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Benefit of larger aperture for doubles, although obstructed

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#1 John Fitzgerald

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Posted 21 May 2022 - 01:05 PM

I wonder if it would be worth it to replace my current 6 inch f/8 apo with a C11, for my double star observing?  The current new price of my apo is around 6k.  C11 is quite a bit less.   I know the views would no longer be "refractor like".  Would image quality suffer a lot, though?    Any replacement would need to fit in my 8 foot dome on my G11S mount.

I don't have very much visual experience with SCTs.  Lots with refractors and Newtonian reflectors.



#2 Astro-Master

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Posted 21 May 2022 - 01:32 PM

I wouldn't sell the 6" APO, just buy a good used or new C11 and put it on your 6" mount and see for yourself.  I'm betting the 6" APO will have better image quality with its better optics, no image shift, and focus backlash.


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#3 Astrojensen

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Posted 21 May 2022 - 02:30 PM

In my, admittedly somewhat limited experience, image quality would suffer dramatically. At least on almost all nights. Once in a while, you might get a rare alignment of conditions, where you would see good images, but it would be rare. I had a good C8. On two occasions in as many years, I could use it at 800x and get a sharp image. Fantastic views! On almost every other night, it was soft and ill-defined, even at dramatically lower magnifications. 

 

In my 6" f/8 ED, I get good images on a far more regular basis, year-round. Even on mediocre nights, I can often get flashes of sharp images, where I can get to the diffraction limit, and sometimes beyond. In my 6" APM, 255x is a routine magnification for doubles, used almost every session. I've had MANY nights, where I could go to 500x - 700x and see a sharp image, allowing me to regularly detect elongated doubles WAY below the Dawes limit. This basically never happened in the C8, except on those rare, exceptional nights. 

 

In short, in terms of viewing economy, getting observing bang for the buck, the 6" ED has proven to be a phenomenally good investment. Probably the best I've ever done in my astronomy career. Especially for double stars.

 

I would STRONGLY ADVISE AGAINST SELLING YOUR 6" APO!!! It's a good idea to get a C11 and see for yourself, how it will perform for you, but selling the 6" to finance it would be the height of foolishness, in my experience, and I think you would greatly regret it. 

 

Especially since you're interested in double stars, where aesthetics play a significant role (at least for most, myself included). 

 

 

Clear skies!

Thomas, Denmark 


Edited by Astrojensen, 21 May 2022 - 02:31 PM.

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#4 John Fitzgerald

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Posted 21 May 2022 - 03:22 PM

I would not be selling the 6" apo to finance a C11.  I'd probably find a good used one.  I simply wonder if it would be worth the effort.


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#5 Rutilus

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Posted 21 May 2022 - 03:49 PM

I agree with Thomas. I have a C9.25, and I much prefer using my 6" f/8 achromat refractor

for double stars. I find that the SCT gives a messy diffraction ring pattern that tends to make

observations of close doubles difficult. For all other things in the sky, I find the C9.25 very good,

but it's not my cup of tea when it comes to double stars.



#6 Ihtegla Sar

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Posted 21 May 2022 - 04:05 PM

Have you seen this thread?

https://www.cloudyni...tars-on-a-diet/

I have a 10" GSO Classical Cassegrain that I bought recently for planetary, lunar and doubles in my light polluted backyard since I'm still waiting for my TEC 140. Before that my main scope at home was an FC100DL. I haven't had the CC long enough to really answer your question but based on the few times the weather has been good enough, I can say that double stars are a lot prettier in the 4" Tak.

There are double stars that are too dim to be seen in the Tak that can be seen in the 10" CC. And those dim doubles look nice in the 10" CC. But given my seeing in the Pacific Northwest under the jet stream and near the mouth of the Columbia River Gorge, I have been having trouble splitting tighter doubles than I can split in the 4" Tak.

Last night was the first clear night in a month and seeing was surprisingly good so I worked on dialing in the collimation on the 10" CC. I still can't collimate on an in focus star because the seeing wasn't that good but I was able to see the poisson spot and a few diffraction rings just outside of focus and touched up collimation until everything was concentric with the poisson spot was centered to the best of my ability to discern a difference.

Then I took a look at Izar. It split nicely but wasn't nearly as pretty as it is in my 4" Tak. In the 4", both stars are nice round balls on nights like last night. In the 10" the primary star was a somewhat fuzzy ball with edges that shimmered in the seeing. I doubt I will ever experience seeing good enough where I live to see an airy disc in a 10".

Weather is supposed to be clear again tonight so if I don't have the energy to drive a couple hours and camp, I will be out splitting doubles with the 10" CC again tonight.

But a 6" has a lot more aperture than a 4", so I wouldn't think an 11" SCT would be better for doubles unless you were going for doubles that were too dim to be seen in a 6" or too tight to be split with the 6" (and you have the seeing to split such doubles). Ask me again once I finally get the TEC 140. I've been waiting quite a while.

#7 Astrojensen

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Posted 21 May 2022 - 04:06 PM

I would not be selling the 6" apo to finance a C11.  I'd probably find a good used one.  I simply wonder if it would be worth the effort.

That's a more tricky question to answer. Only yourself can answer it, in fact. Fortunately, it's quite easy to find used C11's for cheap. Especially if you can find an old Nexstar with dead electronics. 

 

There's no question that it'll gather a LOT more light than the 6". That's not going to be subtle. 

 

 

Clear skies!

Thomas, Denmark


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#8 John Fitzgerald

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Posted 21 May 2022 - 04:15 PM

It would have to be the right deal.  I would only want the OTA.  I have a 14 inch Losmandy dovetail.  I have finders, diagonals, etc.  It would probably have to be within a day's trip out and back to pick up, to avoid shipping, and breakage of the corrector plate.



#9 Astro-Master

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Posted 21 May 2022 - 06:33 PM

I have 5 scopes I use in my light polluted back yard.  A Meade 10" F/10 SCT. a C8, an Intes 180mm Mak-Cass F/10 with 1/9 wave optics, a 6" ES Mak-Newt F/4.8, and a Stellarvue 105mm APO.

 

I can only remember one night with the Meade 10" when I got a real nice view of the double double in Lyra, but the Intes Mak-Cass always has better images than the 10".

 

The C8 is my newest scope, and I've only used it twice on double stars.  It was better than the 10" but not as good as my three smaller scopes.

 

The 180mm Intes Mak-Cass is my best scope for double stars, but sometimes the secondary star will be hiding on the first diffraction ring.  The scope is heavy, built like a tank, and needs the big CGEM II mount, so it's more trouble to set up.

 

The 6" Mak-Newt and the 4" APO are my most used scopes for double stars.  The 4" APO always has nice images even when the seeing is below average, I can usually use 300 to 400x or more on an average night, and the 6" and 4" can use the smaller CG5 mount that stays in the upper back yard.

 

IMHO, after using all the above scopes on double stars, your best scope for double stars by far is your 6" APO, but that C11 will be nice on Globular Clusters, Planetary Nebula, and the fainter DSO's.



#10 John Fitzgerald

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Posted 21 May 2022 - 07:49 PM

<snip>

 

IMHO, after using all the above scopes on double stars, your best scope for double stars by far is your 6" APO, but that C11 will be nice on Globular Clusters, Planetary Nebula, and the fainter DSO's.

I have a 12 inch Dob for DSO. It doesn't track, and only has analog azimuth circle and digital angle finder to get close to things that I might I have trouble star hopping to.  I was thinking of the C11 on my G11 mount in the little dome.  Newtonians and domes don't mix well, unless the dome is nearly on the ground. 

 

I think the 12 inch Dob is diffraction limited, but I never get good enough seeing to prove it.  The best views I have ever had of Gamma Leonis, Castor, and Porrima have been with that Dob with a 50mm off axis mask, giving about f/35.  Perfect diffraction rings.


Edited by John Fitzgerald, 21 May 2022 - 07:51 PM.


#11 ngc7319_20

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Posted 22 May 2022 - 12:51 AM

There is some rule of thumb that the effective resolution of an obstructed scope (in terms of the equivalent refractor) is roughly the aperture minus the obstruction size.  (You can stare at MTF curves for both and see why it is more or less true.  It has to do with lots of light going into the first Airy ring for the C11.)  So the refractor equivalent of a C11 is roughly 11" minus the 3.7" obstruction = 7.3".  So I would not expect C11 to be that much of a jump from the 6" APO in terms of effective resolution.  Maybe something like a C14 would be a better increase.

 

In terms of collecting area, and seeing faint stuff, the C11 would be a huge improvement.  But that's not what you were asking about.



#12 Jon Isaacs

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Posted 22 May 2022 - 12:51 AM

I think it depends on you, the seeing and the targets of interest.

 

If you're interested in double significantly under 1.0", if the seeing allows doubles under 1.0" on a frequent basis, then a larger aperture scope can be worthwhile.

 

I'm not sure that an SCT is the ideal scope, you might be better off with your 12 inch Dob and adding a tracking platform and making sure it is fully cooled.

 

I've pretty much given up using refractors because my 10 inch Dob is just so much more capable. If I'm feeling extra perky, I setup the 13.1 inch F/5.5. Granted my largest refractor is a 120 mm F/7.5 FPL-53 Doublet but even a 6 inch has a Dawes limit of 0.76" and a Rayleigh of 0.90".  For the 10 inch, the Dawes is 0.46", the Rayleigh 0.55". For the 13.1, 0.35" and 0.42".

 

Another advantage of a larger scope is the ability to see faint doubles, particularly under light polluted skies. My eyes need all the help they get.

 

Finding what works for you is important. I put setting up the 120 mm as slightly easier than the 10 inch. A 6 inch F/8 would be more effort than the 10 inch. I like the 10 inch size, the 11 inch would be similar. In 1" seeing it it provides clean splits and if the seeing turns out to be super, I'm ready.

 

Jon



#13 C.Hay

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Posted 22 May 2022 - 03:03 AM

Have you considered a Mewlon 210 or 250? I find my Mewlon 180 is, for whatever reason, miraculously unaffected by seeing and dramatically more capable on everything, doubles included, than my 5-inch apochromatic refractor. Perhaps the analogy holds for a comparison between your 6-inch and the Mewlon 210. The 250 gets seriously expensive and heavy and may well start to be hit by aperture-related seeing. But the 210 feels like a sweet spot by the numbers, can be found second-hand, and is nice to handle with its 8kg weight, all concentrated at the back end thanks to the absence of a Schmidt plate.

 

CS, Christopher


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#14 oldscope

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Posted 22 May 2022 - 02:47 PM

In my, admittedly somewhat limited experience, image quality would suffer dramatically. At least on almost all nights. Once in a while, you might get a rare alignment of conditions, where you would see good images, but it would be rare. I had a good C8. On two occasions in as many years, I could use it at 800x and get a sharp image. Fantastic views! On almost every other night, it was soft and ill-defined, even at dramatically lower magnifications. 

 

In my 6" f/8 ED, I get good images on a far more regular basis, year-round. Even on mediocre nights, I can often get flashes of sharp images, where I can get to the diffraction limit, and sometimes beyond. In my 6" APM, 255x is a routine magnification for doubles, used almost every session. I've had MANY nights, where I could go to 500x - 700x and see a sharp image, allowing me to regularly detect elongated doubles WAY below the Dawes limit. This basically never happened in the C8, except on those rare, exceptional nights. 

 

In short, in terms of viewing economy, getting observing bang for the buck, the 6" ED has proven to be a phenomenally good investment. Probably the best I've ever done in my astronomy career. Especially for double stars.

 

I would STRONGLY ADVISE AGAINST SELLING YOUR 6" APO!!! It's a good idea to get a C11 and see for yourself, how it will perform for you, but selling the 6" to finance it would be the height of foolishness, in my experience, and I think you would greatly regret it. 

 

Especially since you're interested in double stars, where aesthetics play a significant role (at least for most, myself included). 

 

 

Clear skies!

Thomas, Denmark 

Sage advice from Thomas. I have never found an SCT that matches my 4.5" f/15 refractor (now 119 years old ... the scope, not me!). A quick filtered list of double stars that I can split, as listed in Stelle Doppie, yields 7853 pairs. I don't have that many nights in my lifetime to examine them all. Who needs a bigger scope?

 

Bart, NYC


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#15 Astrojensen

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Posted 22 May 2022 - 03:53 PM

 

Who needs a bigger scope?

Well, there's one thing about large aperture scopes, that make them very appealing: They put a lot of rapid binaries in reach.

 

If you have the kind of seeing that actually allow resolving them, that is. 

 

 

Clear skies!

Thomas, Denmark


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#16 Konstantin 1980

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Posted 24 May 2022 - 01:11 PM

It all depends on local visibility conditions. I wouldn't trade a 250mm newton for a 150mm refractor for anything. Even if this refractor is perfect. For double stars, a 250 mm newton will be very far ahead of a 150 mm refractor. In any case, in my conditions of visibility. I have never had the desire to remove the 250mm telescope and put in a 150mm. To me, this is absolutely stupid. I have seen many examples of perfect visibility for a 250mm telescope. And what's more, I would gladly change a 250 mm telescope for a 500-600 mm telescope. Also, many nights he would work at 80-90 percent of his ability to detail small objects. I was very sorry, watching in a 250 mm telescope, that I did not have a 500-600 mm telescope. I have repeatedly witnessed that the diffractive disks of stars in a 250 mm telescope were absolutely motionless, as were the diffraction rings.


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#17 John Fitzgerald

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Posted 24 May 2022 - 01:40 PM

Konstantin, 

Seeing good enough to fully utilize my 152 mm of refractor aperture rarely occurs here, maybe a week of nights per year, mainly in late summer. The refractor is in an 8 foot ExploraDome.

 

I have a 10 inch Newtonian and 12 inch Newtonian also, and seeing almost never allows for much more than half their resolving power.  Also, the Newtonians have more tube current and temperature change issues.  The 12 inch is in a roll off roof observatory.   The 10 inch is in my semi heated garage when not in use.

 

The only thing the Newtonians do better for me on double stars is allow access to some fainter, relatively wide pairs, like Xi Peg.  I was asking about the C11 because it will fit in my dome, and it  is f/10. Both my Newtonians are f/5, plus paracorr is usually in use, so effectively f/5.75.

 

I am thinking I may pass on a C11, unless someone is selling one cheap,  like a Nexstar with dead electronics.



#18 Astrojensen

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Posted 24 May 2022 - 02:19 PM

 

I have repeatedly witnessed that the diffractive disks of stars in a 250 mm telescope were absolutely motionless, as were the diffraction rings.

I have seen this twice in a fine C8, on very hot summer nights. I've not seen it in my 6" so far, but it has been very close to perfect on a few occasions, just not quite there. 

 

You regularly have excellent seeing conditions, but that's most certainly not the norm in most places. 

 

You should definitely get a larger scope, if you get the chance, because I'd love to read those observing reports!

 

 

Clear skies!

Thomas, Denmark 


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#19 Ihtegla Sar

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Posted 24 May 2022 - 05:28 PM

I've only seen in focus diffraction rings in a four inch. I've never been able to see in focus diffraction rings in anything larger but the next size up I have is 8" so I've never tried in a 5" or 6".

Would love to have seeing good enough to see in focus diffraction rings in my 10" CC. Seeing has been better than usual the past three nights and I was able to dial in collimation a little better and get some nice views of double stars, including the seven star system Stf 1830-31. The 10" showed all seven conponents nicely, including the mag 13 star labelled G in stelle doppie, whereas my 4" only shows six components clearly in my light polluted skies. But even with the unusually good seeing (for my area) it was nowhere near good enough to resolve in focus diffraction in a 10" scope.

#20 Spikey131

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Posted 27 May 2022 - 05:45 AM

I frequently observe with a C8 side by side with a 4” refractor.  I live in the NE USA.  I chase a lot of doubles.  Most nights, the stars are prettier in the 4”, but double stars split easier with the 8”.  The only group of doubles for which the 4” is more effective at achieving a split are relatively bright, close pairs with disparate magnitudes.  That clean airy disc and sharp diffraction rings make finding a dim secondary next to a bright primary easier.  But for everything else, the C8 is better.  If the seeing is exceptional, the C8 diffraction rings are sharp and the views are nearly as good as the refractor.
 

I also observe with a 5” refractor.  It is in between the other 2.  I can usually split omega Leo with the C8, but it is a challenge with the 5”.

 

I try looking at doubles with my 12.5” dob, but it is a rare night that the seeing will be good enough to make it the equal of the C8.  This result might be skewed by the fact that I tend to only set up the dob when the transparency is good for DSOs, and this is usually accompanied by poor seeing.




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