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Charlie HeinAdministrator
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GCE filters from DGM Optics
      #5644329 - 01/26/13 12:39 PM

GCE filters from DGM Optics

By Al Milano


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David Knisely
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Reged: 04/19/04

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Re: GCE filters from DGM Optics new [Re: Charlie Hein]
      #5645370 - 01/27/13 01:54 AM Attachment (67 downloads)

Al Milano wrote:

Quote:

At a quick glance, they might be considered close to a 'Broadband' type of light pollution filter - As opposed to say, a UHC-type (for example). I suppose the only popular filters that they could be compared to, would be the 'Deep Sky' types. However the GCE's actually do improve the view on galaxies in my heavily light polluted backyard. And, Iíve never heard or read about any skyglow-type, or other LPR filters being able to accomplish this.




Then perhaps the author hasn't heard or read nearly enough about these filters. It has been commonly known since the late 1980's that the broad-band "LPR" filters like the Lumicon Deep-sky can indeed help the view of galaxies to some small degree. Usually they end up notching out the common emission lines of mercury and sodium vapor while passing as much of the rest of the visual spectrum as possible. This can sometimes result in a small but definite increase in the contrast for some of the larger and more diffuse galaxies like the ones the author looked at. The improvement tends to be greater under darker skies than under conditions where the skyglow is more notable. Indeed, the DGM Optics GCE filter was rigorously tested and reviewed way back in 2007:

CN REPORTS: DGM Optics GCE Filter

While some benefit on some galaxies was noted in that test, it was also noted the improvement was mild and was not universal. Indeed, the GCE filter tended to be a bit less effective under some mild to moderate skyglow than the Lumicon Deep-sky filter was. In fact, under moderate to severe skyglow, the broadband LPR filters tend to lose what effectiveness they have as the skyglow overwhelms the faint detail in galaxies even when filtered. I am glad the author had some success with the GCE filter, as it does help sometimes. However, while it works to some mild degree in some circumstances, for those expecting a lot, they may be disappointed. The GCE doesn't provide nearly the same level of enhancement on galaxies that the narrow-band and line filters do on emission nebulae. In short, they won't work miracles, so don't expect too much. To get the most out of galaxies, the "gasoline" filter is still probably the best bet. Clear skies to you.


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Lightning
sage


Reged: 07/04/10

Loc: Canberra, Australia
Re: GCE filters from DGM Optics new [Re: David Knisely]
      #5653560 - 01/31/13 02:50 AM

One thing that may have made a point of difference in the author's experience (other than a lack of alternative filters that is...) is that he was using them with binoculars - the 10x50s being the most common for him.

So with that in mind, a slight drop in background brightness may have greatly improved the constrast for number of objects.

I know my ~30nm O3 filter approximately "draws even" on stars under my highly light polluted sky. I would likely get better performance with the GCE on clusters while most nebulae wouldn't get quite the contrast boost I already gain from the O3.

Cheers,
Cam


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Danman53
Vendor (DGM Optics)


Reged: 11/17/11

Re: GCE filters from DGM Optics new [Re: David Knisely]
      #5658827 - 02/02/13 08:04 PM

Quote:

Al Milano wrote:

Quote:

At a quick glance, they might be considered close to a 'Broadband' type of light pollution filter - As opposed to say, a UHC-type (for example). I suppose the only popular filters that they could be compared to, would be the 'Deep Sky' types. However the GCE's actually do improve the view on galaxies in my heavily light polluted backyard. And, Iíve never heard or read about any skyglow-type, or other LPR filters being able to accomplish this.




Then perhaps the author hasn't heard or read nearly enough about these filters. It has been commonly known since the late 1980's that the broad-band "LPR" filters like the Lumicon Deep-sky can indeed help the view of galaxies to some small degree. Usually they end up notching out the common emission lines of mercury and sodium vapor while passing as much of the rest of the visual spectrum as possible. This can sometimes result in a small but definite increase in the contrast for some of the larger and more diffuse galaxies like the ones the author looked at. The improvement tends to be greater under darker skies than under conditions where the skyglow is more notable. Indeed, the DGM Optics GCE filter was rigorously tested and reviewed way back in 2007:

CN REPORTS: DGM Optics GCE Filter

While some benefit on some galaxies was noted in that test, it was also noted the improvement was mild and was not universal. Indeed, the GCE filter tended to be a bit less effective under some mild to moderate skyglow than the Lumicon Deep-sky filter was. In fact, under moderate to severe skyglow, the broadband LPR filters tend to lose what effectiveness they have as the skyglow overwhelms the faint detail in galaxies even when filtered. I am glad the author had some success with the GCE filter, as it does help sometimes. However, while it works to some mild degree in some circumstances, for those expecting a lot, they may be disappointed. The GCE doesn't provide nearly the same level of enhancement on galaxies that the narrow-band and line filters do on emission nebulae. In short, they won't work miracles, so don't expect too much. To get the most out of galaxies, the "gasoline" filter is still probably the best bet. Clear skies to you.




Hi David,

One distinction I would note between the GCE vs a traditional Wideband (Deep Sky type) filter is that they all use a wideband design which are typically just a 50-55 nnm wide bandpass, where as the GCE is a true notch by design. Some WBs are a blocked in the red others have high TX% at 656.3. While I didn`t have a Deep Sky when I designed the GCE I did have a prototype which I used as an A/B and found the GCE to be a bit brighter than my prototype WB.

As a comparison quick glance here`s an overlay of the GCE and a typical Wideband. Granted anything beyond Ha and lower than 450nm is largely insignificant, there is still a bit more overall energy throughput.

I think part of the reason they work well with small binos is the low power-to-aperture ratios and widefield. I`ve setup a pair of raw filters (GCE) on the inside entrance of my very cheap Tasco 10 x 50mm as I kind of what if. I also tested the NPB and VHT with 2" over-the-aperture to compare performance. And while both the NPB and VHT did enhance bright nebula, the GCE provided by far the most pleasing view aesthetically and hardly effected background stars. I am looking forward to summer viewing with the GCE binos where I think they will provide some excellent views of large diffuse nebula or milky way dust lanes. I`ve taken a pair of 2" GCE`s and just "flashed" them in front of my eyes where it really brings out the dark lanes. The effect with binos should be very nice.

As far as galaxies, you correct, alas, in that the only real cure is aperture and gasoline ..... that said what I have seen so far with my limited experience is that binos seem to bring out the best with the GCE. I`m currently working with Omega to find the most cost effective way to offer raw filters which can be placed behind the eyepieces of selected binos which have accessibility to the EPs. Maybe even pre-coated EP lenses. Could be interesting!

best regards,
Dan


RED = GCE
BLUE = Wideband


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Danman53
Vendor (DGM Optics)


Reged: 11/17/11

Re: GCE filters from DGM Optics new [Re: Lightning]
      #5658853 - 02/02/13 08:16 PM

Quote:

One thing that may have made a point of difference in the author's experience (other than a lack of alternative filters that is...) is that he was using them with binoculars - the 10x50s being the most common for him.

So with that in mind, a slight drop in background brightness may have greatly improved the constrast for number of objects.

I know my ~30nm O3 filter approximately "draws even" on stars under my highly light polluted sky. I would likely get better performance with the GCE on clusters while most nebulae wouldn't get quite the contrast boost I already gain from the O3.

Cheers,
Cam




Hi Cam,

Also the GCE rejects the same light-pollution WLs as all nebula/LPR filters, so it does enhance nebula. Although not as the NPB or an OIII, but it does so while very much preserving background stars, where I found with my 10x50s the NPB was just too much for such small aperture. I never even tried my OIII`s.

best regards,
Dan McShane


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faackanders2
Carpal Tunnel


Reged: 03/28/11

Re: GCE filters from DGM Optics new [Re: Danman53]
      #5658872 - 02/02/13 08:30 PM

The plot is quite a bit different with lots more transmission for the GCE.

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Danman53
Vendor (DGM Optics)


Reged: 11/17/11

Re: GCE filters from DGM Optics new [Re: faackanders2]
      #5659163 - 02/03/13 12:02 AM

Quote:

The plot is quite a bit different with lots more transmission for the GCE.




They`re just design plots, not actual spectral scans. Although the actual spectral results with the GCE are almost identical to the theoretical, and typical widebands easily average 96% thru the passband. The GCE averages about 97% thru the visible, outside the rejection notch. So they`re a fairly good representation of the (2) designs.


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David Knisely
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Reged: 04/19/04

Loc: southeastern Nebraska
Re: GCE filters from DGM Optics new [Re: Danman53]
      #5659283 - 02/03/13 01:58 AM

Hi there Dan. You posted:

Quote:

One distinction I would note between the GCE vs a traditional Wideband (Deep Sky type) filter is that they all use a wideband design which are typically just a 50-55 nnm wide bandpass, where as the GCE is a true notch by design. Some WBs are a blocked in the red others have high TX% at 656.3. While I didn`t have a Deep Sky when I designed the GCE I did have a prototype which I used as an A/B and found the GCE to be a bit brighter than my prototype WB.




Well, my old Lumicon Deep-sky has a primary passband which has a FWHM of about 71.5 nm (94% max transmission) with fairly steep filter skirts, while my newer Orion Skyglow has a FWHM of 64 nm (98.5% max transmission). Your plot of the GCE shows about a 63.7 nm FWHM figure, but in spectroscopically comparing my two broad-bands with the GCE you sent me for review, the GCE has somewhat less steep skirts than either of the broadbands. Its primary "notch" is also not quite as wide as that of the Orion Skyglow and also not quite as aggressive as that of the Deep-sky, which, I think, is why the regular broadbands were somewhat more effective under mild suburban skyglow than the GCE was. It was often tough to see which filter (if any) helped galaxies more under dark sky conditions, but out of all those I tested, the GCE had a small but definite edge in performance for about 60% of the galaxies I surveyed. The improvement wasn't exactly huge, but it was there. Clear skies to you.


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