Imaging filter for visual?

[Ratlet]

I feel the answer is no, but I can't work out why...

I've got an SVBONY dual band filter (7nm pass).  I've been toying with the idea of getting an Oiii filter but wondering if I can use this instead.

[vlaiv]

2 hours ago, Ratlet said:

I feel the answer is no, but I can't work out why...

I think you gave the answer in the second half of that sentence.

There is no reason why not - and hence, answer is yes.

Dual band filter will pass both Ha and OIII - and while Ha is almost invisible to dark adapted person - so is the sky glow at that wavelength. Filter will act as OIII filter for 99.9% of the time (unless there is really bright Ha source - like M42, where it could make small difference over regular OIII in what it shows).

 

[Ratlet]

29 minutes ago, vlaiv said:

I think you gave the answer in the second half of that sentence.

There is no reason why not - and hence, answer is yes.

Dual band filter will pass both Ha and OIII - and while Ha is almost invisible to dark adapted person - so is the sky glow at that wavelength. Filter will act as OIII filter for 99.9% of the time (unless there is really bright Ha source - like M42, where it could make small difference over regular OIII in what it shows).

 

Cheers @vlaiv, you've saved me a chunk of money.

I guess it felt a little too good to be true, like having your cake and eating it.

[Louis D]

As long as the bandpass(es) are within the range of human vision (photopic or scotopic, depending on the viewing conditions), photographic filters should work fine.  An example of one that would not work visually would be a Ca-K filter which operates at 393nm, or far violet, which is basically not perceivable by our eyes.

I use a Meade Green interference filter visually which was sold as part of a photographic set for LRGB photography.  It's probably my best green filter by far.  It has high transmission and very sharp cutoffs at the blue and red ends.

Note how tight, bright, and pure the Meade Green is compared to my other green filters:

The other two Meade interference color filters work well as blue and red filters as seen below:

Edited April 26, 2023 by Louis D
Added images to show passband of Meade Green

[Franklin]

I have a 1.25" Baader OIII 10nm visual filter and I also have a 2" Baader OIII 8.5nm CCD filter and have used both visually and they both work visually. The 8.5nm CCD offers a darker sky background than the 10nm visual.

[John]

I think it will work but it won't necessarily be an indication of what a good visual O-III filter can do.

 

 

[vlaiv]

2 minutes ago, John said:

I think it will work but it won't necessarily be an indication of what a good visual O-III filter can do.

 

 

Why do you think good visual OIII filter will best it?

[PeterW]

I can confirm that visual use of a CaK filter is a waste of time…. Modern cataract surgery adds back UV filtration, so that won’t help. As noted anything in the visual range should work fine, just a matter of how narrow the bandwidth is and thus what effect it will have one the contrast. I’ve used 5.5nm hydrogen beta filters on some binoculars to make some galactic nebulae visible from an urban location. Christopher Hay has done comparisons of h-beta and OIiI for visual observing and the narrower tend to do a better job, even though conventional wisdom would advise such filters are only for big scopes and dark skies….

https://www.cloudynights.com/topic/722322-pushing-the-filter-envelope-observing-galactic-nebulae-with-handheld-binoculars-under-suburban-skies/
 

Peter

 

[Ratlet]

I'm mostly just in it to get a flavour.  SVBONY have a sale on at the moment of some items on a price list and they had a couple oiii filters that were 50% off.  They are 18nm cut.  I had a look and cloudy nights suggested that 10nm was best and I thought that 7nm is pretty close to 10nm.

I've got a 10" Dob I want to try it on as well as a 5" Newtonian.  Had good experiences with both scopes and a 1.25" svbony UHC on planetary so wanted to see if there was appreciable difference.

Also the fact various chats on here have made me realise my 130pds with a 30mm uff will squeeze the entire veil into the fov which is quite exciting.

[bosun21]

I had both the SVbony UHC and OIII filters which I was relatively happy with until I borrowed the Astronomik UHC and OIII filters from a friend. The difference was apparent straight away with the surrounding sky being darker and the target brighter. Soon thereafter I sold both the SVbony filters and purchased both the Astronomiks. They are indeed more expensive but I learned with my eyepiece journey the old saying “buy cheap buy twice”. This definitely applied to me in this case.

Edited April 26, 2023 by bosun21

[John]

41 minutes ago, bosun21 said:

I had both the SVbony UHC and OIII filters which I was relatively happy with until I borrowed the Astronomik UHC and OIII filters from a friend. The difference was apparent straight away with the surrounding sky being darker and the target brighter. Soon thereafter I sold both the SVbony filters and purchased both the Astronomiks. They are indeed more expensive but I learned with my eyepiece journey the old saying “buy cheap buy twice”. This definitely applied to me in this case.

That was the point that I was trying to make.

I have seen quite often people put off from using O-III or UHC filters though getting rather uninspiring results from less effective ones. When you use good ones which have an effective specification for visual use (which do tend to cost more 🙄) the impact can be significantly greater on receptive targets - like night and day really.

 

Edited April 26, 2023 by John

[vlaiv]

Performance of the filter depends on two parameters:

1. width of filter band - which is usually expressed as FWHM width

2. Peak transmission.

SVBony visual OIII has FWHM of 18nm and while Astronomik does not list FWHM of its visual filter - it is safe to say that it is less than that.

Here is transmission graph overlaid one on top of the other. Blue is Astronomik, green is Svbony.

Both will be bested by Astronomik OIII-CCD which has 12nm or even narrower one which has 6nm bandpass.

Similarly SVbony photographic OIII/Ha has 7nm bandpass and it will perform better than 12nm Astronomik CCD one, but a bit worse than 6nm Astronomik OIII-CCD

Bandwidth determines contrast of the target while peak transmission determines brightness (and few percent there does not make visible difference - we need about 7% difference in intensity to notice any difference at all).

[Louis D]

One problem I've had with Chinese made OIII filters in the past is that they tend to have poorer quality control.  I'm not saying that is the case any longer, but it really falls to the branding company to ensure the quality of what they offer under their name.

Note below how my decade old Zhumell OIII filter (Chinese made) is right shifted.  It actually made for a excellent comet filter on Comet ZTF to make it stand out better from my light polluted skies.  I'm actually quite happy with the $10 I paid for it now that I know it makes for a good comet filter.  By way of comparison, my Lumicon OIII filters basically masked it completely.

My new Svbony UHC is actually quite decent compared to my 25 year old Lumicon UHC, especially considering I paid $22 for it.  The slight blue-violet bleed isn't visually noticeable by my older eyes.  The view of the Orion Nebula looked remarkably similar through both.

[Ratlet]

QC is always a concern buying direct from China.  This far I've only had minor issues thankfully and they've been resolved by AliExpress quite quickly.

I think svbony are trying to make a push to higher quality gear which is part of the reason I went for the sv220.  I was thoroughly impressed by the eyepieces (the sv215 is an absolute delight)  from them and figured they would do a reasonable amount of due diligence.

It performed well I think in the one night I got out imaging with it (despite only 11 minutes of data).

Really I'm not looking for a revelation but another opportunity to boost my understanding of how things work.  It'll be interesting to compare and contrast the narrower cut Vs the much broader UHC.  Doesn't need to be good, just good enough.

Cuiv the lazy geek did some testing and seemed to get stung a while back so it appears that QC of high performance filters is challenging.  It's a shame that they are so difficult to objectively test at home.

I really appreciate the spectrographs you post @Louis D.  I've got a folder full of them stashed on my computer.

[Mr Spock]

I have an original Lumicon OIII. It says on the box 486nm 0.3%, 496nm 93%, 501nm 90%, 656nm N/A

Can't say I've seen much through it. But, LP is really bad here so not that surprising

[PeterW]

The other thing that’s little notes is the amount of blocking that filters provide away from the transmission line, the degree they block the light pollution and thus the resulting contrast. Even a fraction of a % can affect thingsZ very few companies publish the data, more expensive filters are likely to do better.

 

Peter

[vlaiv]

15 minutes ago, PeterW said:

The other thing that’s little notes is the amount of blocking that filters provide away from the transmission line, the degree they block the light pollution and thus the resulting contrast. Even a fraction of a % can affect thingsZ very few companies publish the data, more expensive filters are likely to do better.

 

Peter

This is true for Solar Ha etalons, but I'm not sure if it is that important for visual filters and light pollution.

Difference being magnitude of light involved - both on band and off band, and also the fact that there is something called JND - just noticeable difference which is about 7% for visual. We need something to be 7% brighter to barely notice it is brighter.

I think that you won't notice increased brightness of the background if blocking part of filter differs fraction of percent.

Again, for contrast - if we have say 7nm FWHM and roughly 300nm of whole range, that is ~ x43 difference. Even if our eyes were equally sensitive to whole 400-700nm range (and they are not, they peak at around 500nm in scotopic vision - so perfect for OIII) - it would take difference of about 0.16% in transmission (0.16% * 43 = ~7%) to even have a chance to notice contrast difference (again due to need to change contrast by ~7% to start noticing).

In reality, due to sensitivity to light not being uniform - this is probably more like 1-2% rather than 0.16%.

 

[Louis D]

The only concern I might have with photographic filters is when using them for solar observing.  Some photographic line filters strongly leak in the IR wavelengths.  This is not an issue photographically because they are usually paired with a UV/IR blocking filter when imaging in the visual spectrum because test images will clearly reveal this leakage.  However, the human eye can't perceive this leakage beyond a mild discomfort while viewing.  Thus, be careful and knowledgeable when using photographic filters for solar observing and take appropriate safety measures.

[jetstream]

@Ratlet Do yourself a favour and buy a top OIII from either Astronomik or (New)Televue. The difference between a very good OIII and a mediocre one is staggering from dark skies like you have. Pickerings Wisp just wont be seen in your 10" dob/top OIII it will show very nice detail.

Top OIII are just wide enough to catch both OIII lines completely and have very high transmission. Transmission matters contrary to some beliefs about our ability to detect it. IMHO

Gerry

[jetstream]

4 hours ago, jetstream said:

@Ratlet Do yourself a favour and buy a top OIII from either Astronomik or (New)Televue. The difference between a very good OIII and a mediocre one is staggering from dark skies like you have. Pickerings Wisp just wont be seen in your 10" dob/top OIII it will show very nice detail.

Top OIII are just wide enough to catch both OIII lines completely and have very high transmission. Transmission matters contrary to some beliefs about our ability to detect it. IMHO

Gerry

Just to add, the 10" dob will show at least as good as this mine does better here. I have an old, excellent Lumicon OIII that shows this but the new TV OIII is a bit better. This is a borrowed illustration. Exit pupil matters to maximize views and I find that for some unknown reason 100 deg EPs excel on objects like this. The "Majesty Factor" perhaps? (Nagles idea).

Edited April 27, 2023 by jetstream

[PeterW]

It’s more that at night your eyes are colourblind and although the sensitivity rolls off away from the green, if you have a nice bright bit of light pollution and attenuate it 99.5% it’ll still be bright and possibly swamp your faint nebula. With your eyes being logarithmically sensitive it matters if you have 0.1% or 0.01% or better away from the transmission. Narrow  transmission and good blocking give the best contrast enhancement, though this can kill the stars off pretty severely too.

Baader are doing a lot of innovating in this space, keep them in mind too. 

Peter

[jetstream]

The new Baader CMOS Hb filters are rumoured to be excellent visually. My next filter purchase will be one of these.

Edited April 27, 2023 by jetstream

[Space Hopper]

Crikey :   € 295 . 00 for a filter 😮

It would need to be excellent.

[John]

33 minutes ago, Space Hopper said:

Crikey :   € 295 . 00 for a filter 😮

It would need to be excellent.

The TV Bandmate 2 Nebustar 2 inch is £299 now. It is really excellent though, by all accounts.

[PeterW]

10 hours ago, Space Hopper said:

Crikey :   € 295 . 00 for a filter 😮

It would need to be excellent.

You ain’t seen what Chroma filters cost… they recently put up their not insignificant cost a bit….

Peter