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Mauro Da Lio
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Reged: 09/12/04
Posts: 223
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Quote:
You still aren't reading what I am saying! The red filter I have does *pass* H-alpha, but is *not* an H-alpha interference filter! It is a simple dye glass color filter with some of the characteristics of the Kodak Wratten #23a light red filter, and is intended *only* for viewing lunar and planetary detail (such as the dark albedo markings on Mars). Its transmission at H-alpha is not very high (probably around 40% to maybe 60% at best), so while it does show the brighter H-alpha features, it cuts out enough light to hurt the view of the fainter ones. However, the red passband of a filter like the DGM Optics NPB is close to 90%, so there is little attenuation of H-alpha. This makes the somewhat fainter red features more easily visible. These fainter details were not seen in the simple red filter because the red filter does not have the transmission that the dual-band filters do.
Thus you mean that you did not see all the red spots in the red filter only because it transmits only (probably) 40-60% at Ha. But they must be certainly visible in the Ha filter, because they are seen in the OIII+Ha filter. Am I interpreting correctly?
BTW which features among those listed in the picture (A-M) are seen red in the OIII+Ha? Are there others?
PS since in post #2050151 you defined your filter to be "equivalent" to W23A ("I have looked at M42 using my "Mars" filter (a Wratten #23a equivalent red filter), ..."), please could you be clearer on this point (eg brand and model)? I have a W23A and I can repeat your tests (In Fact I already did it a month and a half ago, but not in ideal conditions I do not recall if I mentioned them here).
Edited by Mauro Da Lio (12/16/07 05:59 AM)
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David Knisely
Postmaster
Reged: 04/19/04
Posts: 6787
Loc: Beatrice, Nebraska
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Mauro Da Lio posted:
Quote:
Thus you mean that you did not see all the red spots in the red filter only because it transmits only (probably) 40-60% at Ha. But they must be certainly visible in the Ha filter, because they are seen in the OIII+Ha filter. Am I interpreting correctly?
Yes, the simple red filter just doesn't have a high enough transmission to let the fainter features show up. Many of the fainter red features *do* show up in all the filters which have a red secondary passband with high transmission (my old Lumicon OIII, the DGM NPB, and the Lumicon H-Beta).
Quote:
BTW which features among those listed in the picture (A-M) are seen red in the OIII+Ha? Are there others?
It is hard to describe it from memory, but the features marked A, B, C, and D, all show up with at least some faint reddish color (especially in the dual band H-Beta filter, where even M43 shows a little red). The red is not uniform along the features, but each does show at least some pinkish or reddish hues in or close to the locations you have marked. The brightest red portion is probably at and between the B and C arrows. The E-F-G feature appears as one long band in the H-Beta filter and does show some red, although again, it is not uniform along the length of the band. The location marked N is faintly red, but is also where the red starts to fade somewhat, as in the interior of the M42 "loop", there is little color visible. The features I, L, and M show little if any color in my 10 inch. Without filters, unless I use a very large aperture, I don't see the nice reddish hues I did with the filters (they are more pale pinkish or tan colorations when they are visible).
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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
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Mauro Da Lio
sage
Reged: 09/12/04
Posts: 223
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Quote:
Yes, the simple red filter just doesn't have a high enough transmission to let the fainter features show up.
Which is the Brand and model of your filter?
I have not been able to find transmission data for the Wratten 23A (light red) but I found these for the 25 (red) and 29 (deep red). At 656 nm they have transmisson of 88% and 90% respectively. The 29 is very interesting: it could be a very cheap substitute for a Ha filter for testing Ha visibility on nebulae.
Edited by Mauro Da Lio (12/16/07 06:10 PM)
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David Knisely
Postmaster
Reged: 04/19/04
Posts: 6787
Loc: Beatrice, Nebraska
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Quote:
Quote:
Yes, the simple red filter just doesn't have a high enough transmission to let the fainter features show up.
Which is the Brand and model of your filter?
I have not been able to find transmission data for the Wratten 23A (light red) but I found these for the 25 (red) and 29 (deep red). At 656 nm they have transmisson of 88% and 90% respectively. The 29 is very interesting: it could be a very cheap substitute for a Ha filter for testing Ha visibility on nebulae.
It is a very old inexpensive filter that was sold with a common small telescope about 20 years ago, and it is unmarked (I have no idea who made it). From the way it improves Mars, it looks to be similar to the Wratten #23a but is not extremely high in red transmission. Newer filters are significantly better in their red transmission, as one filter company markets one "Wratten #23a" equivalent with a near 90% H-alpha transmission, so that one might indeed make a good H-alpha filter.
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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
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Mauro Da Lio
sage
Reged: 09/12/04
Posts: 223
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Quote:
It is a very old inexpensive filter that was sold with a common small telescope about 20 years ago, and it is unmarked (I have no idea who made it). From the way it improves Mars, it looks to be similar to the Wratten #23a but is not extremely high in red transmission. Newer filters are significantly better in their red transmission, as one filter company markets one "Wratten #23a" equivalent with a near 90% H-alpha transmission, so that one might indeed make a good H-alpha filter.
How did you estimate the transmission at Ha?
In any case can you find a W23A/W25/W29 and check again? I Have a W23A, I think that the 29 could be perfect for the assesment of Ha visibility (the 0.2-0,4 nm band as for the Sun is not necessary, and the greater band helps seeing stars and literally finding the way to objects)
BTW you should also check the 23A compared to Ha or the 29... it is very very meaningful (but see yourself).
Edited by Mauro Da Lio (12/17/07 02:25 AM)
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Mauro Da Lio
sage
Reged: 09/12/04
Posts: 223
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Here is an interesting elaboration of M42. I took the picture by Clark, imported it in Mathematica, split the RGB channels and then I made a contour plot of the R channel. The contours are ~1 magnitude apart: the red foreground in the picture means surface brightness >22; the first yellow inwards >21, and then progressively to the Huygenian region at ~14-15.
Depending on what is the actual limit for the eye, one should see the nebula -in a Ha or W25 or W29 filter- filling one of the inner contorus. I think the blue (maybe cyan?) should be the limit.
Edited by Mauro Da Lio (12/24/07 07:19 AM)
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Mauro Da Lio
sage
Reged: 09/12/04
Posts: 223
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Quote:
Yes, the simple red filter just doesn't have a high enough transmission to let the fainter features show up.
...
It is hard to describe it from memory, but the features marked A, B, C, and D, all show up with at least some faint reddish color (especially in the dual band H-Beta filter, where even M43 shows a little red). The red is not uniform along the features, but each does show at least some pinkish or reddish hues in or close to the locations you have marked. The brightest red portion is probably at and between the B and C arrows. The E-F-G feature appears as one long band in the H-Beta filter and does show some red, although again, it is not uniform along the length of the band. The location marked N is faintly red, but is also where the red starts to fade somewhat, as in the interior of the M42 "loop", there is little color visible. The features I, L, and M show little if any color in my 10 inch. Without filters, unless I use a very large aperture, I don't see the nice reddish hues I did with the filters (they are more pale pinkish or tan colorations when they are visible).
Here is the contour map with the locations arrowed. Right is the original photo.
There are many comments:
1) The red filter. Although the estimated transmission of 40%-60% may look little, in fact in terms of magnitude losses 40% means a loss of 0.88 magnitudes compared to a filter with 90% transmission (2.5 Log(40/90)). 60% means a loss of 0.66 magnitudes.
On the other hand, the contour map shows that the reported red spots span several magnitudes, from ~14-15 (C) down to >22 (actually near 24) the spot M. Compared to a Ha 90% filter, the image of the nebula should shrink by less than one contour in the 40%-60% red filter. So, it looks somewhat odd that a filter that absorbs 0.6-0.9 magnitudes makes disappear features over several magnitudes interval (I understood that with the red filter only the brightest features remained visible). The same reduction of brightness can be produced by using ~35% more magnification. Does the nebula shrink the same way? Which contour represents the visible portion of M42 in Ha? Which contoru represents the visible portion of M42 in the red filter?
2) The faintest spots themselves are a puzzle. Especially M, which (I understand) should be visible in color with scopes with very large aperture. It is at surface brightness of ~24 (to the camera, add 2.5 magnitudes for reduced Ha sensitivity of the eye). If it were seen as a real red that would mean that there is no scotopic interval at all and that everything we see, we see in real colors.
3) The Hbeta filter. The feature near C (the "wall" that borders the Huygenian region slightly above the arrow) is indeed the brightest Ha emission. I see it red in a red filter (no surprise it is mesopic). I see it Green in the OIII filter. With no filter it is circa white however, because the OIII lines (actually less powerful, but the eye is more sensitive to it) grossly balances the Ha.
Now the test with Hb+Ha *cancels the OIII line*. It lets pass only Hb together with Ha and color is no longer balanced. There is no surprise we can see one of the two (Ha, Hb) prevail (as long as we stay in the mesopic range and we really see them). However, if we see red in Ha filter we do not think the feature is red. We think we see only the red part. So, why should we think that a certain spot is (revealed to be) red when the most important color component (OIII) has been cancelled? Like the case of Ha only, in this case we should better think that we see only two components of the color of the nebula (the missing one being the most important) and that *of the two we see* red prevails is certain spots. Any claim that real red is seen through filter that cancels OIII is intrinsically flawed. More: any claim of seeing the real colors through filters is flawed (unless they let all the meaningful lines pass).
4) So, what is the region visible in Ha? How does it shrink with increasing magnification?
Edited by Mauro Da Lio (12/25/07 04:14 PM)
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