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reflector vs sct question

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#1 donnie3

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Posted 31 January 2013 - 09:01 PM

this may be a dumb question but im going nuts trying to figure out what to get for dso (visual only) orions 12" solid tube goto dob vs celestrons 11" regular sct. if you had a choice, what one would you buy for deep sky objects and why. thanks, donnie.

#2 coopman

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Posted 31 January 2013 - 09:04 PM

What would you mount the C11 on?

#3 KerryR

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Posted 31 January 2013 - 09:22 PM

The 12" solid tube dobs are major pigs to get out the door, never mind into a car or down stairs.

11" sct's are surprisingly heavy for the small package, but the smaller size makes them pretty easy to move around.

Your gonna have to mount the SCT olidly, no small chore, nor no small expenditure.

If you already a suitable mount, the SCT would probably give you more transport options, which is worth considering.

If you're going to be able to leave the 12" dob set up in a convenient location, the price differential makes it a potentially better choice.

#4 donnie3

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Posted 31 January 2013 - 09:49 PM

kerryr, it will not travel. ive had a 12" dob before and built a cart for it and it worked out just find the only thing im concerned with is what scope would be better for dso. i have a celestron cg5 goto ( its new ) just not sure that 11" would be stable enough, a lot of real experienced people on these forums clam its really not too bad for visual. i would hate to spend $1800 then find out its too shaky for me, i already have a meade 8" sct. and like it but i wont more aperture. the 9.25 would be a nice scope but $900 more for just 1.25" of aperture i just cant see.

#5 korborh

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Posted 31 January 2013 - 10:03 PM

The CG5 will be too shaky for C11 and not stable.

#6 Skyshooter

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Posted 31 January 2013 - 10:23 PM

If it were me I would opt for the 11" SCT. Not that the 12" Dob would be an inferior scope, quite the contrary actually. I used to own a 10" F/7 Cave Astrola that was hands down one of the best scopes I'd ever owned for visual DSO's. I sold it because it was a beast to set up and take down. Had I owned an observatory I'd most likely would have kept it. The C11 will give you plenty of aperture in a compact package. As they say, "the best scope is the one you USE". A nice newtonian with a zambuto mirror would be something I would dearly love but with my work schedule it probably would see less starlight that my 90mm APO or my 8" SCT all due to convenience. If I were retired it may be different. The answer to your question is dependent on your particular situation. What's best for you may be different for the rest of us. Sorry to ramble but hope this helps you with your decision. Best of all, enjoy your new scope !

P.S. I just saw you only have a CG5 to mount it on. I agree with the above comments, it's not a sturdy enough mount for the C11. You could always sell the 8" SCT and fund a new mount. Good luck.

Cheers,
Ed

#7 David Pavlich

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Posted 31 January 2013 - 10:34 PM

kerryr, it will not travel. ive had a 12" dob before and built a cart for it and it worked out just find the only thing im concerned with is what scope would be better for dso. i have a celestron cg5 goto ( its new ) just not sure that 11" would be stable enough, a lot of real experienced people on these forums clam its really not too bad for visual. i would hate to spend $1800 then find out its too shaky for me, i already have a meade 8" sct. and like it but i wont more aperture. the 9.25 would be a nice scope but $900 more for just 1.25" of aperture i just cant see.


I'm an SC fan. However, for what you've asked for namely the better scope for deep sky stuff, the Newt is the better choice. One inch more of aperture, no big deal, but a smaller central obstruction so it'll be a little better on planets when you get the urge. And it tracks, so there is no problem there.

For the SC, you'll need a mount. Ideally, something like the Atlas will make for a nice visual setup. I have a 10" Meade that I mount on a CG5-GT. It works alright for visual. I use it for my outreach events. Yea, the CG5 is a bit light, but with good balance, it does just fine.

But considering your request, the Newt will be the better of the two. And it will reach temp equilibrium quicker.

David

#8 gmartin02

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Posted 31 January 2013 - 10:50 PM

The CG5 will be too shaky for C11 and not stable.

You can mount a C11 on an AS-GT - if it is very carefully balanced (and not very windy), it will work acceptably well on the mount. Also, a SCT microfocuser from Starlight Instruments helps for less shake when focusing.

Balance is critical when loading that much weight on the little mount. As long as it is balanced OK, alignments & GOTOs are not a problem. Since you don't touch the telescope except for the focuser, what would make it shake?

Here is a pic of my C11 on a CG-5 that I had for a while - I also used to mount a 10" f/5 steel tube Dob on the CG5 (for over 50 observing sessions over a 3 year period), and compared to that setup, the C11 was much more stable :)

Attached Files



#9 korborh

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Posted 31 January 2013 - 11:04 PM

The CG5 looks cute in that picture :)

#10 coutleef

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Posted 01 February 2013 - 07:51 AM

For dso viewing and considering the scope will not travel, the dob is a better choice optically.

I am not sure that goto and tracking will be as good with the dob however than what you get with the sct.

Why not get a cpc1100

#11 rmollise

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Posted 01 February 2013 - 09:18 AM

The CG5 will be too shaky for C11 and not stable.


Actually, as I have said here frequently the CG5 (or VX) will be just fine for visual use based on me using this combo frequently for almost the last five years. ;)

#12 MikeBOKC

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Posted 01 February 2013 - 09:33 AM

Why not go for the truss Dob instead? Can break down into several less cumbersome parts for transport.

#13 rmollise

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Posted 01 February 2013 - 09:38 AM

Take it from me, unless you can carry or roll a truss tube scope out of the house, you will never use it at home. Disassembling one for the trip to a dark site is OK. Doing so for a backyard session? Not so much. After I converted my 12 to a truss, I basically never used it at home ever again. ;)

A 12-inch solid tube might be heavy, but once it's out the door and plunked down in the backyard, you are good to go. A 10-inch solid tube Dobbie is even quicker.

#14 Eddgie

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Posted 01 February 2013 - 10:18 AM

If you can store the scope so that it can be moved on a cart or use wheelbarrow handles, then by all means, go with the Dob.

The dob offers multiple benefits over the SCT.

First, the coma will be about the same, but you can correct this with a Paracorr in the Newt so that the view is esentially coma free.

The 12" scope will have the ability to generate a bigger true feild, even with the Paracorr installed. This is a major weakeness of the 11" SCT. You are limited to a 1 degree field in the SCT, whereas you can go quite a bit wider in the Dob.

You can get a bigger exit pupil in the Dob. Oh you can buy a 60mm Plossl for the SCT, but it is like looking though a tiny hole in a wall. With the Dob, you can get this big exit pupil using modern wide field eyepeices.

Some will say that you can use a reducer in the SCT, but if you attempt to use 2" eyepeices with the reducer, the scope will work at less than even the 11" aperture.

Remember this. For non-stellar (extended objects) such as galaxies and Nebula, both scopes will show the object with the exact same brightness if you use the same size exit pupil.. The only difference betwen the C11 and the 12" dob will be that for a given exit pupil, the object will appear sllightly larger in the dob. But it will be just as bright an any two telescopes, regardless of aperture, if they are used at the same exit pupil.

But for clusters, the bigger scope will give slighty better limiting magnitude, but more importantly, it will allow better framing of larger showcase objects becasue of the wider field.

For general use, a big Newtonian is hard to beat (it is hard to call the Go-To Newt to me a "Dob" becuase dobs are usually un-driven, and this is an Alt-Az mounted Go-TO.

You get a wider, flatter field, a bigger exit pupil using modern widefields, and coma free viewing if you put a Paracorr in it.

So, if you have a garage or covered outdoor storage so you can use wheelbarrow handles to roll it out, get the dob.

Don't let anyone fool you... Setting up a dob is no more difficut than setting up a C11. And if you can wheelbarrow it out, it is even easier.

And for deep sky, the Go-To Newt is far more flexible.

For deep sky, the quality of the mirror is not as important as for planets, but the C11 would likely have a better mirror.

Still, for the cost of a C11 and a decent mount for it, you can buy a Go-To Newt, a Paracorr, an observing chair, and a buy hand full of good qualtiy eyepeices.

And later, if you want to turn it into a planetary killer, you can have someone refinish the mirror for about $800.

#15 rmollise

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Posted 01 February 2013 - 10:25 AM



Some will say that you can use a reducer in the SCT, but if you attempt to use 2" eyepeices with the reducer, the scope will work at less than even the 11" aperture.


Ed, Ed, Ed, Ed, Ed...

I hate to keep harping on this with you. But it's not true. Any "aperture loss" is completely insignificant. The reducer and the 2-inch set up works very well indeed.

Otherwise, your advice is spot on...except...the coma in an SCT is NOT comparable to that in a dob. A Dobsonian of the usual focal ratio for a 12-inch, which is what we are discussing, is f/4.5 and will show appreciably more coma than an SCT.

Not that there is anything wrong with a 12-inch Dobsonian. One can be an excellent telescope for visual use. But there's no need to gild the dadgum lily! :gramps: :shakecane:

#16 Eddgie

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Posted 01 February 2013 - 10:45 AM

But it's not true. Any "aperture loss" is completely insignificant



Well, it may be insignificant to you, but it was appaling in my C14 and C8, so for me, it is a major factor.

But that is me, and you are you, and we both get to make the chioces we like.

My only point is to make people aware that the aperture is reduced.

They then get to consider if this is important to them.

I mean why buy an 8" scope if it is only going to work as a 7" scope? If they want a bigger field, maybe they should be looking at an 8" Newtonian.

So you can keep saying that it doesn't matter, and I will keep educatiing people on the compromises different descisions they make with their equipment can involve.

Only they can decide if the compromise is meaningful or meaningless.

But they should be informed about it.

#17 Eddgie

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Posted 01 February 2013 - 11:06 AM

And by the way, the 12" Go-To Newt that Orion sells is f/4.9, not f/4.5

The coma of the standard all-spheical SCT is in fact the same as the coma of an f/5 reflector for the same size true field.

If you have a doubt about this, you can ask anyone on the forum that has a copy of the book Telescope Optics to check the data. Page 85, section 9.3, second paragraph.

The plots are on page 86.

Not only this, but the coma in the SCT is also defocused because of the very steep field curvature of the SCT design.

So, when comparing apples to apples, both scopes would have the same coma, but the coma in the SCT would also be out of focus, while the Newtonian has a field that is four times flatter

And for a few hundred dollars (total cost of the telescope, a 31m Nagler, and Paracorr less than a C11 OTA alone) you can have a bigger Go-To telescope with a bigger, flatter, coma corrected field.

And once again, you can use a Parcorr. This will still give the ability to get a much wider true field out of the 12" Orion Go-To Newt than you could get out of a C8.

#18 rmollise

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Posted 01 February 2013 - 01:42 PM

And by the way, the 12" Go-To Newt that Orion sells is f/4.9, not f/4.5

The coma of the standard all-spheical SCT is in fact the same as the coma of an f/5 reflector for the same size true field.


More like f/6, and the Orion scope is an f/4.9, so both scopes do not have the same coma. ;)

#19 GeneT

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Posted 02 February 2013 - 10:42 PM

For about 10 years, I viewed with a C8 only. For the past, a 12.5 inch Dob. With good optics, mirror cool down, yada, yada, both SCTs and Dobs are excellent telescopes. Flip a coin--or buy both. :grin: Your local astronomy club probably with have members who own both. If you attend a viewing session, you could view through both.

#20 Jon Isaacs

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Posted 02 February 2013 - 11:54 PM

Otherwise, your advice is spot on...except...the coma in an SCT is NOT comparable to that in a dob. A Dobsonian of the usual focal ratio for a 12-inch, which is what we are discussing, is f/4.5 and will show appreciably more coma than an SCT.



If you look around I think you will find that the Chinese 12 inch Dobs are all F/5 or very close to it. 1500mm focal length, 300mm aperture.

As far as the Coma + Field curvature of the SCT versus Coma of the F/6 Newtonian, I can't say. I do know that the field curvature is significant in the SCT and that the coma in a Newtonian is correctable with a Paracorr.

I think that for most owners the choice probably has less to do with the optics and more to do with the way one observes with the two instruments.. People tend to favor one or the other and point to the optics but it has less to do with the optics than it does with the scopes themselves.. To wit:

Schmidt-Cassegrain Telescopes are essentially black boxes, you get what you buy and there is not much one can do to change or improve them.

Newtonians are open architecture designs, a tinkerer's dream... The optics can be sent off to Carl Zambuto, Steve Swayze, Ed Jones and refigured is they are not as perfect as one hopes. The secondary can be swapped for a difference size, better quality.. and many of these things the owner can do themselves..

Personally I am a fan of Newtonians... I like the fast focal ratios that make big, bright, wide fields of view possible. My 12.5 inch is F/4.06, add the 31mm Nagler, the Paracorr and wander around sky just looking. I like the fact that one can employ active cooling to achieve rock solid thermal equilibrium, I like the small COs possible..

All that said, if I had to choose between an 11 SCT and a 12 inch GOTO Dob, I would be very tempted to go with the SCT just because I believe the technology is more mature and more robust. SCTs are well suited to GOTO, Dobsonians, not so much.

Jon

#21 rmollise

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Posted 03 February 2013 - 09:58 AM

F/5 is not f/6, and field curvature is correctable with the f/6.3 r/c.

#22 Jon Isaacs

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Posted 03 February 2013 - 10:12 AM

F/5 is not f/6, and field curvature is correctable with the f/6.3 r/c.


F/5 is also not F/4.5 and coma is correctable with a Paracorr. :) Not much one can do though about those 30% plus Central obstructions.. :ubetcha:

But like I said, if I had to choose between a commercial GOTO SCT and a commericial GOTO Dob, I would probably go with the SCT... I must confess that I have been keeping my eye out for a C-5 to replace the one I gave away a few years ago. Not long ago an original Nexstar 5 showed up on CL for well under $200, it seemed like a reasonable deal.

Jon

#23 rmollise

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Posted 03 February 2013 - 11:21 AM

OK Jon...well, let's just agree to disagree. If we are...not really sure. :shrug:

#24 azure1961p

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Posted 03 February 2013 - 01:51 PM

I love the use of animates here.

Pete

#25 Jon Isaacs

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Posted 03 February 2013 - 01:54 PM

OK Jon...well, let's just agree to disagree. If we are...not really sure. :shrug:


Oh no, not agree to disagree, what fun is that? :whee:

I will let you and Eddgie battle out the question of field curvature and coma versus coma and eyepiece astigmatism.

To my mind, the comparison a non-issue. If one is looking for a larger aperture scope capable of bright, wide fields of view, a Newtonian is the obvious choice, it's a wonderful added dimension. Otherwise, the field curvature of an SCT only is an issue at the lowest magnifications and at the higher magnifications, the ability to use less expensive eyepieces at F/10 along with an acceptable amount of coma (comparable to a F/6 is what most seem to agree upon) with the SCT are definitely advantages.

In the big picture, I think the difference in the optics are not the guiding factor, both are reasonable, neither is perfect. The ease of use, the convenience, the size of the scope, probably come down on the side of the C-11. If the choice were between the Meade 12 inch SCT and a 12 inch Dob, I think the answer would be different.

Jon






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