were substantially smaller, it would not be gone.  He suggested the experiment of putting a small tab of tape on the field stop of an all-positive eyepiece like a Panoptic (which is below the bottom lens) to see where the color fringe gets thinner and disappears.

3) Chromatic smear, or prismatic smear of a star image as the star gets closer to the edge of the field.  This is the most common form of chromatic aberration in eyepieces and it can be reduced or eliminated with additional lenses in the system.  That may or may not be practical if the eyepiece is already large and heavy.  This one is from all the elements in the eyepiece, whether positive or negative.

3 hours ago, Ruud said:

Gee, Louis, that looks awful! It's difficult to call that a ring of fire, it's more like a rainbow disk!

@Don Pensack

Google kind of fails me when I search for “chromatic aberration of the exit pupil” (with quotation marks to avoid partial matches). I find two categories of references:

The first is about chromatic aberration of a Galilean telescope using non-achromatic lenses. That’s not what we are talking about. (It’s odd, btw, that these links refer to exit pupils of Galilean telescopes. Galilean eyepieces do not form an exit pupil in the sense of a region behind the eye lens through which all rays pass).

The second category involves discussions on cloudynights.com. The links referred to are associated with fat and hideous brown rings inside the exit pupil. That is not what I’m addressing in my post here.

For a quick impression have a look at this search result:  link

Another thing I am not talking about is chromatic aberration in single positive rather than negative-positive eyepieces. Most of us already know that when an eyepiece has chromatic aberration, it tends to get worse toward the edge of the field, so the stop shows it more clearly than any other part of the field.

I briefly considered asking a mod to change the title of my post, but really the well defined coloured ring close to the field edge of neg-pos eyepieces is what we call a ‘ring of fire’. This may even be the case in most places.

I’d like to refer you to this thread, in particular the second post:

And yes, this ring does not always need to be cyan or blue. It really depends on what kind of chromatic aberration the Smyth lens needs to correct for. And of course, the less chromatic aberration the Smyth lens has to correct for, the thinner the ring will be.

 

You should not refer to the thin color ring at the field stop, blue or otherwise, as the "Ring of Fire".  This was a mistake made by some observers who had never seen it and assumed it was the same thing.  It is obviously not.

Your link does show the issue with a 31mm Nagler, which was the eyepiece that prompted the coining of the term a couple decades ago.  As Ernest pointed out, it is "chromatic aberration of the exit pupil".

The thin ring of color at the edge of the field shows up in some 50° all-positive eyepieces, as well as many 60-70° eyepieces without a negative field lens.  So it is not necessarily just caused by the use of a negative field lens in an eyepiece.

It is different than the lateral chromatic aberration in most eyepieces.

Excellent pix!  Especially the 30mm ES.  There is a very slight SAEP there, too. The ring at the edge extends farther into the field on the 31mm Nagler.

Over the years, the very thin colored line at the field stop that I've seen in eyepieces have also varied in width.  The line at the edge in the new TeleVue Apollo 11 is about the thinnest I've seen.

That's really never much of a problem at night, of course, but chromatic aberration in the form of lateral prismatic effects is an issue in some wide/ultrawide/hyperwide field eyepieces.

Photographing the issues can be hard when it is in the form of Edge of Field Brightening (EOFB), though Bill Paolini has captured some images that reveal it in at least one eyepiece.

It goes to show there is no perfect eyepiece.

7 hours ago, Mark at Beaufort said:

The short answer is No. I have had various other UHC filters - Skywatcher, Sky's the Limit and Baader UHC-S but found that the Lumicon and TeleVue gave better contrast. There has been a great deal of debate on Lumicon filters because the company was originally started in Livermore, CA then was sold out and the next batch was not as good. Since then stage 3 has provided a new series which I understand is good. Perhaps @Don Pensackin the States can confirm this situation.

Lumicon has had 4 owners and there have been at least 5 different makers of the filters.

The first owner was 1979-2001

The second owner was 2001-2012  The filters were VERY high quality, but expensive.

The 3rd owner was 2012-2016  She got bitten by an unscrupulous filter provider

The 4th owner is 2016 till now.  At first, he was selling off old stock, but moved into another provider in 2018.  They were relabeled "Gen.3" to differentiate.  I can see that.  Calling them Gen.9 would have been really confusing to customers. LOL.

The Baader UHC-S, by the way, is a misnomer.  It's a broadband filter of about a 62nm bandwidth.  it's one of the very best broadbands, but it's not a narrowband UHC-type filter.

Ditto the Astronomik UHC-E, 49nm bandwidth, which, though narrower than the UHC-S is more of a broadband than a narrowband UHC-type.  I'd call it a "medium bandwidth" filter.

This site allows you to superimpose any filters you want that have been tested:

https://searchlight.semrock.com/?sid=a08a1af9-84ee-49d2-959d-153d7e7c0eb8#

Note that all the filter graphs displayed are as they were tested in the lab, NOT what the manufacturer claims.  You can see he has tested a LOT of filters.

14 minutes ago, Gregoire said:

I

Could I ask you what brand of OCA gave you better results?

 

 

All.

But I note that Harry Siebert of Siebert optics makes a 1x OCA, which is a marvelous thing for yielding lower powers.

If the binoviewer is primarily used for Moon and planets, though, a higher power OCA might be preferable.

One thing to note about these filters.

They work by lowering the background brightness several magnitudes while dimming the nebula maybe only 0.1 magnitude.

The contrast enhancement is how they work.

As the magnification increases, the background in the scope dims, as does the nebula.  Above a certain point, the darkening of the background by the filter is small because you can't get darker than black,

but the nebula has dimmed.  This lowers the improvement in contrast and the filters stop being particularly effective.

With narrowband or O-III or H-ß filters, this occurs at about 10x/inch of aperture in the telescope.  That means they will be used at low power and if even your only 2" eyepiece is your low power, then get a 2" filter.

You can always thread it onto a 2" adapter to use with 1.25" eyepieces.

Now, broader filters, like a broadband, can be used at a bit higher magnifications without dimming the objects quite as much--maybe up to 12x-13x/inch of aperture.

But their effects are, at best, somewhat subtle, and only improve contrast a tiny bit.

5 hours ago, jetstream said:

Hows the scatter with these Don?

excellent.  it is nearly identical to the Noblex/Docter 12.5mm 84°.

It is not an eyepiece you would use for scanning through the sky, though as the design has some angular magnification distortion, seen as "globe" or "rolling ball" distortion.

other than that, which is different than most astronomical eyepieces, it's a fine eyepiece.