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Understanding Filters and which ones to use


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I have recently been reading up on filters and I am hoping to clear up some unanswered questions I have about them. I started off buying the basic colour set that Celestron has to offer with the different Wratten colours like #12 (deep yellow), #21 (orange), #25 (red), #58 (green), #80A (blue), as well as some other ones, which I have been using for observational viewing of planets and stars like Antares, Hadar and Betelgeuse.

I then started looking at other types of filters to begin using them for Astrophotography. I have since acquired the Celestron OIII Narrowband and the a UV/IR filter.

I know the next steps to go would be to acquire a SII, H-Alpha and possibly a H-Beta, however what I don't understand is the wavelength measurements. If the Narrowbands say 40nm or 8nm, which one would be the better one to go for?

Also, which brand would be more suited for Deep Space Astrophotography of Nebulae?

I will be looking at using Black and white photos with each filter in place then stacking the photos using Registack and Photoshop, as they bring out more detail.

Any other suggestions would be greatly appreciated.

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Hi Anthtone,

In the case of H-Alpha the wavelength of this line is 656 nm (nano-meters). A pass band of 40 nm would let photons between 636 nm to 676 nm through if centred at 656 nm.

The narrower filters will transmit less total light, but more will be hydrogen line.

I would guess that the narrower pass bands are probably more suited to bright stars rather than nebulae, but someone else may have more to add.

Ben

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I have recently been reading up on filters and I am hoping to clear up some unanswered questions I have about them. I started off buying the basic colour set that Celestron has to offer with the different Wratten colours like #12 (deep yellow), #21 (orange), #25 (red), #58 (green), #80A (blue), as well as some other ones, which I have been using for observational viewing of planets and stars like Antares, Hadar and Betelgeuse.

I then started looking at other types of filters to begin using them for Astrophotography. I have since acquired the Celestron OIII Narrowband and the a UV/IR filter.

I know the next steps to go would be to acquire a SII, H-Alpha and possibly a H-Beta, however what I don't understand is the wavelength measurements. If the Narrowbands say 40nm or 8nm, which one would be the better one to go for?

Also, which brand would be more suited for Deep Space Astrophotography of Nebulae?

I will be looking at using Black and white photos with each filter in place then stacking the photos using Registack and Photoshop, as they bring out more detail.

Any other suggestions would be greatly appreciated.

are you happy with the quality of the celestron set?

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Thanks Ben.

That makes sense on when to use them too.

Saw a forum thread on here on making an automatic filter wheel. Suspect this might have to be a project to tackle in the near future as I get these filters for quick swap in and out.

Anth

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Hi Oymd.

Very happy with the sets I have got from celestron. I did originally go the cheap options on some of the colour filters and quickly learnt, that you get what you pay for with them. i.e. easy scratching on the cheaper ones; outside housing made of plastic, which when adding to the eyepieces feel rough to screw on, in comparison to a nicely machined metal filter housing.

Anth

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I know the next steps to go would be to acquire a SII, H-Alpha and possibly a H-Beta, however what I don't understand is the wavelength measurements. If the Narrowbands say 40nm or 8nm, which one would be the better one to go for?

Also, which brand would be more suited for Deep Space Astrophotography of Nebulae?

Since it's a narrow-band filter, the number is probably the bandwidth: so the range of wave lengths it lets through. You can give that number a context if you recall that the visible spectrum is roughly from 750 nm to 380 nm. So that's a 370 nm range. The 8 filter is about 2% of that range and the 40 nm filter is about 11% of that range. My guess is that the narrower bandwidth filter is a photographic one. It's very narrow, so it will likely produce a view that appears very dim to the eye and will only work well in long exposure images (where the narrow bandwidth will translate to greater contrast).

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oymd... depends on how you are using the filters. As I mentioned I use the coloured filters directly for viewing planets and stars using eye to telescope.

for nebulae they wouldn't have as greater depth of contrast nor detail... that is why I have got ones for viewing and then looking at the ones for astrophotography.... i.e. correct tools for the right job :)

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oymd... depends on how you are using the filters. As I mentioned I use the coloured filters directly for viewing planets and stars using eye to telescope.

for nebulae they wouldn't have as greater depth of contrast nor detail... that is why I have got ones for viewing and then looking at the ones for astrophotography.... i.e. correct tools for the right job :)

Ah, but not everyone finds coloured filters useful for planets. Others find they help bring out certain colour contrasts. Personal taste, I reckon. I'm not sure how they'd help in viewing stars, though...

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You don't say what mount and scope you are using but I gather with narrowband filters in OIII and SII you would need to track for a reasonable time to get enough light in through such a narrow bandwidth, particularly with smaller apertures.

I think Ha with a wider (say 35nm compared to 7nm) bandwidth should be useful blended with the R in LRGB. I've gone for the LRGB first with a manual filter wheel (about £65) and have just got the 35nm Ha filter to experiment as my next step. Advice has been widely variable so thought I would dive in and see for myself.

I wanted to get into the proper narrowband action too but I think with my small f5 scope on an AZ GOTO I stand very little chance so I'm concentrating on LRGB + Ha (for now!!!)

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Think I can now add my signature in M37.

I am using Celestron C8-SGT XLT with EQ5 Goto Mount.

Backend I am switching between:

-10:1 Bintel Crayford focuser, 2" 45 Degree and then a 2" to 1.25" reducer for visual viewing and

-a 6.3 Focal reducer, visual back, plossl 17mm and 32mm lenses, then with DSLR tele-extender with Canon T-Ring and a Canon 600D DSLR for astrophotography

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