Draco dwarf easier with a 6"?

[akarsh]

Hi

I managed to observe the Draco dwarf galaxy yesternight from Texas Star Party. However, it seemed to be easier to notice it in a 6" Orion SkyQuest dob than in an 18" Obsession f/4.5 dob. The eyepieces used were the same on both scopes -- 20mm Pentax XW and 25mm University Optics HD Orthoscopic. I also used a 31mm Nagler on the 18" (could not do so on the 6" because it did not have a 2" barrel.)

The best view seemed to come from 6" + 20mm Pentax. Of course, there is a substantial amount of subjectivity in this entire exercise, but it looks like my Obsession dob doesn't have good contrast. I'm going to try and flock the truss poles and the like in a better manner, make a light-shield and the like... but do you folks have any comments on this? Any suggestions on how to improve the Obsession's contrast?

Or am I approaching all of this incorrectly / do I have a mental bias towards the 6"?

Clear Skies!

Regards
Akarsh

[Ivan Maly]

The tube ("tube") of this 18" is more open to stray starlight. It would take a sizable, complete circular shield (baffle) on the top to make it a scaled version of the 6" Sky Quest.

My 16" has an upper cage modeled after an Obsession, i.e. a standard type. It will need a sizable baffle tube on top to be a scaled version of my 12" Sky Quest (already a much stockier tube than the 6"), and to begin approaching the degree of baffling in my 12" SCT, which I use without a shield, it would need one no less than 1 m in length! That's just intuitively.

[akarsh]

The tube ("tube") of this 18" is more open to stray starlight. It would take a sizable, complete circular shield (baffle) on the top to make it a scaled version of the 6" Sky Quest.

My 16" has an upper cage modeled after an Obsession, i.e. a standard type. It will need a sizable baffle tube on top to be a scaled version of my 12" Sky Quest (already a much stockier tube than the 6"), and to begin approaching the degree of baffling in my 12" SCT, which I use without a shield, it would need one no less than 1 m in length! That's just intuitively.

Thank you Ivan, that makes sense. I have a 1½ foot long home-made light shield that the previous owner of my dob (John Tatarchuk) gave me. I wasn't using it, but I haven't noticed substantial increase in contrast upon "blinking" it. It's also torn apart. Maybe I'll try putting it together and using it in the coming nights to follow-up.

I should also try and put black flocking on my truss poles -- they have some reflective aluminum left exposed. Do you have any other suggestions?

(So it was a good idea to bring a 6" along ;-D. People around me were wondering why I brought a 6" when I had a 18" also on the field.)

Regards
Akarsh

[Ivan Maly]

Oh, you have exposed aluminum there... Yes, that I would paint over at leisure. The negative results of your experimental approach with the shield are interesting. That said, a length that is equal to the diameter is the absolute minimum to expect any effect theoretically. That's against stray light from the sky; I assume the shield was built originally to be used against local light sources in a backyard situation.

[akarsh]

Oh, you have exposed aluminum there... Yes, that I would paint over at leisure. The negative results of your experimental approach with the shield are interesting. That said, a length that is equal to the diameter is the absolute minimum to expect any effect theoretically. That's against stray light from the sky; I assume the shield was built originally to be used against local light sources in a backyard situation.

Hi Ivan

The light shield was made to avoid star light from the sky -- not local light sources. John Tatarchuk, the previous owner of the scope, almost exclusively observed from Bortle 2 or better skies. It does seem to be at least 18" long, which makes sense.

I wouldn't read too much into the experiment of "blinking" the shield, because I was not doing it in a very controlled manner.

Yes, I should paint that exposed aluminum. Thanks for your pointers!

Regards
Akarsh

[akarsh]

Interestingly, I observed Palomar 15 a few hours ago, and the 18" clearly won over the other smaller scopes. The object was detected in the 18" as a slight increase in background brightness, and the exact location was confirmed using a POSS2 image. The observation could be reproduced repeatedly quite easily, by tapping the telescope a bit. In a 12" Hardin Optical dob, the object was hardly visible, and if at all, was felt on few occasions. In the 6" SkyQuest, even the reference stars were very faint.

Since this too, is a contrast-limited observation, I expected the 12" to yield. The 12" is cerainly not baffled well, but yet, this seems to indicate that there is value in larger aperture even for such low-contrast objects (as is to be expected).

Clear Skies
Regards
Akarsh

[Ivan Maly]

It is always some combination of brightness and contrast that is critical for visual detection. Larger-aperture scopes win on brightness, and longer (whether in physical length of a simple tube, or in equivalent focal length with simple Cassegrain baffling) will win on contrast, sensu stricto. The visually perceived contrast though is a function of brightness. Galaxies taken statistically are fairly uniform in what they are and what they need, especially the NGCs due to their discovery method, but the nearby dwarfs are outliers.

I regard my 12" SCT and 16" f/4.5 truss Newt as equivalent, and 12" f/5 tube Newt as significantly weaker than either.

Crude as this approach is, a bigger truss Dob is usually better than anything else, even if more of its detection power is wasted due to the poorer baffling. Having carried the said SCT up the stairs the other day I know I won't soon use a bigger one (it is A LOT heavier than my 16" truss), and adding serious baffles to the transportable truss Newt is unappealing, because its assembly after a long drive already feels cumbersome enough, and the best transparency means stiff winds here.

But if you are inclined to experiment with, say, a long end tube and a few baffle rings with holes for the trusses, I would be most curious to know the results. Obtained in a very controlled manner, of course ;)

[Marko]

I would not have thought a 12" scope would be able to detect Pal 15. That object was the last of the Pals on my list and it took many nights of observing over several years till I finally got it in very dark skies with certainty in my 18" with excellent eyepieces. Once at 8000 feet and once at GSSP 4000 feet. both cases 21.7 type SQM readings. The field itself being clearly identified in each case so I knew EXACTLY where it was and also one of the most difficult of my observations to be sure pulling ALL the stops to be certain the glow was really there.

If you are 'sure' you saw Pal 15 in a 12" ... I would like to borrow your eyes when I go to GSSP, I'll give them back ... honest! LOL
Just joking. I'm nearing 60 years so I do feel that a sub 30 year old may just be able to see Pal 15 in a 12 " from excellent skies as a limiting case.

[akarsh]

Hi Mark

Not sure if it was clear in the report, but I consider Palomar 15 in the 12" as negative. In the 18", I am certain I saw it, because I did not know the precise location before-hand -- just knew that it was in between a pair of asterisms. My observation pointed out that it was closer to one of these than the other, and estimated the size of the glow, which turned out to be correct.

Clear skies

Regards
Akarsh

[akarsh]

Ivan, that idea of baffling inside with holes for the trusses seems rather attractive. I might try it just for fun and try and see if I can detect any improvement. In a controlled manner, hopefully! :D

Clear Skies

Regards
Akarsh

[Marko]

Ok, thanks for the clarity. Pal 15 for me is at my very limit of detection in ideal skies and my 18". I know for sure there are other observers who can detect tiny differences in contrast with lower mag objects better than I so it may be possible in something like a 16" too but in a 12 ... that would be amazing for a 55+ guy like me.

I have a 12" lightbridge and the difference between my 18" StarMaster and the lightbridge is huge. To be fair, my 12" lightbridge has silver poles and I don't use a light shield so that hurts it quite a bit. I only use the 12" for a sort of 'grab and go' and it is a nice scope as it takes so much less space and so on if there is no space for the 18". I think I'll paint my lightbridge poles black and maybe take the shroud out next time as I do have it with me sometimes at 21.5 SQM sort of site when I do astrophotography.

Oh, and you asked about a definition of 'young observer' which I would say it would be a person who is in the stages that typically pre-date the degradation of the eye's clarity. I think that then means typically 30 or younger. At 40 is when the eyes often degrade but that varies of course and some lucky folk have great eyes past 50, just not me.

[Richard Navarrete]

My observation of Pal 15 matches Marko's. We also are observing buddies, but I don't think that has anything to do with it. I've also got an 18" dob. We should try again at GSSP with Steve's 22". So now the question is, what is the minimum aperture someone has used to successfully observe Pal15, and how old they are!

Richard

[akarsh]

I've read that Sue French has seen Palomar 10 in some very small apertures (something like 4"?). Wow, but not entirely unbelievable. These objects are low surface brightness, so a high-contrast refractor might be able to show some of them. Of course, aperture almost always helps (AFAIK). Maybe next time I'm in the west Texas, I'll give it a go in smaller aperture.

Clear Skies
Akarsh

[skyraider]

Akarsh,
Have you done anything to reduce the light that sneaks in between the top of the mirror box and the shroud? Any baffling across the back of the mirror box to stop light from sneaking in that way? I did the latter on my scope when I built a cooling system that pulls air across the face of the mirror. A buddy of mine did both as well as a baffle on the top of the UTA to his Obsession 15, and when you looked down the "tube" of his scope you saw NO stray light coming in. And his scope has great contrast.

[akarsh]

Hey Josh

No, I haven't really baffled my telescope at all. I'm pretty sure that's one of the big problems. I should work on it before my next outing to very dark skies :D

That is a good litmus test -- looking down the scope in a dark sky site. Thanks for the pointers.

Regards
Akarsh