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Dave Lee
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Reged: 02/14/13

Loc: Pinehurst, NC USA
General Double Star Observing Equipment Question new
      #6127508 - 10/09/13 05:38 PM

I am a 'returning to the hobby' guy who used to be focused on Messier/Herschel DSO objects. I am back to the hobby and now have a G11 permanently mounted in my yellow/green back yard. My primary scope is a mid 90's vintage C11 scope of unknown to me heritage. Relative to this forum I have decided to seriously add double stars to my target list.

At this point I am not sure of the 'relative optical quality' of my C11, but I am quickly learning how important the factors of

* precise collimation
* seeing
* temperature equalization

are.

So for purposes of this discussion it seems reasonable to me that assuming that my C11 is 'average' and that I will achieve very good collimation and (using my recently purchased Lymax cooler) achieve decent temperature equalization (at some cost of time using the Lymax) is a good place to start.

So the question now becomes should I add another scope for double star observing. With a birthday and Christmas coming up I could generate up to around $2k for this. If I were to go down this path the choices would probably be a either a high quality, long focus 4 inch refractor (less than $2k unless I would chose the latest Skylight offering) or an ED/APO scope of aperture 120-130 mm.

The additional factors are:

1) I would really like (I think) a scope with a 'sharper and more crisp focus' than what I am experiencing at the moment with my C11. I also have a sense that better contrast than I am now getting (easier to baffle a refractor vs. SCT) is also available to me. But maybe as I get better at temp and collimation, all of this will change.

2) I quite honestly have a desire for a new scope - just the old/male 'want a new toy' thing.

Just fishing for comments - thanks.

dave


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coopman
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Reged: 04/23/06

Loc: South Louisiana
Re: General Double Star Observing Equipment Question new [Re: Dave Lee]
      #6127546 - 10/09/13 05:57 PM

You might want to read this thread:
http://www.cloudynights.com/ubbthreads/showflat.php?Cat=&Board=double&...


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coopman
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Reged: 04/23/06

Loc: South Louisiana
Re: General Double Star Observing Equipment Question new [Re: coopman]
      #6127584 - 10/09/13 06:16 PM

Another one:
http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/5605554/page...


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brianb11213
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Reged: 02/25/09

Loc: 55.215N 6.554W
Re: General Double Star Observing Equipment Question new [Re: coopman]
      #6127647 - 10/09/13 06:50 PM

Well .... your C11 is always going to resolve finer details than any 4" refractor, and the light grasp will be a couple of magnitudes deeper.

However there is a lot to be said for having a 4" class apo as a second instrument - mounted on something like the iOptron Mini Cube Pro it's a highly transportable package yet still has enough clout. A f/7 triplet apo or a f/9 ED doublet is probably better for a general second instrument than a long focus doublet as it is more flexible in that it will show wide field DSOs better than your 11" SCT & will also be more transportable than a f/15 scope.

If you want to do only double star, lunar, solar & planetary then a 4" f/15 doublet will do just fine. But it will require more mounting than a shorter focus scope of a similar aperture.

I have (amongst other scopes) a CPC1100 and a William Optics FLT 110 (4.3" f/7 triplet). The FLT 110 gives good performance with minimal acclimitisation (15 minutes is more than enough) and there is no doubt that it gives tidier images but there is also no doubt that, except when the seeing is exceptionally bad, the actual performance of the C11 is better.

Personally I would stick to 100mm - 110 mm for the refractor as anything bigger loses transportability.


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Dave Lee
sage


Reged: 02/14/13

Loc: Pinehurst, NC USA
Re: General Double Star Observing Equipment Question new [Re: brianb11213]
      #6127831 - 10/09/13 08:32 PM

Thanks for the links. I am working my way back through this forum, but was a ways from either of the two referenced threads.

Brian, FWIW since most of my viewing is in my back yard portability (up to a point) is a minor issue for me. Something that I can easily hoist up onto my G11 (at my mid 60's age) is important, however.

From all that I have read perception of the view in SCT's vs. refractors varies person to person. Given my almost zero experience with refractors (with no easy way to give them a try), the best option may well be a scope close to what I think I want with a high probability of mostly recovering my investment if/when I decide that something else is better. Kind of like renting a scope in a way.

Thanks again for the replies.

dave


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WRAK
scholastic sledgehammer


Reged: 02/18/12

Re: General Double Star Observing Equipment Question new [Re: Dave Lee]
      #6128338 - 10/10/13 03:22 AM

Dave, first tests I made with different CO sizes suggest that a reflector with a small CO of ~0.2 is better than an refractor of same aperture for splitting equal binaries and even unequal ones up to delta_m of ~2mag or may be even a bit more. With a CO of ~0.25 you are in this regard on equal terms with a refractor and with larger sizes of CO some degradation of the image begins and this gets really bad with CO larger ~0.35. For the resolution of close doubles with delta_m larger ~2.5mag refractors are the choice but also here aperture rules means larger aperture may compensate CO. For resolution of very faint doubles fainter than +10mag aperture is decisive and CO is of no relevance if separation is not very small.
So may be a 6" f/10 Newton with CO a tad smaller than 0.2 would be a good choice for really crisp double star observing up to the limits of this aperture.
Wilfried
PS: I myself use a 140mm refractor for double star observing and I am very satisfied with it. I own a C9.25 with good optics but do not like it for doubles due to the large 0.38 CO and I am looking forward to a 8" f/22.5 Dall-Kirkham with CO 0.25 to get a refractor like quality with this aperture with less cost and especially less weight - hope this works out


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Dave Lee
sage


Reged: 02/14/13

Loc: Pinehurst, NC USA
Re: General Double Star Observing Equipment Question new [Re: WRAK]
      #6128415 - 10/10/13 06:37 AM

Thanks for the comments.

I have considered the possibility of buying the pieces and building a Newtonian. Good luck with the DK.

dave


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Ed Wiley
scholastic sledgehammer


Reged: 05/18/05

Loc: Kansas, USA
Re: General Double Star Observing Equipment Question new [Re: Dave Lee]
      #6129669 - 10/10/13 06:40 PM

I use a C11 for imaging and measuring. The trick is precise collimation. I also use a Royce DK. The trick is precise collimation. Do not give up on your C11 until you spend the time to really nail the collimation. Took me two night to really get it right, but it was worth it.

Ed


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brianb11213
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Reged: 02/25/09

Loc: 55.215N 6.554W
Re: General Double Star Observing Equipment Question new [Re: Ed Wiley]
      #6129695 - 10/10/13 06:53 PM

Quote:

Do not give up on your C11 until you spend the time to really nail the collimation. Took me two night to really get it right, but it was worth it.



I certainly agree about the necessity of accurate collimation but the real issues with the C11 (whether standard or Edge version) are that the primary mirror usually fails to track temperature changes even when they're small, overcooling of the upper side of the thin metal tube wall causes tube currents which can ruin the seeing even when the primary mirror is at ambient temperature and the corrector plate is very prone indeed to getting fogged up, a dew heater tape is effectively mandatory & if it's set even a tad higher than is absolutely necessary to prevent condensation, again you get thermals in the light path.


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Dave Lee
sage


Reged: 02/14/13

Loc: Pinehurst, NC USA
Re: General Double Star Observing Equipment Question new [Re: Ed Wiley]
      #6129716 - 10/10/13 07:16 PM

Quote:

I use a C11 for imaging and measuring. The trick is precise collimation. I also use a Royce DK. The trick is precise collimation. Do not give up on your C11 until you spend the time to really nail the collimation. Took me two night to really get it right, but it was worth it.

Ed




I agree on this point. I need to work more diligently than I have so far on this. But I was out of town during the recent new moon/clear weather and now we are fighting a persistent low - darn.

dave


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Dave Lee
sage


Reged: 02/14/13

Loc: Pinehurst, NC USA
Re: General Double Star Observing Equipment Question new [Re: brianb11213]
      #6129732 - 10/10/13 07:28 PM


I certainly agree about the necessity of accurate collimation but the real issues with the C11 (whether standard or Edge version) are that the primary mirror usually fails to track temperature changes even when they're small, overcooling of the upper side of the thin metal tube wall causes tube currents which can ruin the seeing even when the primary mirror is at ambient temperature and the corrector plate is very prone indeed to getting fogged up, a dew heater tape is effectively mandatory & if it's set even a tad higher than is absolutely necessary to prevent condensation, again you get thermals in the light path.




I have been most surprised at how difficult temperature management seems to be in my C11. It seems a good bit tougher than I recall it being 20 years ago in my older 8* Meade SCT (although I didn't do a ton of critical high mag work then either).

I have a Lymax cooler but its effectiveness is mostly TBD at this point. My C11 tube is black and I have wondered if covering it with a towel or something might help lower the temperature gradients (would mostly avoid the tube radiating heat to the sky). Just speculation.

BTW, at this point I am using an unheated dew shield and a blow dryer for dew control. Effectiveness varies but it helps that my observing sessions (in my back yard) tend to be 2-3 hours max. I am able to set up the scope well in advance, however.

dave


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brianb11213
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Reged: 02/25/09

Loc: 55.215N 6.554W
Re: General Double Star Observing Equipment Question new [Re: Dave Lee]
      #6130387 - 10/11/13 03:55 AM

Quote:

I have a Lymax cooler but its effectiveness is mostly TBD at this point. My C11 tube is black and I have wondered if covering it with a towel or something might help lower the temperature gradients (would mostly avoid the tube radiating heat to the sky). Just speculation.



Yes, that's what I do. An hour with the cat cooler then drape a towel over the top of the tube whilst imaging. 10 mins with the cat cooler between imaging sessions.

When imaging at large image scales I do not use the dew heater fitted round the corrector plate. An extra long dew shield (made from foam camping mat) is used & if necessary condenstaion is removed using a warm air blower between sessions.

It's better this way but still not right. Personally I think the "closed tube" design is poor for high power work ... it needs thicker, insulated tube walls, active ventilation and a corrector plate heater (with surface and ambient temp sensors) built in ... would inflate the price tag but still a lot cheaper than an apo of comparable light grasp or resolution.


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exmedia
sage
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Reged: 05/26/13

Loc: Orange County, CA
Re: General Double Star Observing Equipment Question new [Re: brianb11213]
      #6131345 - 10/11/13 03:05 PM

I'll just throw this out there. Consider a large Mak (7" or larger if you can find one). Good light grasp (certainly better than any 4" refractor). Considerably less money than an apo refractor with nearly as good optical precision. The only one I know of is made by Orion.

http://www.telescope.com/Telescopes/Telescope-Optical-Tube-Assemblies/Orion-1...

That's my 2 cents.

Richard


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Ed Wiley
scholastic sledgehammer


Reged: 05/18/05

Loc: Kansas, USA
Re: General Double Star Observing Equipment Question new [Re: exmedia]
      #6132814 - 10/12/13 11:06 AM

I fitted Tempest fans to my C11 edge. I turn them on 1-2 hours before observing. The quality is such that I can do high resolution imaging with the fans turned on. Although I bought the nose fan (good deal as a package) I rarely use it.

Ed


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RAKing
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Reged: 12/28/07

Loc: West of the D.C. Nebula
Re: General Double Star Observing Equipment Question new [Re: exmedia]
      #6138909 - 10/15/13 12:12 PM

I won't knock the Mak's optics. I have owned two beauties: a 6 inch TEC f/12 and an 8 inch STF-Mirage f/10. The optics were outstanding .......... but if you think thermal control is difficult with an SCT, a Mak is ten times worse. That thick meniscus on the front takes much longer to equalize than the thin corrector in front of the SCT.

I was always a refractor guy and love my refractors. I currently have two excellent examples of the design and they do a very good job on doubles. But the best scope I own for doubles is my C8-HD Edge SCT. Yes, the contrast is not quite up to the TEC 140 - but that is more than offset by the 8 inch scope's ability to resolve tighter doubles. Three years ago, I was finally able to split Sirius B and I also finally got Zeta Cancri A-B. Sirius B is almost easy now, but Zeta Cancri A-B is still very tight (1.0 arc second in 2008 and increasing). I have not been able to get a clean split of these with my TEC 140... yet.

I have also owned a C11 and a C925. Neither of these scopes was any better at splitting the tight doubles than the 8 inch and I think they were simply more susceptible to atmosphere and thermal issues.

If the OP wants to try something new, then I might suggest a newer version of his SCT. Try the 8 inch Edge. It doesn't weigh any more than a decent 4 inch apo, but will do much better with tighter doubles.

Cheers,

Ron


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Dave Lee
sage


Reged: 02/14/13

Loc: Pinehurst, NC USA
Re: General Double Star Observing Equipment Question new [Re: RAKing]
      #6139333 - 10/15/13 03:45 PM

FWIW I have just bought (used) a Vixen 140mm NA140SS 'Neo achromat'. See this thread, http://tinyurl.com/ll6fhmu , in this forum for more information.

This is kind of a poor man's Televue in a way. It has an f11'ish (don't know for sure) achromatic objective and a focal reducer in the rear (built in) that brings the optical train down to f5.7. From a visual optical performance perspective it has many of the characteristics of a longer fl achromat but in a smaller package.

I honest did not chose this scope because I felt it was the best choice for me. I have become convinced that 'the best choice' for me is right now indeterminate. I wouldn't know it if I saw it (or in the case of a 175mm AP Starfire couldn't afford it). And this (used) scope was a very good price and I think that I can recover my investment if/when I decide to experiment.

Scope is (literally) in the mail.

dave


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drollere
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Reged: 02/02/10

Loc: sebastopol, california
Re: General Double Star Observing Equipment Question [Re: Dave Lee]
      #6148494 - 10/20/13 02:02 PM

i'm a devoted double star astronomer (see site link in my sig); i use a ~6" refractor, a 10" DK and a 12" SCT for double star astronomy.

in my view the foundation is that you stick with one scope and get used to it. paul couteau said it takes one year of continuous observing to really be able to utilize a visual instrument fully. partly that is learning, and partly i believe the visual cortex adapts to process a standard image more effectively. the "learning curve" when switching from one scope to another is palpable to me. in particular, once you select an eyepiece set that gives you the appropriate imaging, you can judge position angle and separation directly from visual appearance.

unlike most of the posters on CN, for whom technical specifications, thermal physics and optical theory are the predominant frame of reference for choosing an instrument, i feel those considerations are most important in learning how to use best whatever second best instrument you have, given that the best instrument you have will always be your eye.

the points about collimation and cooldown are important, truly. learning the whole mechanical and environmental skill set needed to use a scope efficiently is part of the benefit of sticking to one instrument. but you won't be collimating a refractor, and i think the thermal issues with a commercial SCT can be exaggerated. (if you can't see a thermal plume off the mirror, visible in the extrafocal defocused image of a bright star, then your mirror seeing is at the same scale as the atmospheric seeing.) one consideration routinely omitted is focal length: double star astronomy benefits from magnification in the same way deep sky astronomy benefits from aperture.

it's also remarkable how far people go in for tools to fix behavioral challenges. you can use a lymax, or you can start cooling early, check seeing often, and just wait for those "revelation peeps" that even poor seeing will let slip through. again, the eye can do things a camera cannot, and advice from astrophotographers is of a fundamentally different nature than advice from visual astronomers.

personally, i prefer the esthetic simplicty of a refractor double star image, and royce's claim that his DKs yield "refractor like images" is nonsense. there's also no qualitative benefit to more light grasp or resolution, since you're just going to see the same configurations at greater astronomical distances, with more trouble about aperture seeing. but my point is that the solution to any problem is to keep at it, rather than think your wallet or a new scope is the better solution: commercial astronomical instruments today are of astoundingly good quality.

ceteris paribus, it is definitely more challenging to use a larger aperture, but the challenges can be overcome and that is part of the gratification. again, technical specifications tend to be put in the place of motivation and skill.

think of your C11 as a championship golf course, forego the need to buy many clubs -- and get to work on your slice.


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Dave Lee
sage


Reged: 02/14/13

Loc: Pinehurst, NC USA
Re: General Double Star Observing Equipment Question new [Re: drollere]
      #6148613 - 10/20/13 03:42 PM

Bruce, your comments regarding mastering the observing process (WRT your equipment but not including mechanics like collimation) are interesting and new (to me anyway).

In the meantime I have added a 2nd scope (Vixen 140mm 'Neo' achromat). However, I did get it used and at a price which should allow me to recover most of my investment. It will be interesting to see what I think (first light under clear skies should be tonight).

I guess if I were to follow the golf metaphor I would be building my own scopes as I do build my own clubs (which is a muchsimpler process, BTW). I will say however that I most definitely have fixed my slice (unfortunately not the hook, however )

dave

ps. So far I have only glanced at your website. But this is clearly an extremely rich source of information that I was unaware of. Thank you.


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WRAK
scholastic sledgehammer


Reged: 02/18/12

Re: General Double Star Observing Equipment Question new [Re: Dave Lee]
      #6148780 - 10/20/13 05:52 PM

A 140mm refractor should do a nice job with doubles. If you can for example resolve BU67 1.6" +6.8/9.9mag with it you should keep it as this would be about a limit unequal double for this aperture.
Wilfried


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fred1871
professor emeritus


Reged: 03/22/09

Loc: Australia
Re: General Double Star Observing Equipment Question new [Re: drollere]
      #6148971 - 10/20/13 07:33 PM

Quote:


personally, i prefer the esthetic simplicty of a refractor double star image, and royce's claim that his DKs yield "refractor like images" is nonsense. there's also no qualitative benefit to more light grasp or resolution, since you're just going to see the same configurations at greater astronomical distances, with more trouble about aperture seeing.




Bruce, some good thoughts in your exposition, but I'd like to ask about your experience with the Royce DK, as it's one of various telescopes I'm looking at, because I'm planning a larger scope to supplement my 140mm refractor for double stars. Yes, I want "to see the same configurations at greater astronomical distances, with more trouble about aperture seeing".

In particular, is your comment about the Royce DK not giving "refractor-like images" meant to point out the difference in image appearance (less aesthetic), or is it suggesting that the optics aren't as good as their advertised quality?

As a point of comparison - the DK presumably has a fairly small CO - 0.25 ? - and your Meade 12-inch presumably a fairly large CO - 0.35 ? - so, allowing for the difference in aperture, how do the two compare for observing close and unequal pairs. Those I think are the toughest test for double star observation; and the most likely to become "too hard" as CO increases.

Your thoughts?


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