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2019 Nebula Filters Buyer's Guide

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15 replies to this topic

#1 Starman1

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Posted 26 March 2019 - 05:40 PM

Here is the latest version of the Nebula filters buyer's guide.

Many filters disappeared since the last version, but many more were added.

 

Attached Files


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#2 russell23

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Posted 26 March 2019 - 07:17 PM

Thanks for doing this Don!

 

A filter I've been using the last couple years as a nebula filter is the Astronomik Blue deep sky imaging filter. It has a roughly 90nm bandwidth.

I typically use the filter on emission nebula at smaller exit pupils where broadband and narrowband filters are dimming the image too much, but I do see contrast enhancement with the filter at larger exit pupils as well. 

 

Anyone with an imaging filter set can try the blue filter and see for themselves.  Before I bought the Astronomik filters I had used an Orion imaging filter for the same purpose. 


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#3 Jon Isaacs

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Posted 27 March 2019 - 02:34 PM

Don:

 

I just want to thank you for all the effort you put into compiling this Buyer's Guide. It's a valuable resource on multiple levels.

 

bow.gif

 

Jon


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#4 Allan Wade

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Posted 28 March 2019 - 01:01 AM

Nice guide Don. We are in the middle of a golden age for filters at the moment.



#5 desertlens

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Posted 28 March 2019 - 01:47 AM

I'm with Jon. Thanks Don for this and all of your contributions to this community.



#6 Ernest_SPB

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Posted 28 March 2019 - 05:41 AM

Great work! Thanks

 

bandwidth - main quality parameter - must be in separate column making possible to order by lines of the table

in "Type" O-III (one line) and O-III (both lines) could be separated from one another


Edited by Ernest_SPB, 28 March 2019 - 05:45 AM.


#7 AxelB

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Posted 28 March 2019 - 07:00 AM

Thanks!

This should be pinned. The old list is still useful for discontinued models.

#8 Starman1

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Posted 28 March 2019 - 08:53 AM

Great work! Thanks

 

bandwidth - main quality parameter - must be in separate column making possible to order by lines of the table

in "Type" O-III (one line) and O-III (both lines) could be separated from one another

Bandwidth, if the manufacturer even bothered to list it or show a chart, is in a separate column.

And, there are several different O-III filter types: 1 line (extremely narrow), 2 lines (narrow), 2 lines (wide), and 2 lines (wide) with red transmission.

I think the 1-line O-III filters are photographic filters, but Baader, Celestron, Thousand Oaks, and others sell them as visual filters.

No standardization.


Edited by Starman1, 28 March 2019 - 08:54 AM.


#9 sickfish

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Posted 28 March 2019 - 09:07 AM

Thanks Don.



#10 Napp

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Posted 28 March 2019 - 09:26 AM

Thanks, Don!



#11 Hesiod

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Posted 28 March 2019 - 01:05 PM

Thanks!



#12 harryst

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Posted 06 April 2019 - 06:56 PM

Very nice THANKS for posting it.



#13 careysub

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Posted 13 April 2019 - 12:42 PM

This question is not specific to the visual nebula filters in Don's spreadsheet, but this is a question that arose in my mind when perusing listings of all types of narrowband filters.

 

What is the intended, or accepted, meaning of "bandwidth"? I note that the Baader 3.5nm Ultra Narrowband H-Alpha Filter listed here:

https://agenaastro.c...-1-2459451.html

is described as "Hbw (halfbandwidth): 3.5 Nm".

 

Is this the total width between the half-intensity transmission points? Half the total bandwidth by some other standard? When a filter is described as a "XXnm filter" does the meaning of the XX number have a nominally consistent meaning (whether or not it is actually followed)?



#14 Starman1

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Posted 13 April 2019 - 01:51 PM

Yes, the bandwidth is often referred to as the FWHM bandwidth.

FWHM means "full width, half maximum", so it is the bandwidth at the point halfway between zero and maximum.

Now, that is the most relevant factor on a filter with a pure "square wave" response.

But if the curve of the filter has a Gaussian shape, the bandwidth could continue to widen as the response diminishes,

putting more energy in the OOB (out of band) wavelengths (just call it "noise" if you will).

 

Technically, however, a reduction of 100% transmission to 40% transmission (less than HBW) is only a dimming of 1 magnitude.

You can see response that is 1 magnitude down.

 

Filter coatings are applied in multi-layers, called "cavities".  The more cavities added, the narrower the bandwidth, but if the bandwidth gets too narrow,

the maximum transmission goes down as well.  I've see some great 3nm H-α filters that peaked at about 75% transmission.  The narrow bandwidth was great for H-α contrast

but the low transmission meant a longer exposure.

For visual use, we want high transmission (over 90% means a loss of <0.1 magnitude), but we also want a square wave response and a narrow bandwidth.

 

That can be accomplished in filters, but not cheaply.  More cavities = higher cost due to more materials used and lower production volume.

Tight bandwidth control also means a lot more rejects, lower volume, and higher costs.

If it costs $400k to make 4000 filters, and half of them get trashed, then the unit cost just doubled.

The processes are automated now, and the rejection rate is very low, but add the testing of each and every filter to the mix and suddenly the cost jumps again.

 

About the best achieved at the low consumer prices we see (not the $10k/each price you'd expect in aerospace) is a bandwidth with about a +/-2nm control

and a spread of about the same on the starting and ending wavelengths of the FWHM bandwidth.  That's one of the many reasons a single line filter for visual use

might have a 10-12nm bandwidth.  Add to that the shift that occurs as rays enter the filter more obliquely on short f/ratio scopes, and you're very lucky if the peak transmission falls around the spectral line the filter is designed to enhance.


Edited by Starman1, 13 April 2019 - 01:52 PM.

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#15 careysub

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Posted 14 April 2019 - 10:16 AM

The meaning of the term "Hbw (halfbandwidth): 3.5 Nm" for the cited Baader filter is still confusing with that explanation, since it seems to explicitly say that it is NOT "full bandwidth". But I suppose it really means "half maximum bandwidth" since otherwise it would be a run-of-the-mill  7nm filter and not worth the extra cost for the product.



#16 Starman1

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Posted 14 April 2019 - 11:01 AM

Yes, I think you're right.  The error is in translation, I'm sure, and what they really meant was FWHM.

Don




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