Let's talk filters

[Merlin66]

Orion is a very big astronomical supplier in the US, sells SW mounts etc. re-badged to "Orion"

The human eye is still sensitive up to 700nm.

Visual H alpha observations of the Sun ( using extreme narrowband etalon filters) are regularly conducted by many amateurs.

CCD/ CMOS cameras are based on silicon technology which is sensitive up to 1100nm.

 

[Ricochet]

1 hour ago, Stardaze said:

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?

 

1 hour ago, Merlin66 said:

The human eye is still sensitive up to 700nm.

Visual H alpha observations of the Sun ( using extreme narrowband etalon filters) are regularly conducted by many amateurs.

You are both right. The response of the eye depends on the lighting conditions. During the day the cones dominate and the response is photopic (black line in the graph below). Here, H-alpha emission is visible. At night the rods dominate and vision becomes scotopic (green line). Under these conditions H-alpha emission is invisible, hence why UHC filters are mainly concerned with the H-beta transmission. Between these two extremes, vision is mespoic and so you might still have some red sensitivity. 

Also, the following graph shows the scotopic response curve compared to the Hydrogen/OIII emission lines.

[Stardaze]

9 minutes ago, Ricochet said:

 

You are both right. The response of the eye depends on the lighting conditions. During the day the cones dominate and the response is photopic (black line in the graph below). Here, H-alpha emission is visible. At night the rods dominate and vision becomes scotopic (green line). Under these conditions H-alpha emission is invisible, hence why UHC filters are mainly concerned with the H-beta transmission. Between these two extremes, vision is mespoic and so you might still have some red sensitivity. 

Also, the following graph shows the scotopic response curve compared to the Hydrogen/OIII emission lines.

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?? 

[John]

Orion (USA) is a re-brander. Today a lot (but not all) of their stuff is made by Synta who also own the Skywatcher brand and Celestron now as well. Orion branded stuff tends to be on the expensive side in the UK compared to the same product under Skywatcher or Celestron branding. In the past Orion have carried products made by Vixen in Japan and Intes in Russia. Below is a maksutov-newtonian that I used to own. It was made by Intes in Russia, their name for it is the MN61 but my example was re-branded a black livery by Orion and called "The Argonaut". I've owned a couple of their filters, the Skyglow broadband which I found did not make much difference and the Ultrablock UHC type which I found quite effective. I understand that the Ultrablocks can be very variable in their transmission ranging from the pretty good to the rather mediocre.

Who would have thought that the subject of deep sky filters was as complicated ?

 

[johninderby]

Yes as Orion branded stuff is shipped to the US first and then on to the UK it adds a lot of extra costs unlike Skywatcher that ships directly from China to the UK.

[bomberbaz]

18 minutes ago, Ricochet said:

 

You are both right. The response of the eye depends on the lighting conditions. During the day the cones dominate and the response is photopic (black line in the graph below). Here, H-alpha emission is visible. At night the rods dominate and vision becomes scotopic (green line). Under these conditions H-alpha emission is invisible, hence why UHC filters are mainly concerned with the H-beta transmission. Between these two extremes, vision is mespoic and so you might still have some red sensitivity. 

Also, the following graph shows the scotopic response curve compared to the Hydrogen/OIII emission lines.

I really would like to compare the astronomik uhc which allows HA transmission and something like the TV bandmates which cuts it entirely. 

An increase in contrast from the astronomik would point to the HA band being picked up, the level of difference would be indicative of the sensitivity. 

BTW I understand what you say above and I am not disputing this, just curious as to how much the shift away from cones to rods is.

I guess this would likely vary from person to person also depending on the eyes. 

[Stardaze]

47 minutes ago, bomberbaz said:

I really would like to compare the astronomik uhc which allows HA transmission and something like the TV bandmates which cuts it entirely. 

An increase in contrast from the astronomik would point to the HA band being picked up, the level of difference would be indicative of the sensitivity. 

BTW I understand what you say above and I am not disputing this, just curious as to how much the shift away from cones to rods is.

I guess this would likely vary from person to person also depending on the eyes. 

Traditional wisdom was that cones had no use at all in the dark. With the advent of led lighting, which has a much larger content of the blue end of the wavelength (for most applications they’re fundamentally blue chips with a phosphor coating to product a warmer hue), it’s been perceived that the eye is more sensitive in that end of the spectrum. To this end, it has challenged that traditional belief and now it’s felt that cones do also play some part in night vision. 

Edited June 14, 2020 by Stardaze

[bomberbaz]

5 minutes ago, Stardaze said:

Traditional wisdom was that cones had no use at all in the dark. With the advent of led lighting, which has a much larger content of the blue end of the wavelength (for most applications they’re fundamentally blue chips with a phosphor coating to product a warmer hue), it’s been perceived that the eye is more sensitive in that end of the spectrum. To this end, it has challenged that traditional belief and now it’s felt that cones do also play some part in night vision. 

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.

[John]

Does averted vision bring rods or cones into play ?

It certainly works !

[Stu]

5 minutes ago, John said:

Does averted vision bring rods or cones into play ?

It certainly works !

Rods are the ones that get used with averted vision John. I remember it from Cones are in the Centre, both Cs!

[Stardaze]

Definitely is the case. The very fact that the bandmates seem to have more contrast from what I understand, having omitted the alph B, does prove that we can see some of the infra red end around 750nm, it’s just that all graphs depicting the human eyes range seem to cut off at around 650nm? It’s a moot point really, but interesting.

[Stu]

Just found the pics of my filter boxes, I got lucky on all three with high percentage transmission. These are keepers for sure.

[John]

They look very good Stu

 

 

[Stardaze]

I might be talking a load of tosh looking at those filters. So if hydrogen alpha is 656nm, it's just within the accepted spectrum of the human eye. Maybe there's no benefit in blocking further into the infra red spectrum?

[Don Pensack]

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.

[Don Pensack]

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.

Edited June 14, 2020 by Don Pensack

[Don Pensack]

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.

 

[John]

I had a DGM NBP filter for quite a while. It was very effective as I recall. I just found that I was using an O-III much more often so I let the NBP go.

Then I missed having a UHC of course

So I got a couple of lower cost ones (ES UHC and Meade 4000 Narrowband) which are not bad but don't have quite the impact that the NBP had. If I find myself reaching for UHC more often I'll have to get another NBP.

 

 

 

[bingevader]

On 13/06/2020 at 18:01, Philip R said:

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. 

No qualms here either. Delighted to be able to see the Veil Nebula with the 8" dob. Couldn't see it without the filter! Mine wasn't second hand either, so paid the whole 60 odd quid, or what ever it was then.

[bomberbaz]

thanks for that @Don Pensack, that explanation makes sense although I am probably not going to splash out the cost of a another filter to test it out. 

[Ricochet]

23 hours ago, Stardaze said:

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?? 

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.

[Stardaze]

8 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.

What are your own personal favourites?

The DGM sounds interesting, especially for M42. Might see if one crops up secondhand. Definitely going to go bandmate 2 for the O-III but tempted to scrimp a bit with a UHC for now, upgrade it at some point later. Ordered an APM 13 earlier so plenty of time yet till the darker nights.

[Ricochet]

23 minutes ago, Stardaze said:

What are your own personal favourites?

The DGM sounds interesting, especially for M42. Might see if one crops up secondhand. Definitely going to go bandmate 2 for the O-III but tempted to scrimp a bit with a UHC for now, upgrade it at some point later. Ordered an APM 13 earlier so plenty of time yet till the darker nights.

I've not had enough to have personal favourites, but I would go for the Astronomik OIII over the TV Bandmate II OIII. I can't see any obvious difference in the specified spectra and the searchlight data for the 4 filters tested looks more like batch variation rather than a design difference. If you're looking at 2" filters then I think it is better to save yourself the £35 that it costs for the filter to be shipped from Astonomik in Germany, to TeleVue in the USA, and then back to the UK.

For the UHC filter, TV have clearly specified a filter without the H-alpha spike. In this case I would happily pay the "TV premium" because it is a different product to the one that Astronomik are selling under their own branding.

[Don Pensack]

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.

[Stardaze]

I've bought an Astronomik 2" O-III secondhand which has just arrived. What's the best practice for cleaning a filter? I've blown some dust off but it could do with a clean really.