small SCT vs big Dob - Paradox?

[Rodstar (Rod)]

Big J, I think you are heading in the right direction with the 10 inch Bintel Dob.

When I started, I began with an SCT. It was a great scope, especially in terms of its go-to function. BUT when comparing with other scopes I was often frustrated with the image quality compared with a newtonian or refractor. An SCT configuration involves four surfaces between the origin and the eyepiece, if you include the corrector plate (front glass) and the diagonal. That definitely impacts upon the quality of the image. So, a 10 inch dob will produce noticably better visual results than a 10 inch SCT.

You can hook up an Argo Navis to a dob, and give it "push-to" functionality. That works really well....gives you a database of 30,000 objects to keep you busy. So, the idea of dobs being "dumb" is not necessarily the case! An argo + digital setting circles will set you back about $900, so it isn't cheap, but is money very well spent.

The eyepieces that come with the Bintel scope will provide a good start for you. They are usually Plossl's which have about a 50 degree field of view, and are fine to start with.

Good luck!

[rat156]

Hi BigJ,

As an owner of an 8" SCT, and an 80ED, if you're interested in astrophotography, forget all of these options, including a dob.

When considering astrophotography the first and foremost consideration is the mount. You need a high quality tracking mount with low periodic error. Unless your main interest is in solar system objects, in which case the main consideration is a high quality fast frame rate camera.

For $1500 you won't get a mount that's easy to use for astrophotography, not saying it can't be done, but it's just way harder, most people give up. The more PE in your mount, the more sophisticated the camera has to be.

My SCT was the first scope I bought, it's go goto and helped me learn heaps, it was fun looking at stuff rather than looking for stuff. That said I was blown away when I recently had the privelage of looking through a 12" version of the same scope at a dark sky site, aperture rules on the faint fuzzies. For the bright stuff, really the contrast enhancement of a Newtonian over an SCT is really not a big issue.

It's a hard choice, but I'd be steering towards a goto Meade Newtonian, LXD75 in 10" if the budget goes that far. It has a goto mount, which will track OK, and the fast optics would aid in astrophotography, as would the widefield. When you want to move up (it happens to us all) you can simply upgrade the mount to a better GEM.

Lastly see what you can get on the secondhand market, there are bargains out there.

Cheers
Stuart

[KG8]

Here is an interesting looking deal out of the trading post.

Telescope Professional, 6in
refractor with equatorial mount and motor drive, as new, cost over $2000, sell $950 STRATHALBYN

Not my style but 6 inches of refractor would give you some nice images eh?

[csb (Craig)]

Yeah, I saw that ad.

A professional scope worth $2000!?

What determines a scope being professional (Ha Ha!)- rhetorical.

[mick pinner]

the original point of the thread small SCT v big Dob is just not a viable argument in either dollar value or actual hardware. these two examples of scopes would not cross paths in any aspect.
a small SCT is just that and has limited light grasp so therefore reasonable solar system views and some brighter DSO's although it has the electronics to allow some limited astrophotography and the ability to find objects that the user may not know how to find.
the dob has a larger light grasping ability so therefore more objects become visible however without extra equipment eg tracking platform for planetary work astrophotography at deep space level is really a waste of time.
l have learned that unless you are only interested in visual astronomy bigger is not better, the right tool for the right job is more important.

[MortonH]

I've previously owned an 8" SCT and various refractors. After a bit of a hiatus from astronomy I decided to buy a decent telescope a few months ago. At first I 'expected' to buy another SCT, but the more I read the less I was convinced, so I chose the 'safe' option of a cheap, second-hand Bintel Dob.

I have been amazed by the quality of the images. Beats my old SCT any day. And I was pleasantly surprised that nudging the scope to keep objects in the field of view doesn't really bother me at all (of course, others may disagree). I too am considering an upgrade to the 10" model.

Bottom line for me? You can buy a decent sized Dob for less than $700. If you decide you'd rather have an SCT instead, sell it and you'll only lose a couple of hundred dollars.

I do miss the SCT for easy piggyback photography, but for now visual is just great. If I upgrade to the 10" I'll probably get Argo Navis too, all for less than the price of a smaller SCT.

Good luck, whatever you decide.

Morton

[xelasnave]

Good choice.
If you do decide to do some astro photography, the 10 inch will go on an eq 6 (good mount I feel for the money $2000 approx) add a guide scope 80mm (best is only $1500) but $500 buys a good one. Guide and captuer thru either .. to a DSLR 6 meg $700 .. I said I would never do astro photos but they are easy compared to what I first thought and it gets you in somehow. You are probably only $3500 away from astro photos.


I have a 12 inch dob but its just too big for the eq6 mount but I am sure the eq6 would handle a 10 inch easy... later get the mount etc you will have a decent capture or guide scope in the 10 inch.. and you will see heaps with the 10 inch

Dont be in a rush to get EPs standard issue is ok for a fair while, think about what to get after you get a feel for what you like..planets and Moon as to DSO for example.

good luck
alex

[bkm2304 (Richard Brown)]

Originally Posted by BigJ

Hi,

I'm new to this group and considering my first real 'scope (my 30 year old 600mm Tasco with table-top- tripod doesn't count!).

I have been doing a lot of reading, and seem to have come to the conclusion that for general planetary, lunar and DSO work, an 8" Dob is ideal, especially considering the price. I'm considering a Celestron 8" Star Hopper. But some Celestron Schmidt-Cassegrains caught my eye

Now, here's what I don't quite understand: I have seen a lot of smaller Schmidt-Cassegrain's for sale, which are quite small (eg Celestron Nexstar 4 or 5), but inordinately expensive compared to the Dobs. Granted, some of the expense is because they tend to have fancy go-to mounts. Bus aside from that, if you were so into astronomy that you were prepared to spend $1,500 on a scope, why would you choose a 4" or 5" SCT over the light gathering power that an 8" Dob would give you?

Surely, optically speaking (and also since aperture wins), the view of distant nebulae, galaxies and star clusters would be inferior...?

Hi BigJ

As the proud owner of an SCT (Meade 10" LX200R) and a Meade 16" Lightbridge Dob, I can give you my experiences. The SCT is an F10 which is known to be superior to the shorter focal ratio scopes as far as planetary and lunar, high magnification viewing go. Comparing the F10 10" with the 16" F4.5 as i have done by literally running between the two scopes with the same eyepiece, this is true. But where the F4.5 16" comes into its own is in the deep sky object work. Using a 17mm eyepiece and looking at the same deep sky object - in this case it was eta carinae, there was no contest - the 16" F4.5 showed richer, more detailed view than the F10 SCT. Not that the F10 was poor, it was great, but the 16" had it for breakfast. On the other hand I look at planets through the F10 because of the crystal clarity I get compared with the 16".

The SCT I have is indeed a go to and when you master its foibles it is an amazing thing. I am not a "starhoppr" but like to get to the object and look at it rather than slowly work my way to it. The Dob on the other hand requires an addition of digital Setting circles which will cost another $1,000 minimum.

So cost of a 10 or 11" goto SCT from either Meade or Celestron will set you back between $5,000 and $7,000 depending on the extras you get. Yes this is due to the goto but also due to the increased number of surfaces in the light path that have to be configured. The cost of a 12" Dob ranges from $1,000 to $2,000 with serious scopes of the same apeture costing even more.

Then there are the refractor people. Small apetures (relatively speaking) are compensated for by incredible clarity and crispness of image to say the least. But the law is the law and a 100mm refractor worth $4,000 will never ever resolve more than a 10" Dob worth $800.

So your choice of scope depends on what you are into. For Generalist stuff you can't go past a 10" or 12" Dob or a 100mm refractor. But if you want to see faint things without the challenge of the hunt, then a goto is a worthy investment.



Richard

[KG8]

Originally Posted by BigJ

I have been doing a lot of reading, and seem to have come to the conclusion that for general planetary, lunar and DSO work, an 8" Dob is ideal, especially considering the price.

I have been doing a little reading around on this subject too and have come to the conclusion that it pays to really figure out what you want to do with your scope before you purchase. Here is an informative article on some of not so obvious aspects of scope size and mount type, I have posted a few extracts below.

http://www.ayton.id.au/gary/Science/...old%20contrast:

planetary observation:
"seeing" conditions impose the greatest limitation on a telescope's usable magnification & resolution, and at times of average seeing conditions which is most of the time, the resolution of planets will be as good in a 5" as a 16", it is only on those handful of rare nights of excellent seeing that a 16" can be used to its fullest.
in addition, the lack of a guided telescope such as by an equatorial mount with motor drive, limits useful magnification to 200-250x as you will otherwise need to be continuously moving the telescope by hand creating vibrations. This applies to Dobsonian mounts in particular...

telescope threshold contrast:
unlike light-gathering power, telescopes of increasing aperture quickly approach a limit to its threshold contrast - the ability to display an object of given brightness on a given background sky brightness...

deep sky:
the ideal magnification for general viewing of faint objects is 10-15 x telescope aperture in inches (5x aperture in cm)
objects become too dispersed if magnification is higher, sky brightness intrudes if magnification is lower. However, many say that for a 10" SCT, 50-60x magnification is most useful with occasional use of 100x ...

[MarkN]

G'day BigJ,

Re the Bintel EPs: They'll get you on the way but that's about it. When I got my 10" they provided two plossls (9 and 15 mm) and a 32 mm (2" format).

The 32 has a load of edge distortion and you'll probably want to start saving for something decent in this range. The 9 and the 15 are better but I don't imagine the good blokes at Bintel would claim they are world beaters. Again, they will get you started.

You won't regret buying the 10" and there is always the option down the track of adding Argo Navis for about a G. Still "push-to" but you will know what you're looking at!

Mark.

[Rodstar (Rod)]

KG8, I think you may need to keep on researching.....some of your conclusions seem way off the mark, at least from my experience of the issues you have discussed. Hve you had much experience looking through those apertures you have described?

My 20" dob blows out of the water any view of any object through a 10" SCT in any conditions.

I don't have tracking on my 20" dob, but I comfortably view objects at 360x, and have no trouble nudging the scope along without undue vibrations.

In reasonable conditions, the scope can handle 500x or more without image deterioration.

As I pump up the magnification (within limits), the effect is to darken the background, therefore creating greater contrast. The sky appears much darker, and the object far more contrasty at a higher magnification than the formula you have suggested, perhaps by a factor of 5x. This works extremely well for galaxies in particular.

My 10" SCT performed acceptably on any night up to about 150x, and would often go over 200x in good conditions.

I do, however, agree that it is important to decide what the scope is to be used for. I remain of the opinion (having bought an SCT as my first scope) that the best first scope purchase is a dob in the 8 - 10 inch range. It will whet the apetite by providing sufficiently good views of deep sky objects, ensuring the user is hooked for life, whilst preserving the bank balance for inevitable purchases that continue, on and on!

[Petzza]

Hi to all, I hope you don't mind me jumping in as I'm in a similar position as BigJ in that I'm shopping for my first real telescope. I'm looking at a 10" Dob. but want to know if there is any great difference, apart from the mounting of the tube, between a Skywatcher (Saxon) and a Bintel (GSO). Are the primary mirrors of the same standard are the optics better in one than the other. Thanks.

[74tuc]

I believe KG8's research and the site quoted are pretty "spot on".

Rod,

The magnifications that you quoted are a testement to the high optical quality of your 'scope and say nothing of the contrast reduction due to seeing - espacially with large aperature telescopes - please see "Myth #5" in the telescope myths referred to on your web site. Without its adaptive optics the 10M Keck telescope has the resolution of a 100mm telescope - if the seeing conditions produces cells 100mm in diameter (Frieds parameter r0 = 100mm).

Addition:

Interesting to note that the magnifications you mentioned (360x and 500x) are 18x and 25x the aperture in inches (see myth #5).


Jerry.

[shredder]

Hi BigJ,

I think you are getting the right advice there. You can't play with astrophotography, look on the forum, many people (including me) try and fail.

I had an 8"SCT, a good small refractor, and sold them both to buy a dob, and dont regret it. From the light poluted back yard they all perform badly, the difference is surprisingly little, but from a dark site you cant beat size. Another advantage of the dob is ease of setup, you dont need to line it up, level it, point it south etc etc, just pull it out and throw it on the ground. Oh and as an aside, people often mention how SCTs are easier to transport because they are smaller. Well that has some truth to it, but not a whole lot. With the sct there is also the mount, the forks, the dew shield, the dew zapper, the power packs, and a whole bunch of other kit that gets forgotten when comparing the basic scopes. I find I need far less gear with the dob.

Visually the biggest difference is in focal length. The SCT generally has a much longer focal length. So its good for high power close ups, but bad for wide angle, large views (you will never bother looking at the pladies with it). The dob is the reverse, good for low power, wide angle, and not so good for high power (partly for the short focal length, partly cause you need to hand move it, which is not easy at high power).

In terms of colimation, a refractor is obviously best. For my money the SCT is worst, they are fidley to do, and from my experience always in need of doing. While you also need to colimate large dobs, you can buy simple tools to do this, its quick and relatively accurate. Though in truth with both you get used to it over time.

But as has been said before the biggest difference is between a motorised mount and non motorised. And my this I dont just mean goto. I mean tracking, without a motorised mount you continually nudge the scope along, try calling friends over to have a look and chances are whatever it is has drifted out of view etc etc. With a motorised mount it generally stays put for long periods of time, you can go have a cuppa and come back to find it more or less on the same thing you left it on. Makes a big difference if you are taking a partner with you to show things to etc. I tend to view alone, so its no big deal to me, but I do notice it a lot when I dont.

Your next question is do you want a standard dob or a truss dob (such as a LightBridge). For 8" get the standard as they arent that big, for anything bigger go for the trus.

The eyepieces in the GSO/Bintel dobs, are fine for beginners to intermediates, no need to buy expensive ones till you get the feel for it. They arent perfect, but it will be a long time before you see the difference and value in an expensive similar one.

Cheers

M

[KG8]

Originally Posted by Rodstar

KG8, I think you may need to keep on researching.....some of your conclusions seem way off the mark, at least from my experience of the issues you have discussed. Hve you had much experience looking through those apertures you have described?

You might want to re-read my post rodstar, those are not my conclusions but those of the author of the web site I posted. And yes I have looked through those apertures, I was up at an astro weekend just last month and observed several clusters and deepsky objects through both an 8" meade and a quality 16" dob. There was not a huge difference between the two I thought, but the seeing was not that good either.

[Rodstar (Rod)]

Thanks for your comments 74 Tuc.....I will defer to your greater knowledge of optics....

Is the difference we are talking about resolution v detail? With the greater light grasp of a larger aperture telescope, it is my experience that the detail in deep sky objects will be greater. It has been my experience that as I look through scopes of increasing aperture through various telescope sizes and types in the range 80mm, 4 inches, 5 inches, 6 inches, 8 inches, 10 inches, 12 inches, 14 inches, 16 inches, 18 inches, 20 inches and 25 inches, that the detail evident in DSO's increases. Most galaxies in 10 inches of aperture are mere smudges, in 20 inches spiral arms become obvious with direct vision.

I don't have a website!??? Do you mean Peter Read's??? I couldn't find "Myth 5", to which you have referred, but I did have a chuckle at some of the "legends", including relating to the assassination of JFK!

KG8, I take your point re the comments being those of the author of the website. Glad you have had some observing time. Hopefully sometime soon you can enjoy some excellent seeing and transparency, and have the chance to witness how truly stupendous the views through a large aperture telescope can be!

I had a very funny experience at SPSP15, when John Sarkissian (CSIRO, Parkes) and Alan Dyer (Sky & Telescope) spent a few hours with me observing through my scope. John is a very experienced astronomer (although he spends a lot more time with "the Dish" than with a garden variety backyard telescope). Anyway, I took John and Alan through a tour of some choice southern hemisphere objects, and with each object, John expressed more and more astonishment. It was a great laugh, he was completely floored by the views of the Homunculous, M104, Centaurus A, the Tarantula, etc. He had never seen anything like it before, and his reactions were so funny.

Hope you find a scope that brings you much pleasure.

[Satchmo]

Originally Posted by KG8

I was up at an astro weekend just last month and observed several clusters and deepsky objects through both an 8" meade and a quality 16" dob. There was not a huge difference between the two I thought, but the seeing was not that good either.

With 4 X the light gathering and increased resolution the 16" should provide a stunning increase in object brightness and detail. Your lacklustre experience suggests a bright background sky and poor seeing conditions ( and perhaps even some general observer inexperience ) which will cripple the subjective perception of any large scope.

Mark

[ballaratdragons (Ken)]

Originally Posted by KG8

I was up at an astro weekend just last month and observed several clusters and deepsky objects through both an 8" meade and a quality 16" dob. There was not a huge difference between the two I thought, but the seeing was not that good either.

There is a huge difference! A beginner is even astonished at the brightness and detail in the larger. I still am.

You must have been looking thru pea soup

[74tuc]

Hello Rod,

Apologies about referencing the web site as yours - I referred to the signature part of your post. In that site they refer to "More telescope myths" and "Myth #5" is at:
http://www.televue.com/engine/page.asp?ID=141and and after doing a copy and paste job here it is:

Myth #5: The highest useful magnification is 50x per inch of aperture.

What is "useful"? Although small telescopes little affected by the atmosphere may give pleasing images even up to 100x per inch of aperture, no more detail is seen than at 50x per inch. On the other hand, large instruments, more affected by atmospheric seeing, may top out at 20x or 30x per inch. In practice, a 3- or 4-inch refractor may work well at 200x, but it is rare indeed that any size instrument benefits from more than two or three times that magnification.

Please note: "On the other hand, large instruments, more affected by atmospheric seeing, may top out at 20x or 30x per inch"

What Televue says is so true.

Myth #5 leads to an interesting truth best described by asking the question: How can one see the Encke division in the rings of Saturn with a 6" 'scope ? This division is about 0.05 arc secs and is 10 times lower than the rubbery x50(Aperture in inches) figure !! BTW the x50 figure is derived from Daws limit - a practical fiddle - on the separation of double stars. See Mike's excellent planet pics. What is most important is an animal called the "Contrast Transfer Function" or less descriptively "Modulation Transfer Function" (MTF). The overall MTF is set by the Telescope optics and seeing. Large telescopes will show more if you can remove the effects of seeing (Registax etc.) whether or not you see any more detail is upto the seeing conditions and sky transparency.

Addition:

Of course a larger aperture will show more and more nebulosity but resolvable detail? Compare what you see with the exquisite detail in the pics of Trimarchi, Ward, Bond, Gendler &c.

Hope this helps explain things,

Jerry

[BigJ]

74tuc, I think the link you were after was http://www.televue.com/engine/page.asp?ID=141 the URL hypertext didn't come through properly.