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Daniel Mounsey
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Reged: 06/12/02

Re: refractor best for doubles? new [Re: Daniel Mounsey]
      #5508104 - 11/07/12 09:50 AM Attachment (59 downloads)

And more often, this is what the refractor will look like.

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Daniel Mounsey
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Re: refractor best for doubles? new [Re: Daniel Mounsey]
      #5508152 - 11/07/12 10:25 AM Attachment (45 downloads)

The solution is an aperture mask, which will greatly clean things up to make the image look like #2. Two important things will happen though.

1) reduced angular resolution.
2) light loss for seeing fainter stars.

The solution? Make a list of double stars that are within the power range of the aperture and enjoy, problem solved.

Sure, there will be some nights where you'll wish you had a bit extra, but that mask is a great solution.


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dpwoos
Pooh-Bah
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Reged: 10/18/06

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Re: refractor best for doubles? new [Re: Daniel Mounsey]
      #5526166 - 11/18/12 01:38 PM

Here in Vermont (not thought to have very good seeing) I regularly get clean, beautiful splits of iota Cass in my homemade 10" f/6 dob at 120X. I accept that a top-flight refractor can split doubles at lower magnifications than a significantly larger reflector, but it ain't a huge difference - more like 20-30% less mag at the most. I think that any 10" reflector that can't keep up with a smaller refractor within that range has some problem(s).

Edited by dpwoos (11/18/12 01:40 PM)


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Daniel Mounsey
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Re: refractor best for doubles? new [Re: dpwoos]
      #5528741 - 11/19/12 08:28 PM

Your point is well taken. Actually what the images mainly imply are thermals that many amateurs encounter. I remember using my 14.5" dob(which I sold) on doubles with and without the off axis mask at a regular event which rarely encountered good seeing. The results with the smaller off axis aperture mask were much better. Where the aperture comes in handy though are on fainter doubles and tighter doubles but that's only if the circumstances allowed. My 10" DK easily outperformed my 6" apo by splitting finer and tighter doubles, no if ands whats or bts about it.

The general issue though is that aperture is over rated, mainly because the average observer will most likely adopt a reflective design. Because of the inherent issues they encounter, it requires special attention that most observers would not be aware of in fact I know several veteran observers here on CN who still don't even know.


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dpwoos
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Re: refractor best for doubles? new [Re: Daniel Mounsey]
      #5529249 - 11/20/12 12:59 AM

I know that my 10" dob is excellent, and yet I also know that it is not as up-to-snuff as a $4,000 apo. My collimation is imperfect, and my coatings are imperfect, and my thermal control is imperfect, and my stray light control is imperfect, and the larger exit pupil means that my imperfect eyesight also comes into play. However, I also know that the views of Jupiter that I had this evening surpass anything that a much smaller (but more perfect) apo can provide, and the fact that I can easily split 1 arcsecond doubles means that, in my mind, my homemade reflector is one heck of an instrument. I can live with the fact that the apo can split a 2 arcsecond double at 70x whereas my 10" requires 90x!

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Asbytec
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
Re: refractor best for doubles? new [Re: dpwoos]
      #5529288 - 11/20/12 01:44 AM

Well, I promised Mike Cee I'd promote his 10" refractor as the best scope on doubles.

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Ziggy943
Post Laureate


Reged: 08/11/06

Loc: Utah
Re: refractor best for doubles? new [Re: mikey cee]
      #5532093 - 11/21/12 11:51 AM

Quote:

I don't need perfect seeing to detect the dark space in 72 Pegasi.....but it sure would make a perty picture! Mike




Awesome view of your scope.


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Asbytec
Guy in a furry hat
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Reged: 08/08/07

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Re: refractor best for doubles? new [Re: Ziggy943]
      #5532135 - 11/21/12 12:14 PM

Daniel, that's zeta Cancri. (Edit: just realized it even says so in the corner...LOL)

More often than not, the obstructed views of zeta Cancri I get are more refractor like according to your image comparison. The difference you illustrate is a bit harsh toward obstructed scopes. I understand you were driving home a point, but that comparison is scary for reflector owners. The message is refractors are pretty, reflectors are UGLY. That's just too biased.

No doubt refractors have a very aesthetic view and beautiful diffraction patterns. It's probably (and sadly) true many reflectors operating out there need proper collimation and cooling. But, that's no fault of a superb design. If the aperture debate is off the table, well the argument ends here. There are a lot of variables to cover, including the nature of seeing. I disagree the solution is to stop down the aperture and settle for a wider Dawes limit. However, doing so when conditions suggest it will help is fine. An aperture mask is a tool, not a solution.

But an obstructed scope can put up nice images with nice diffraction patterns, too. Cooled and collimated, of course, as you correctly imply. And if so, they will look like your second image, more often than not.

Edited by Asbytec (11/21/12 12:21 PM)


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Daniel Mounsey
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Re: refractor best for doubles? new [Re: Asbytec]
      #5532976 - 11/21/12 09:27 PM

Norme, after looking at it some more, I actually agree, it is a bit too harsh. Let me see if I can find something a bit more accurate. Thanks for mentioning this. I don't want to mislead anyone too much.

Edited by Daniel Mounsey (11/21/12 09:35 PM)


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Daniel Mounsey
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Re: refractor best for doubles? new [Re: Daniel Mounsey]
      #5533011 - 11/21/12 09:56 PM Attachment (29 downloads)

How about this for on Albireo instead. Hard finding accurate images hmmm...

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Asbytec
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
Re: refractor best for doubles? new [Re: Daniel Mounsey]
      #5533031 - 11/21/12 10:11 PM

LOL, Daniel. Spikes!

Well, if a refractor was looking over a clothes line, eh?

(Chuckling silently to myself.)


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Daniel Mounsey
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Re: refractor best for doubles? new [Re: Asbytec]
      #5533043 - 11/21/12 10:19 PM

LOL!!! Maybe you could share something that gives an idea of the appearance in the eyepiece. The Albireo image is too saturated.

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Asbytec
Guy in a furry hat
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Re: refractor best for doubles? new [Re: Daniel Mounsey]
      #5533055 - 11/21/12 10:32 PM Attachment (32 downloads)

If you looking for something with spikes, I'd have to drop by the mall.

Well, cooled and perfectly collimated and obstructed, maybe something like...


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zvaragabor
member


Reged: 01/25/11

Re: refractor best for doubles? new [Re: Asbytec]
      #5546693 - 11/30/12 06:20 AM

Do you think a 90/1000 refractor is good for double star observing?

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azure1961p
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Reged: 01/17/09

Loc: USA
Re: refractor best for doubles? new [Re: zvaragabor]
      #5546733 - 11/30/12 07:30 AM

A 90mm is fine and some folks are forever content and even prefer it over larger though obstructed telescopes. I prefer my 6 and 8 over a smaller refractor as the available list of doubles resolvable in both seperation and color is substantially longer. The bigger you go the less u have to pick and choose but those beautiful BRIGHT doubles can then appear blown out and glaring through bigger apertures. Just some thoughts. My 70mm is dandy on doubles but I prefer my 8".

Pete


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zvaragabor
member


Reged: 01/25/11

Re: refractor best for doubles? new [Re: azure1961p]
      #5546737 - 11/30/12 07:35 AM

Thanks azure. Unfortunately 6" (and more) is out of my budget, and it even needs a larger mounting, which weighs and cost even more. I want a lightweight and low-budget scope. I even thought of a 4"/1000.

Edited by zvaragabor (11/30/12 07:37 AM)


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WRAK
scholastic sledgehammer


Reged: 02/18/12

Re: refractor best for doubles? new [Re: zvaragabor]
      #5546951 - 11/30/12 10:37 AM

With a 4" refractor you have many 1000 interesting doubles within the range of your scope and it es very easy to use - certainly a good choice for DSO.
Wilfried


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azure1961p
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Re: refractor best for doubles? new [Re: WRAK]
      #5547003 - 11/30/12 11:25 AM

Youmight find Orion's 6" f/8 reflector dob very budget friendly. Probably more affordable than the refractor. I appreciate your budget concerns. At anyrate either system well made is at least good.

Pete


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zvaragabor
member


Reged: 01/25/11

Re: refractor best for doubles? new [Re: azure1961p]
      #5547220 - 11/30/12 01:23 PM

I own a 10" reflector dob, so a 90mm refractor would be a huge step back. But my dream is to observe doubles with a long focal length refractor. A refractor with aperture 90-100mm, and f/ratio f12-15 would be the best, but unfortunately it's a rarity in Hungary. The closest match is a Meade DS-90 (90/1000), a Soligor 3.7"/1000, or a Celestron 100/1000. All the tree are within budget, but the longest f/ratio is the meade. I know the bigger aperture means a slightly better resolution, but I symphatize with the meade.

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azure1961p
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Re: refractor best for doubles? new [Re: zvaragabor]
      #5547663 - 11/30/12 06:47 PM

In that case get the refractor by all means. It compliments what you have in the 10" and if ever you feel the need for more resolution reach for the ten. Both s opes would actually strike quite a fine balance. It IS fun going between my refractor and reflector. I thought you were looking for one to do it all. And a lot of folks who are seasoned observers actually are fine with four.

Good luck with your choice. You were fine anyway I just thru in my two cents.


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