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Filters and Image Intensifier Eyepieces

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#1 StarStuff1

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Posted 23 July 2009 - 05:29 PM

Last night. It got mostly clear around 11. Earlier it had rained and the air was very humid. At best only a trace of the Milky Way was visibIe. Maybe magnitude 3.5 overall. I tried the 80mm achromat with focal reducer for a total reduction down to f/2.6. NGC 7000 was visible but ho-hum. The Pelican was just not there. The Gamma Cyg area was weak. Other than the Lagoon the Scorpius and Sag areas were not very impressive either.

Just for grins I took out the Ha filter and put in an OIII filter. Many more stars were visible but the views of nebulas were worse than before. Next I took the OIII out and put in a Lumicon Deep Sky filter. WOW! MANY more stars were visible in Cygnus. The noise seemed quite subdued
in the IIE. No Ha regions were immediately noticeable or maybe I was just overwhelmed by open clusters and the vast number of stars visible in such a small, hand held instrument working at about 9x. Of course, the majority of these stars were previously subdued by the Ha filter.

I worked my way down the Milky Way to Aquila and was shocked at how easily dark nebulas were visible on such a not so great night. Scanning on down through Scutum, Sagittarius and the tail of Scorpius the views were absolutely mesmerizing. Globular clusters galore, open clusters and star groupings everywhere. The best view of the Scutum Star Cloud I have ever had was years ago from a mountain top on a good night with my 14x70 Fujinon binoculars. Last night was better! Dark nebulas all over the place. I took the focal reducer out and re-installed the Deep Sky filter again and re- visited the MY at 13x. The views were even better at the slightly higher magnification. Quite impressive. I should have gotten out
my 5-in f/6.5 refractor but it was then after midnnight and even though I was excited I was also getting tired.

For a year and a half I have been uising the IIE with Ha filter about 90% of the time to observe nebulas, seeing numerous objects I had never seen before. Unfiltered it gives great views of globular clusters, faint open clusters and galaxies. It seems that I tried the IIE with OIII and Deep Sky filters once or twice before and they did not impress me with nebulas. Good ol' dumb country boy me did not realize how great a different filter would perform on objects other than Ha regions.

Not anymore. Now I realize the IIE is a far more flexible and useful instrument than before.

My question to you guys with the Collins or a BIPH or IIE: what filter have you used other than Ha and how did they do?
:question:

#2 Maureen

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Posted 23 July 2009 - 06:52 PM

Hi Terry,
With the terrible weather here, I've had limited opportunity to observe but have discovered that what was posted in another thread is correct. I have found that the #25 red filter and an IR filter really suppress moonlight so that basically, if the weather would cooperate, I could observe for the entire month. Will definitely try every filter I own in various combinations with both scopes- if it ever clears up.
Maureen

#3 highfnum

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Posted 23 July 2009 - 08:16 PM

I got nice improvement with a NAHG reject filter
this is a sodium mercury reject -- (not for nebula- Ha is king)
but for great veiws of milky way

also in hot summer time a IR rejects cuts noise down

Most "Human filters" suck on II's they cut the wrong parts of spectrum for II use

#4 jdbastro

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Posted 24 July 2009 - 01:29 AM

I've tried stacking TWO H-alpha filters and gotten a little more detectable nebulosity out of nebula. This pulls out the N American better in heavy light pollution. Also in darker skies I noticed that the Angel Fish nebula (head of Orion) showed up better with stacked H-alpha filters. I've had better luck stacking filters with different bandpasses, e.g. 7nm (Baader) and 13nm (Astronomik). It seemed that stacking matching filters showed little if any change. Not sure why.
I've also tried a narrowband SII (Sulphur) filter with no success on nebula.
I use the Astronomik CLS filer for general deep sky in severe light pollution.
My IIE is the Collins I3 G3TF.

#5 PEterW

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Posted 24 July 2009 - 09:32 AM

The lumicon DS filter has a good block of the main light pollution lines, but no blocking of the blue end and the red end of the spectrum, leaving plenty of stellar light to be massively boosted by the II. I'd be interested in the performance of the II+DS under nasty light pollution, does the DS give as good performance??

...makes these devices just that bit more useful.... now we just need to find a 'galaxy filter'.... see how the DS filter peforms..

PEterW

#6 Douglas

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Posted 24 July 2009 - 09:33 AM

I know that noisejammer (Bruce) was experimenting and had great notes comparing the Kenko R-1/R-60 to the Lumicon Deep Sky Filter. Hopefully he will jump in and post his findings.

- Doug

#7 Maureen

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Posted 24 July 2009 - 09:53 AM

I combined (by accident) the 9nm H alpha filter with the #25 red filter. Really made the Crescent nebula pop out but there was too much noise for me, however, I don't remember how much (if any) moonlight was present, so I'm going to experiment under various conditions - if it ever clears up!
Maureen

#8 StarStuff1

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Posted 24 July 2009 - 10:15 AM

Last night was a bit better than the night before, less humidity. Oddly, the dark nebula with the DS filter didn't "pop" as well as before. Dunno why? I used a 50mm f/2.2 lens, an 80mm working at f/2.7 and a 127mm refractor working at f/4.6. The best overall views were with the 50mm.

Coincidently Maureen, I also combined the the DS and with a #25 red filter and that combo seemed to combat skyglow pretty good. Not nearly as good as the Ha but one could see at least twice as many stars and globs stood out much better.

#9 PEterW

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Posted 24 July 2009 - 04:12 PM

Stacking the H-alpha filters should give the combined result about the same as the narrower one (unless the centre wavelengths are not the same). Adding two identical ones will reduce the bandwidth, the effect of which depends on exactly where the middle of the bandpass is with relation to the H-alpha line.

Have a check of Christian Buils measurements of some of the available Astrofilters.
http://www.astrosurf...ters/curves.htm

The astronomic UHC looks to be similar to the Lumicon Deep-Sky, with both visible and Halpha transmission. As seen most of the UHC/OIII filters block the h-alpha light, so give bad performance with an II. I reckon that stacking the DS with a Baader UV/IR could help with mopping up the 750(ish)nm Sodium emission that the II will see.

The Lumicon Halpha pass coupled with the Baader UV/IR ought to produce a 'wide band H-alpha filter, be useful to see if it gives you both the nebulae and more stars?
As II are sensitive to around 900nm, you want to make sure you know their 'out of visible' behaviour, to gauge their effectiveness in blocking the light pollution, but letting through everything else. From the plots on the webpage you can see that the behavious outside the visible doesn't correlate to their visible performance. Of course there are lots more filters out there, who knows what better combinations we can dream up!

I might be able to help with filter measurements if there is interest.

All the best

PEterW

#10 noisejammer

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Posted 24 July 2009 - 09:23 PM

Doug flagged this thread to me - as he noted, I've been experimenting a little with filters for my Biph.

The conditions were hazy but reasonably steady. Light pollution was awful so that none of my targets were visible through either of my scopes, even when a filter (Lumicon DS or UHC) were employed. Limiting magnitude was probably around 2.5, perhaps even worse.

I then installed my Biph on the LX200 at about f/5. I concentrated on objects in the deep south and observed M4, M6, M7, M8, M16, M17, M20, M22 and (I think) Cr332.

With the exception of M16, it was obvious that the emission nebulae performed best in the 9 nm H-alpha, however the H-alpha filter completely suppressed stars that are frequently found in these structures. This had the effect of stripping a lot of context from the image. Without a cluster to guide my eye, I found that picking out subtle detail was more difficult.

Secondly, I found that there was very little difference between the view presented by the Deep Sky and R1/R60 filters when observing clusters. This surprised me quite a lot, not least because there is a huge price differential (something like $200 vs $30). The R1/R60 filter had the effect of enhancing the nebulosity without suppressing the embedded stars - a very pleasing result.

Plans - I intend to combine the Lumicon Deep Sky with a Kenko R1/R60. I _think_ this will work like a wide Ha filter, but we are experiencing eclipse-weather.

Secondly, I want to try the Kenko R64 (640nm, $30) and _maybe_ a B&W 091 (630nm, $160 because it's a special order.) I think the lower cut off will suppress light polution more effectively which may help. In spite of what some vendors may tell you, both are available in 48mm. An even better choice would be Hoya R62 (620nm), but I haven't found a supplier yet.

Observation - someone commented that they couldn't see much nebulosity in a UHC filter. This is because the UHC suppresses H-alpha light (but OIII and H-beta go through.) The GaAs photocathode at the front of Gen3 II's has very limited sensitivity to the blue-green part of the spectrum, so you really might find that some stars appear unnaturally faint.

Last thought for now - my experience has been that II's really respond to short f-numbers. It seemed reasonable that if I was going to suppress some of the light coming in, I would want to get the best out of whatever remains. I constructed a reducer by gutting an unused Meade f/3.3 unit. I have tried it at f/1.8 on occasion, but f/2.5 - f/3 is more sensible. Provided you install the filter before the reducer, you can get away with interference filters or coloured glass. You loose in image scale, but you gain in image brightness - it's horses for courses.

Clearest
Bruce, Toronto

#11 PEterW

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Posted 25 July 2009 - 02:27 AM

The R64 looks good. The 650nm will eat into the h-alpha too much. Seems like a lower cost version of the lumicon h-alpha filter. The hutech ir blocker seems to but off a bit early than neccessary

If the DS is the same as the r60 then it looks like the OIII region transmission isn't needed.

PEterW

#12 noisejammer

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Posted 25 July 2009 - 10:06 AM

The R64 looks good. The 650nm will eat into the h-alpha too much.


PEter, I agree with this comment. I should have typed B+W 091. I have corrected my original post.

I found some spectral plots of the filters I discussed. The Hoya R62 passes 90% of the H-alpha (656.3 nm) while blocking everything below 600 nm. This is roughly the same as the B+W 091. The R64 passes 75% of the H-alpha, but the skirt will give you just a little better isolation from the sodium lines at 589 nm.

For those who are interested, the Hoya R-series filter data is available here.. The B+W 091 and 092 data is available here

Clearest
Bruce, Toronto

#13 highfnum

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Posted 25 July 2009 - 11:29 AM

I have some luck with a 675 bp 20 (pb= band pass)
it shows ha stuff and more stars
from omegaoptical
but ha shows nore of nebula -- nothing for nothing !

#14 noisejammer

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Posted 05 August 2009 - 09:45 PM

Hi all

Here's something of a follow up to my earlier post. I purchased a B+W 091 (dark red) filter to suit one of my camera lenses, and a second with a standard 48mm thread. Judging from the photometric plots, these are essentially the same as the Hoya R62 and Wratten 29 filters. 50% transmission is at about 620 nm. Sodium light is completely blocked and to the unaided eye, even my heavily light-polluted sky appears near-black.

It's a day after full moon, there is some cloud but judging from the quality of star images there is relatively little atmospheric water. (Not surprising after yesterday's deluge... but that's another story.)

First experiment was with my camera lens and the 091. It's odd - the sky is illuminated but the Biph evidently has a lot of dynamic range left. I looked at bushes in the park (visually very dark) and the image quality looks really good. I then swopped the 091 for an IDAS filter and quickly decided that the 091 was working quite a lot better.

Next experiment was using my scope at f/5. I checked visually but cannot see M8. M4 and M22 were difficult. I then tried the Biph with 091 filter and achieved the expected benefit, namely that the sky was reasonably dark but that stars remain visible. I picked up some nebulosity in M8 but it was clearly being drowned out by the moonlight. The cluster embedded in the nebula was obvious.

Next, I replaced the 091 with a H-alpha and observed M8 again. The cluster disappeared - but so did the nebulosity!

I'm quite surprised at this - the $23, 091 filter seems to outperform my 9nm H-alpha filter... an interesting result. I don't really believe this yet - maybe something else was happening at the same time.

The next phase of this experiment will need to wait for a week or so when the moon isn't flooding the sky in the early evening. After that it's Starfest and having a go under reasonably dark skies.

Clearest
Bruce, Toronto

#15 StarStuff1

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Posted 06 August 2009 - 02:47 PM

Good info, Bruce. Please let us know how it works when the Moon is out of the way.

#16 StarStuff1

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Posted 06 July 2010 - 04:47 PM

Bruce,

Have you tried and compared the 091 filter to your Ha filter since last Summer? If so I'm curious (and probably others) as to your results.

Thanks

Ps: last night I tried out an IR filter, the Lumicon DS and Lumicon OIII filters again. I was not able to stack these in any combination since they all were threaded only on one side. However, the DS was the clear winner showing the MW dark lanes very well in an 80mm f/1.9. The NAN was very faintly visible in the DS, hinted at in the OIII, and absent in the IR.

Terry


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