Narrower narrowband?

[Thalestris24]

Hi all

Just thought I'd put this out there to try and get some opinions. From time to time I've been imaging in Ha with a 2" Baader 7nm. It's fine except that I still get lp gradients which is very annoying. I'm wondering whether doubling up would likely help? There are the Astrodon 3nm or 5nm but they are fiendishly expensive, especially in the larger size.

Any thoughts/experiences? I'm quite happy with what I can get with the the 7nm - it's just the lp....

Cheers

Louise

[johnrt]

A narrower filter will indeed help. The 3nm Astrodons are expensive, but they are very, very good. I think there was another brand doing a 3nm, Chroma? I'm not sure how they compare in price or performance though.

[Thalestris24]

2 minutes ago, johnrt said:

A narrower filter will indeed help. The 3nm Astrodons are expensive, but they are very, very good. I think there was another brand doing a 3nm, Chroma? I'm not sure how they compare in price or performance though.

Hmm... What do you think about doubling up on the 7nm? It would be cheaper! Also, I'd hate to fork out on a narrower filter only to find I still had bleed-through... The Chroma filters are still expensive - £775 for a 2" 3nm Ha  I think I'd rather live with the lp than spend that much!

Thanks though

Louise

[johnrt]

2 minutes ago, Thalestris24 said:

Hmm... What do you think about doubling up on the 7nm? It would be cheaper! Also, I'd hate to fork out on a narrower filter only to find I still had bleed-through... The Chroma filters are still expensive - £775 for a 2" 3nm Ha  I think I'd rather live with the lp than spend that much!

Thanks though

Louise

Do you mean stacking x2 7nm filters on top of each other?

[gnomus]

I have the 3nm Chromas on one of my rigs and Astrodons on another.  There is no doubt in my mind that they are better than the wider filters.  The Ha especially has a wonderful 'creamy' quality to it, and they require little in the way of post-processing.  I have stopped DBEing them, for example.

The two rigs are different scopes and cameras, but I am just as happy with the Chromas as I am with the Astrodons.  The Chromas are a little cheaper than the Astrodons, but not substantially so.  You can get the Chromas from Bern at Modern Astronomy.

[gnomus]

4 minutes ago, Thalestris24 said:

Hmm... What do you think about doubling up on the 7nm? ....

I don't think that would work.  You would still only have a 7nm filter but with 4 surfaces now instead of 2 and reduced light transmission.   (At least I think so.)

[alacant]

Hi. How about a Wratten deep red #29 long pass in front of it? This one or the same on AliExpress: if you can wait until after their new year celebrations... HTH.

[gnomus]

Cost is a killer on these filters and I dithered for a time before getting them.  The 5nm filters are cheaper, Louise and people do go back and forth on 3nm vs 5nm. 

[Thalestris24]

4 minutes ago, alacant said:

Hi. How about a Wratten deep red #29 long pass in front of it? This one or the same on AliExpress: if you can wait until after their new year celebrations... HTH.

No sure how that would help - it's a relatively broadband filter?

[alacant]

1 minute ago, Thalestris24 said:

No sure how that would help

It cuts out all lp.

[Thalestris24]

8 minutes ago, gnomus said:

I don't think that would work.  You would still only have a 7nm filter but with 4 surfaces now instead of 2 and reduced light transmission.   (At least I think so.)

It was just a thought... As I say, I'm fine with the quality I get from the 7nm and I can't afford 3 or 5nm filters really. I get no lp if I image in infra red but obviously no nebulae or colour either.

[Thalestris24]

Just now, alacant said:

It cuts out all lp.

If the narrowband Ha doesn't cut it enough, I don't see how a long pass filter would either. Sure, infra red filters cut it but then no Ha.

[Physopto]

One point that is often missed when talking about different narrow band filters is the transmission level of the different makes. a 3nm by two different makers can have widely differing transmissions. Astrodons are quoting around 98% if memory serves whilst Baader ?  Not sure at all. The only real way is to test each filter with a bandpass analyser. I once had a very narrow band UV filter for 285nm that was quoted at 70+%. I got very poor results and was at a loss to explain it whilst testing new equipment. I managed to check it out at another university, on their machines and found its transmission was  around 6%. We got our money back and went elsewhere.

Derek

[Thalestris24]

8 minutes ago, Physopto said:

One point that is often missed when talking about different narrow band filters is the transmission level of the different makes. a 3nm by two different makers can have widely differing transmissions. Astrodons are quoting around 98% if memory serves whilst Baader ?  Not sure at all. The only real way is to test each filter with a bandpass analyser. I once had a very narrow band UV filter for 285nm that was quoted at 70+%. I got very poor results and was at a loss to explain it whilst testing new equipment. I managed to check it out at another university, on their machines and found its transmission was  around 6%. We got our money back and went elsewhere.

Derek

The Baader 3.5nm are certainly more affordable but can't find a transmission spectrum for them... Anyone tried them?

[SilverAstro]

First - I dont know, no experience ! but :-

You raised a very interesting question ! so I went to the Baader site to look for its spec, in particular what the tolerance was on the position of the skirt responses and/or on the width and centre of the passband  No joy so far, perhaps there is some literature that came with it ?

What I am thinking is that if for ex the the two skirt responses began to fall off at a slightly different knee wavelength then the two will combine to give a (very slightly ?) narrower pass band. Significant ? I dont know without their spec ! If there is tolerance on the centre then that also would contribute to a narrowing of the overall response. There is room for hope there maybe because I imagine that 5nm is wide enough to accommodate all the Ha if the 5nm is positioned slightly to one side or other a bit ??

Worth a try !! umm can you borrow one or three 

The effect of stacking the out-of-band ressponses would (probably?) be negligible as there is probably very little getting through there anyway.

As for £775 wow, I'd start saving for a new house somewhere else

 

[Thalestris24]

13 minutes ago, SilverAstro said:

First - I dont know, no experience ! but :-

You raised a very interesting question ! so I went to the Baader site to look for its spec, in particular what the tolerance was on the position of the skirt responses and/or on the width and centre of the passband  No joy so far, perhaps there is some literature that came with it ?

What I am thinking is that if for ex the the two skirt responses began to fall off at a slightly different knee wavelength then the two will combine to give a (very slightly ?) narrower pass band. Significant ? I dont know without their spec ! If there is tolerance on the centre then that also would contribute to a narrowing of the overall response. There is room for hope there maybe because I imagine that 5nm is wide enough to accommodate all the Ha if the 5nm is positioned slightly to one side or other a bit ??

Worth a try !! umm can you borrow one or three 

The effect of stacking the out-of-band ressponses would (probably?) be negligible as there is probably very little getting through there anyway.

As for £775 wow, I'd start saving for a new house somewhere else

 

The Baader 3.5nm are half the price but, as I say, couldn't find any info on the transmittance. As far as I can tell, there aren't any other filters that could help if combined with the Baader 7nm. I'm not sure that a 3nm or 3.5nm filter would help against the lp brightness. I think I'm doomed! Oh well. I suppose I should maybe try harder with the post processing...

[Gina]

If the LP is narrow-band like sodium or mercury lighting, it won't reduce the LP any more but it will reduce wideband light such as from the moon and maybe LED lighting.  Personally I haven't any experience of light pollution other than the moon (and aircraft).

[alacant]

51 minutes ago, Thalestris24 said:

I don't see how a long pass filter would

Cut out the lp before it gets to the Ha. Just an affordable alternative. If it really isn't strong enough, you've another filter to have a go with and you can get great effects...

HTH.

[ronin]

Stacking 2 should make no difference except to reduce the light throughput, Baaders are apparently rated at 90% at the peak so that would drop to around 80% (0.9 squared).

Any "narrowing" would be to chance - slight difference in where the peak lies or one just happened to have a width of 6.5nm not 7nm.

I see there is a Baader 3.5nm filter around in the Ha: BaaderEnF

Little info on it and not sure how the cost compares to the Astrodon item either.

The Baader 7nm has transmission up at 1150nm, not sure if that could be a problem but doubtful.

One possible problem is that the available curves will be the theoretical ones issued by Baader, what the actual filter does could be another matter. Some time back there was a post where a selection of filters were tested at OIII. Quite fun, one missed the OIII wavelength peak altogether and only caugh some owing to the width. The Baader OIII transmission claims about 87% at peak the measured one in the comparison gave fractionally under 70%.

I assume you have seen: Ast-Baader-Ha

[gnomus]

Ronin - that link you provided at the end of your post makes a very strong case for the 3nm Astrodons.

[Xplode]

1 hour ago, ronin said:

Stacking 2 should make no difference except to reduce the light throughput, Baaders are apparently rated at 90% at the peak so that would drop to around 80% (0.9 squared).

Any "narrowing" would be to chance - slight difference in where the peak lies or one just happened to have a width of 6.5nm not 7nm.

I see there is a Baader 3.5nm filter around in the Ha: BaaderEnF

Little info on it and not sure how the cost compares to the Astrodon item either.

The Baader 7nm has transmission up at 1150nm, not sure if that could be a problem but doubtful.

One possible problem is that the available curves will be the theoretical ones issued by Baader, what the actual filter does could be another matter. Some time back there was a post where a selection of filters were tested at OIII. Quite fun, one missed the OIII wavelength peak altogether and only caugh some owing to the width. The Baader OIII transmission claims about 87% at peak the measured one in the comparison gave fractionally under 70%.

I assume you have seen: Ast-Baader-Ha

Interesting to see the difference between the Baader and Astrodon filters, but since they are different bandwiths it's not really conclusive.

I would love to see a test between Baader 7nm/3.5nm and betwen Astrodon 3nm/Baader 3.5nm

I have a Baader 7nm and i'm thinking about getting a Baader 3.5nm so if i get one i will make sure to compare them.

[Thalestris24]

15 hours ago, Gina said:

If the LP is narrow-band like sodium or mercury lighting, it won't reduce the LP any more but it will reduce wideband light such as from the moon and maybe LED lighting.  Personally I haven't any experience of light pollution other than the moon (and aircraft).

It's yellow street lights, Gina. Nothing seems to suppress the light completely - it always seems to show up whatever I do  I suppose I'll just have to live with it. You're so fortunate to be out in the country!

Louise

[ollypenrice]

I can only echo Steve's comments about the Astrodons, 3nm in my case. Compared with the Baader 7 you get much smaller sars, far more local contrast and a remarkably smooth look. Once given a stretch the images look almost finished. In a back to back comparison during bright moonlight the 7nm data was scrap while the 3nm was perfectly serviceable.

Olly

[Thalestris24]

1 hour ago, ollypenrice said:

I can only echo Steve's comments about the Astrodons, 3nm in my case. Compared with the Baader 7 you get much smaller sars, far more local contrast and a remarkably smooth look. Once given a stretch the images look almost finished. In a back to back comparison during bright moonlight the 7nm data was scrap while the 3nm was perfectly serviceable.

Olly

Thanks, Olly, though I'm not really willing to fork out on an Astrodon... Besides, I suspect I'll still have the same problem since the longer exposure required by the 3nm would still let the lp in since it's clearly getting through regardless of the filter bandwidth. Wish I could move to somewhere better but that's not going to happen - unless I win the lotto jackpot, of course!

Louise

[ollypenrice]

37 minutes ago, Thalestris24 said:

Thanks, Olly, though I'm not really willing to fork out on an Astrodon... Besides, I suspect I'll still have the same problem since the longer exposure required by the 3nm would still let the lp in since it's clearly getting through regardless of the filter bandwidth. Wish I could move to somewhere better but that's not going to happen - unless I win the lotto jackpot, of course!

Louise

The price is horrendous but exposure times are not longer. I remember reading an account of why this is but can't remember the explanation. However, I find I collect a satisfactory 3nm signal in less time than I need for the 7nm.

Olly