5" Takahashi Apo vs. 18" Obsession dob show-down

Dear DSF,

I don't know whether this is specific to my dob (which currently has a layer of stubborn dust on the mirror and on which I frequently use a large exit-pupil eyepiece), but when it comes to contrast, my 18" Obsession fails miserably compared to my friend Hemant's 5" Takahashi refractor.

Of course, when it comes to the Shakbazians, Abell galaxy clusters, or resolution of globular clusters, the 5" would be no match for the 18". So to give fair competition, we chose only those objects that were diffuse and whose observation would be limited by contrast rather than intensity or power. Here are the results -- I mention which telescope performed better on each observation. Comments are invited!

Veil nebula: 18"
Bright pillar in the "Pillars of creation" in M16: 5"
Dark pillars in the "Pillars of creation" in M16: 18"
Lagoon nebula: 5"
Helix nebula: 18"
Barnard E nebula: 5"
Andromeda galaxy overall: tie
Inner V-shaped dust lanes pointing towards the core of M31: 5"
Usual dust lanes of M 31: 18"
Triangulum galaxy: 18" by far
Maffei I: Tie
IC 342: The two observers disagree on the results. (I think the 18" was better, Hemant thinks the 5" was better)

(If I do get that re-coat I've been planning on, it'd be worth repeating this to see how much that dust degraded contrast)

Clear Skies

Regards
Akarsh

What is it apart from the true field (so that you are not looking "through" the large objects with no dark background) that you like in the 5" views? Are you now using the baffle/ upper cage extension on your 18"? I know that a somewhat smaller "real telescope" can beat a Dob, but 5 vs. 18" is beyond the extreme as such comparisons go. The foreground star groupings, for example, in the IC 342 field were probably unresolved in the Tak.

Dear Ivan

I am not using a baffle / upper cage extension on my 18". The baffle the first owner gave me is now unusable, so I need to make a new one.
Yes, the foreground star groupings in IC 342 were probably unresolved in the Tak.
What I liked is the contrast that enabled the observation of some of these features. Of course, the view in the Takahashi was much more dim, but the background was also blacker at lower power than in the dob.
Also, I was using very low power on my scope when making some of these observations. If I stepped up the power, of course, there was better contrast in the 18" without compromising the brightness of the image.

Qualitatively speaking, for example, in the Barnard 'E' dark nebula, the dark features were much darker in the Takahahsi than in my 18", making the dark nebula easier to detect and more contrasty. But in the 18", I recall detecting the non-uniform, faint glow of the dark cloud... something that the 5" could not do. But the "overall view" was better in the 5" in our opinion.

Here is my understanding of how / why these observations might make sense. I admit that I do not know enough about the physics, physiology and nuances of visual observing, but I'm going to put here whatever my understanding is, fully aware that it might not be correct.

Suppose we define contrast as the ratio of the intensity (W/m²) of the object (I_object) to the intensity of the background (I_bg, i.e. contrast = I_object / I_bg. The telescope's light gathering power does not distinguish between the background and the object, so it amplifies both intensities by a factor proportional to the area of the objective. So we have a ratio between I_object x A and I_bg x A. So the net contrast is unchanged by aperture.

It looks like the primary advantage of aperture lies in brightening the objects. The eye is a complicated sensor where detection of an object depends on brightness, contrast and (angular) size (of the magnified object subtended at the eye). As one magnifies the image, the contrast (as defined above) is once again unchanged, because both the sky background and the object's flux are spread over a larger area, rendering both of them dimmer by the same factor. Then how does the increased magnification make 'contrast' better? This indicates that our above definition of contrast is somewhat simplistic, and the eye's sensitivity is non-linear under low light conditions (clearly, this must be the case because there is a lower cut-off on how bright something must be to be detectable). I'd rather call this a dependence on brightness (maybe a good definition of relative brightness is I_object - I_bg). The increased brightness of the object allows us to use higher magnification before we hit the intensity cut-off, enabling us to make the background fainter and the object larger, and in my understanding, this is the primary advantage of aperture.

But when the sky background is visibly bright (like it is when using 31mm Nagler and 26mm Ortho eyepieces on my dob) and when the object is large and low surface brightness, I would imagine that the quality of observation is limited by contrast conforming to our definition above, and not by brightness or by angular size at the eye. For this comparison between telescopes, we intentionally chose objects that we imagined fell into this category.

In a dob like mine, which is not specifically baffled, I would imagine that the stray light leakage is more than in a refractor with excellent baffling (which the Takahashis are apparently known for). So the contrast is poorer, and contrast-limited observations like some of those chosen in this comparison suffer.

The above ideas are based on the following subjective observations:

1. Diffuse nebulae like the Horsehead, Witch head etc. are visible in smaller aperture (4" for Horsehead, and even a 9x50 finder for Witch head); whereas the tiny high-surface brightness galaxies like members of Abell clusters or Hickson groups are nearly invisible in smaller aperture. Whenever low surface brightness is involved, this seems to be the case -- one can go smaller in aperture and still see the objects.

2. An observation of NGC 891 in my 18" vs. a 10" dob showed a dimmer view at lower power in the 18", but I could keep stacking much higher power on the 18" without rendering the object un-observably faint. (It is possible that this is a problem endemic to my 18" dob given that my mirror is coated with a layer of dust, but it seems to be having trouble on low surface brightness like NGC 891. I haven't managed to look through other 18" dobs side-by-side).

Of course, there has been some good amount of research done on visual observations:
http://www.clarkvision.com/visastro/index.html
http://www.clarkvision.com/visastro/omva1/index.html
http://www.uv.es/jrtorres/visib.html
At some point I checked out Roger Clark's book from the library and tried cracking it open, but it was too dense a read for me at that point. Maybe I can understand it now. It might help to understand the use of the Blackwell (or Clark-Blackwell as they are called in this context) curve.

I would be happy if any of you have some insight into these observations. It is of course possible that it all boils down to something specific to my mirror, but I am quoting only what I observe and a plausible explanation -- it could all be wrong :-)

Clear Skies

Regards
Akarsh

Wow. There seems to be more literature and research on these topics than I had imagined. Here's Prof. Roger Clark's website on the matters of contrast and OMVA:

http://www.clarkvision.com/visastro/omva1/index.html

And under a section called 'Controversy', he links to two discussions from Mel Bartels and Nils Olof Carlin that comment on and interpret the Blackwell curves:

http://www.bbastrodesigns.com/visual.html
http://www.bbastrodesigns.com/blackwel.html

If anyone has a good summary of this that is easy to understand, I would be indebted.

(My interest in this theory is partly to get a more scientific understanding of how the various parameters matter for visual observation, and partly so I can code the OMVA curves into my planetarium software so it can automatically recommend the power to use)

Clear Skies

Regards
Akarsh

What you are saying before "But then the sky background is visibly bright..." seemed a summary of Clark's treatment and unquestionable. That next paragraph I understood as a reminder of the impact of stray light, an additional factor. I've always believed it to be the leading cause of the diminishing returns of aperture in amateur instruments. Another effect should be the impact of aperture on dark adaptation.

To illustrate why I nonetheless would not take a professional-style instrument of less than, say, 3/4 of the aperture over the larger Dob, let's look at your example with the Horsehead. The observations with small apertures are praiseworthy, but you can't see the horse's head with them. You can see a formless notch in IC 434. I want power to see the head, the snout, the nostrils, and the strands in the mane.

Ivan,

I'm sorry it took me so long to respond.

I agree with your analysis. I had only a friend's 5" at my disposal, and having seen its excellent contrast, I was tempted to compare the instruments. Had I had access to a larger refractor, it might have been a better choice.

Also, I agree about "a formless notch in IC 434" (BTW, I haven't seen the horsehead in such small aperture, but heard that it is possible). The view in an 18" is superior in many ways. The compared factor, was again, not how much detail you can see in a given object, but how contrasty it looks with respect to the background, and how easy it is to detect etc. For example, I think that when it came to the knots of IC 342, the 18" beat the 5" hands down. We compared it on only those objects where the 5" might stand a chance, and only on those features where the 5" might stand a chance. To be honest, the bright pillar in the pillars of creation was a bit of a surprise.

Clear Skies

Regards
Akarsh