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Old 13-11-2020, 04:32 PM
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Rob6542 (Rob)
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Filters - How to tell which is which

Hi all,
I have obtained a ZWO 7 position 36mm EFW, however the filters were already installed and as they are 36mm frameless there is no writting or markings to tell which is which. Unfortunatley the previous owner can't tell me.
So my question is, is there anyway to tell which is which by looking at them, or will the imaging software (ATP, Deep Sky Stacker, Star Tools, Pixinsight etc etc) be able to tell me from a frame i.e. if I take one frame with one filter will the software be able to tell me that it was a red filter, or green etc, would the histogram give the detail by where the spike is.
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Old 13-11-2020, 05:07 PM
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Merlin66 (Ken)
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I can do it with the spectrograph.....
If they were pre-installed, what does ZWO say???
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Old 14-11-2020, 07:03 AM
JA
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Quote:
Originally Posted by Rob6542 View Post
Hi all,
I have obtained a ZWO 7 position 36mm EFW, however the filters were already installed and as they are 36mm frameless there is no writting or markings to tell which is which. Unfortunatley the previous owner can't tell me.
So my question is, is there anyway to tell which is which by looking at them, or will the imaging software (ATP, Deep Sky Stacker, Star Tools, Pixinsight etc etc) be able to tell me from a frame i.e. if I take one frame with one filter will the software be able to tell me that it was a red filter, or green etc, would the histogram give the detail by where the spike is.
Hi Rob,

A spectrum analysis (of transmitted light) as mentioned by Ken would be the most definitive way, but barring that it may still should be possible to get an idea by observing the transmitted light and/or taking images with the various filters in combination with some deductive reasoning. The software you mentioned may help you somewhat in terms of measuring intensity, but it doesn't know the wavelength, for that part of the puzzle you need some form of reference or basis for absolute comparison. This might be :

1. using an image taken with a known filter of some object, not necessarily astronomical, and comparing it with your set of unknown filter/s and then making some deductions or

2. using some form of known light source transmitting light through your unknown filters and in combination with images or observations and some knowledge of the filter's passband use deduction to help conclude which filter is which.

The job really depends on how many of those 7 positions are populated and with which filters. If it's fully populated then a reasonable starting point would probably be that it has

1. Luminance filter or UV/IR filter
2. Red Filter
3. Green Filter
4. Blue Filter
5. O-III Filter
6. Ha Filter
7. S-II Filter

The next step would be visual inspection to determine the hopefully obvious filters using visible (white light, say diffuse daylight or even artificial broadband indoor light). Using this you should be able to discern the Luminance filter or UV/IR filter as it should appear clear. The RED, BLUE and GREEN filters will appear their respective colours, except that the RED depending on how broad its broad spectrum will also include part or all of the passbands of the Ha and S-II.

Now try to find a laser pointer or at a pinch an optical mouse(don't look in to the laser beam). A laser pointer is preferable as it often specifies its wavelength on the warning label, typically reading something like 650nm and 1mW. Some power tools such as mitre saws, etc also have such lasers fitted. I have one from a table saw that is a 650nm wavelength and 1mW. Now using the 650nm laser light source SHINING ON TO A WHITE SURFACE (don't look in to the beam) insert one of your unknown red coloured filters into the beam path. The S-II filter (672nm) will pass very little if any of the 650nm wavelength depending on its bandwidth whereas the Ha filter (656nm) will pass considerably more. OK so that's the S-II filter dealt with (hopefully). Now to tell the difference between the RED filter and the Ha. It should be possible to tell that difference using visible light as the RED filter should pass more visible light that the Ha Filter. That should then elucidate the difference between the RED and Ha filters, leaving only one filter "unknown", which if you are lucky by the process of elimination (supposing the 7 filter setup I suggested above) should be the O-III filter. One caveat some filters have coatings which can hide their "true colour". Always use transmitted light through the filter as your guide, rather than looking at the surface of the filter glass, especially at acute angles where the colour is camouflaged somewhat.

Have fun



Best
JA

Last edited by JA; 14-11-2020 at 04:32 PM. Reason: typo
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Old 14-11-2020, 07:14 AM
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Merlin66 (Ken)
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OK,
What about a simple test....
Remove the filter wheel and use it visually to view your computer screen.
Set up a white page and look through the filters...
The colour you see will be close the the filter transmission.
( Futher refinement - If you set up some coloured screens the filter will show no (or very little) transmission of complimentary colours)
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Old 14-11-2020, 12:30 PM
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Hi Merlin66 (Ken), they were installed by the previous owner and he has removed the software that would have been able to tell me whiuch one he put in where.
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Old 14-11-2020, 12:34 PM
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Thanks JA, Merlin 66, All helpfull stuff and I will give your suggestions a go. All 7 holes are filled and JA you are correct with the filters you listed.
I'll put do my best Abbey from NCIS impersonation and see if I can figure then out.

Last edited by Rob6542; 14-11-2020 at 04:19 PM.
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Old 14-11-2020, 04:20 PM
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All sorted, using your advice JA, plus some additional information on other forums I think I have figured it out and I am 99% confident that the positions are filled in the order you listed.
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