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Observing >> Deep Sky Observing

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Loren Toole
sage
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Reged: 03/23/04

Loc: New Mexico USA
Barlow for deep sky?
      #5869190 - 05/18/13 11:58 AM

I revisit old observing guides periodically for inspiration...

A good example is Walter Scott Houston's "Deep Sky Wonders" book edited by Steve O'Meara. There are numerous references to Houston's use of a Barlow to increase contrast and reduce scattering. This related to his use of a 4" f15 refractor for deep sky. Of course using a Barlow increases eye relief, but why this should be superior to simpler optics is a puzzle. On page 223 there is a revealing quote:
"I have always recommended using a Barlow for boosting magnification.
Eye relief is better and dust on the oculars scatters less light than if an equivalent high power eyepiece is used alone".

Is this in agreement with your DSO observing experience?


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GlennLeDrew
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Barlow for deep sky? new [Re: Loren Toole]
      #5869211 - 05/18/13 12:11 PM

The scattering from dust comment arises from considerations of the relative size of dust bits to the exit pupil. Unless I'm missing something, I don't see the gain afforded by a Barlow here. If a certain magnification is obtained via a short f.l. eyepiece or a longer f.l. eyepiece with Barlow, the exit pupil diameter is the same. And for DSO work, dust-on-the-lens scattering is hardly worthy of consideration anyway (unless quite dirty), what with the general dimness of the subject and the concomitant invisibility of scatter, as well as the eye's poorer resolving power in this regime.

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WeltevredenKaroo
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Reged: 03/05/12

Loc: 31°51'09" S 24°25'47" E
Re: Barlow for deep sky? new [Re: GlennLeDrew]
      #5869311 - 05/18/13 01:20 PM

Loren, you may be making a better case for keeping your eyepieces clean than for introducing an additional pair of light-reflecting optical surfaces. Dust on a large primary surface is more of a cosmetic nuisance than a light-thief—less than 1% in the primary surfaces typically shown in CN equipment ads, to 3 or 4% if the objective is really awful. But dust particles on the field and eye lenses of a small-lensed short focal-length ocular such as a Plössl or ortho can occupy a large percentage of the available lens surface. Defocus a star or the Moon considerably to see if you spot dust specks. If you can, the specks represent only half your problem because only the field lens is in focus. In this case it makes sense to use a Barlow with an eyepiece of double the focal length, approx. double the lens diameter and four times the lens surface area. A Barlow's light loss may amount to 1 or 2%. Ocular dust is less of a problem with large-lensed multi-element models typically employing a Smythe negative element (which is a built-in Barlow). If in doubt, compare star counts in a selected star field using a Barlow and short efl eyepiece, and the same field using an eyepiece double the focal length. I get out the air can and lens brush only when image blur or scintillation flare becomes clearly worse than that introduced by rough air.

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Loren Toole
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Reged: 03/23/04

Loc: New Mexico USA
Re: Barlow for deep sky? new [Re: WeltevredenKaroo]
      #5869445 - 05/18/13 02:22 PM

I do suspect that Scotty was thinking of larger eye lenses when he made this recommendation... That is, a Barlow allows longer focal length eyepieces to be used,which reduces the relative effect of dust particles versus a shorter focal length eyepiece, with much smaller eye lens.

My next question then is: did the eyepieces used during Scotty's time (roughly 50s to 80s) demand an unusual approach,like use of barlows to avoid dust and scattering? I became a DSO observer during this period and learned to use the same eyepieces he used, which were smaller AFOV, less immersive, and simpler designs? A large eye lens was often not my highest priority....


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Astrojensen
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Reged: 10/05/08

Loc: Bornholm, Denmark
Re: Barlow for deep sky? new [Re: Loren Toole]
      #5869658 - 05/18/13 04:15 PM

Quote:

did the eyepieces used during Scotty's time (roughly 50s to 80s) demand an unusual approach,like use of barlows to avoid dust and scattering?




No, but a barlow allows the use of long focal length eyepieces with a large, comfortable eye lens, for relaxed observing, much like modern, long eye relief eyepieces do, also at medium and high power. One should not underestimate the effects of being relaxed at the eyepiece, instead of straining and squinting to look through a tiny peephole. I spend two decades looking through tiny peepholes, so I know what I speak of. I have Zeiss orthos and stuff, but I see much more with my modern, comfortable (and much cheaper!!!!) ES82 eyepieces. Stunning, but true.


Clear skies!
Thomas, Denmark


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Loren Toole
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Reged: 03/23/04

Loc: New Mexico USA
Re: Barlow for deep sky? new [Re: Astrojensen]
      #5873310 - 05/20/13 09:27 AM

Thomas
Agreed on the advantage/comfort of viewing DSOs with larger eye lenses, however are you suggesting the Zeiss are lesser performers due to comfort? While I don't own any Zeiss, I imagine the polish and coatings would trump nearly anything else, larger eye lens or not.

My favorite sweeper (finder) is actually a Chinese manufactured Meade 32 mm two inch Plossl, with a huge eye lens and comfortable eye relief. But I am skeptical that it would perform as well i.e. Contrast or scattered light if compared to an equivalent Zeiss sweeper.


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Astrojensen
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Re: Barlow for deep sky? new [Re: Loren Toole]
      #5874182 - 05/20/13 04:29 PM

Quote:

are you suggesting the Zeiss are lesser performers due to comfort?




Yes. Comfort should not be underestimated. It's not all, either, on the other hand, but I've compared my 10mm Zeiss ortho and my 11mm ES82 very carefully on my 150mm f/8 refractor on deep-sky and I couldn't find a single thing in the Zeiss that the ES didn't show just as well, if not, shockingly, a little better. And with FAR greater comfort and MUCH more true field, while being just as crisp as the Zeiss across most of its twice as large field. Stunning. I really didn't expect the ES to do this well, I really didn't. I honestly expected the Zeiss to be a little better, especially considering its slightly higher magnification, but I saw no difference in transmission and contrast between them. Limiting magnitude was the same and sky background was equally deep gray in both, with the same "no glass in there" feel, which is a sign of great optical quality.

I haven't compared the 10mm Zeiss and the 11mm ES82 on the Moon or planets, but during a recent lunar session with the Telemator and the 11mm ES82, I was shocked by how sharp it was. And the 18mm ES82 is sharper than my 18mm UO ortho, with a better snap to focus and less glare.

It's been months since my once beloved orthos have been out, other than for mere comparisons. I now can't stand the horribly restricted field, other than if I use them in a binoviewer.


Clear skies!
Thomas, Denmark


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Scanning4Comets
Markus
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Reged: 12/26/04

Loc: Ontario, Canada
Re: Barlow for deep sky? new [Re: Astrojensen]
      #5884988 - 05/26/13 01:16 AM

I have never liked Orthos and never will for this very reason. I looked through a Pentax XO last week and couldn't see any more detail in M-13 than when using my XW. The squinting , small field and short ER of the Pentax Ortho made it a very uncomfortable experience. I hated it.

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azure1961p
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Reged: 01/17/09

Loc: USA
Re: Barlow for deep sky? new [Re: Scanning4Comets]
      #5886413 - 05/27/13 12:15 AM

I don't know what it is, but Im fine with even my 4mm ortho and Im thinking of contacting Seibert for a 3.5mm. I'm honestly comfy with these small lenses and nil eye relief. I don't squint, ever. My eye is very very relaxed.
I wear contacts (never for observing) do Im very used to touching my eyes. Maybe that has something to do with it.

At anyrate I LOVE my Orthos from 20mm down to 4mm - particularly the 9, 7, 6 and 5mm.

Pete


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blb
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Reged: 11/25/05

Loc: Piedmont NC
Re: Barlow for deep sky? new [Re: azure1961p]
      #5886735 - 05/27/13 09:36 AM

Quote:

...I don't squint, ever. My eye is very very relaxed.
I wear contacts (never for observing) do Im very used to touching my eyes. Maybe that has something to do with it.



I do not squint either but I do not wear contacts, so I guess that is why I do not like touching my eye and I seam to do that a lot with these short focal length eyepices. That is the main reasion I like the wide field long eye relief eyepieces of today.

Quote:

"I have always recommended using a Barlow for boosting magnification.
Eye relief is better and dust on the oculars scatters less light than if an equivalent high power eyepiece is used alone".



This is where I would agree with Scotty. I have always done the same thing for the same reason. I have never liked being so close to an eyepiece that I was constantly bumping my eye.


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gunfighter48
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Reged: 03/18/13

Loc: Mill Creek, Washington
Re: Barlow for deep sky? new [Re: blb]
      #5889789 - 05/28/13 09:30 PM

After the Milky Way, M31 was my first galaxy. I had a Cave Astrola 8" Newt. Great scope with great optics, but it weighed around 200 lbs with it's pier mount and counter weights. It never went further than my driveway or backyard.

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