SCT or Dob ?

[Ajohn]

It doesn't need to be much of a monster finder scope really. Field of view is what counts. It's not too difficult to knock up a sort of rifle site type arrangement if the finder is a problem. Some people like unity gain finders - telerad for instance. 4 degree, 2 degree and a 1/2 degree circle. The circles can be used for navigation. People have completes the Messier Marathon with nothing else. A dob I owned use a total reflection prism in a length of tube with a spike finishing on the centre of the tube at the far end. It was taken apart as I wanted the prism. It was actually a beam splitting cube so lost light when used this way. There was also nothing to keep the eye central. The same sort of thing can be made from a cheap star diagonal.

John

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[michael.h.f.wilkinson]

Don't quite agree. I can see far more guide stars in my 16x70 finder (4.4 deg FOV), than I can in my 6x30 (which has the larger FOV). Star hopping has become far more accurate, especially with LP, when the naked-eye limiting magnitude might be 4 (or worse). In my finder I can still spot much fainter stars. Very often, the object itself is directly visible in the finder, which makes life very easy indeed. Besides, it is a RACI type, which means no crick in the neck when viewing near zenith (the 6x30 is, not nice at all). I am not knocking Telrads, I have used one in France, I personally just prefer big finders.

Anyway, the 16x70 finder is not that big: this is a lot bigger :

Did not work out that well.

[Luke]

If it was for general deep sky, non-goto observing, no imaging, I'd go for the biggest aperture scope that is practical (price, size, weight etc. okay), which would normally be a dob for me.

A 6 inch SCT I find a bit lacking for general DSO observing under our okay skies at home. At darker sites at star parties, it feels more like an 8 inch scope does at home, and it gives nice views of a lot of deep sky objects. It is a cracking compact telescope that punches well for its weight and I love it, but at home I find I need 8 inch upward scopes to feel like there's lots to see.

I find a solid tube manual dob to be about as low hassle as it gets, very little to do to get it ready, which encourages me to pop it outside to cool down on the nights when it looks iffy. I spent an hour last night getting my imaging gear ready, and felt deflated when it didn't clear and I carted it back in while it rained! I should have popped our solid tube dob outside instead! :-)

[michael.h.f.wilkinson]

Best thing to do is to see the scopes in the flesh (at a star party or local club). You can then see whether the scope suits you or not, size-wise.

[E621Keith]

Don't quite agree. I can see far more guide stars in my 16x70 finder (4.4 deg FOV), than I can in my 6x30 (which has the larger FOV). Star hopping has become far more accurate, especially with LP, when the naked-eye limiting magnitude might be 4 (or worse). In my finder I can still spot much fainter stars. Very often, the object itself is directly visible in the finder, which makes life very easy indeed. Besides, it is a RACI type, which means no crick in the neck when viewing near zenith (the 6x30 is, not nice at all). I am not knocking Telrads, I have used one in France, I personally just prefer big finders.

Anyway, the 16x70 finder is not that big: this is a lot bigger :

http://stargazerslou...monster-finder/

Did not work out that well.

It's not just SCTs, some dobs need big finders too.

The scary thing is, the small one on the back is a 15"

http://www.obsessede...rg/msfinder.htm

I use my Borg 77 as a finder for my C6 when I need to star hop. You see much more in a 3" scope than the standard finder. A RDF takes care of the wide angle aiming

[michael.h.f.wilkinson]

Brilliant! Were you ever tempted to say:

<Aussie accent>

That's not a finder!

This is a finder!

</Aussie accent>

[scarp15]

SCTs aren't particularly well-suited to DSO viewing because of the narrow field of view, and there's no substitute for aperture.

James

Bit of a myth really, most DSO's fit snuggly into the field of view of an 8" SCT, including M31, that is where 2" diagonals and low power wide field EPs perform and / or a focal reducer / corrector. Many smaller objects are actually quite nicely framed to.

This should be true, but you also find a lot of SCT owners splashing out on Naglers, Ethoi etc as well.

Reckon we're all as daft as one another on this one.

Contrast and sharpness to the edge of field is much improved with naglers, panoptics and plossls used in my C8 compared with my previous eyepieces.

[norcalastroguy]

Sorry to jump in late to this topic...I have owned 46 telescopes since 2001, just about every SCT known to man, all the Meade series UP to the 16 monster and 2 Celestrons a 11 & 14" version both with star bright coatings...All in all YOU can't beat a solid SCT, but if I was just starting out and wanted to gain more from a begining stand point I would definately recommend a DOB...the reasons are fairly simple...1. initial expense - You can get alot for your $$$, they are also well suited to help you learn the sky vs a "go-to" system were you are reliant on technology instead of bringing out some charts and finding it on your own...2. Size does matter...Carrying around an 8 or 10" SCT is often times more difficult, that isn't to say the 8 version on a moderate small EQ mount will break your back but the same size in a Dobsonian is quite a bit more easier to transport, set up and collimate and generally speaking the image quality gives you a better overall view with less obstructions ( 1/3 more light gathering )...3 and most importantly, don't get caught up in the hype about this coating vs another type or how well 1 performs vs another! I haven't used a scope type yet that wasn't fun and enjoyable to use, from my 80mm Tasco back in the day ( the Haleys comet verision which I bought to see it and never did ) to the many Meades, Celestrons and Taks Ive viewed through ..IT honestly takes being OUT there to see and take in all the sky that matters the Most...Clear skies all, Jim

p.s. Dobs because of their fast F Ratio will give you a greater field of view vs. any SCT even with a focal reducer AND anytime I have used and still USE one to gain more field they add additional lens elements which reduce the image brightness and over contrast quality, not good on deep space objects!

[Ajohn]

Sense at last from California. I have to agree in many ways for aperture they can't be beaten. I'm not sure I would fancy carrying a 12in about though.

Personally I would wonder about 6in sct. To me a 6in equatorially mounted F6 newtonian with good optics and a sensibly sized secondary would be a better bet. Maybe even an F8. 1/12 pv optics either way. Still small and shouldn't make great demands on the mount. Maybe the same with an 8in but I feel that is the cross over point as that from say Orion would be the F6 version with 1/12 pv optics. It's pretty costly. Performance wise though it might just work out more capable than an 8in sct but I would wonder about that on an EQ5. Might be ok. I don't know. A sturdier mount pushed the price right up.

John

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[E621Keith]

For visual only use on DSOs I'd go for the 8" dob every time. SCTs aren't particularly well-suited to DSO viewing because of the narrow field of view, and there's no substitute for aperture. If budget is an issue then I'd probably avoid the Skywatcher f/4.7 10" dob to be honest. The views are fantastic, but the focal ratio makes it quite unkind to many eyepieces.

James

Bit of a myth really, most DSO's fit snuggly into the field of view of an 8" SCT, including M31, that is where 2" diagonals and low power wide field EPs perform and / or a focal reducer / corrector. Many smaller objects are actually quite nicely framed to.

I agree. There are only a handful of DSO that won't fit nicely into the FOV of a C8. On the other hand, a dob needs short focal length eyepiece to show some smaller DSO and they can be uncomfortable to use.

2. Size does matter...Carrying around an 8 or 10" SCT is often times more difficult, that isn't to say the 8 version on a moderate small EQ mount will break your back but the same size in a Dobsonian is quite a bit more easier to transport, set up and collimate and generally speaking the image quality gives you a better overall view with less obstructions ( 1/3 more light gathering )...

I found the opposite is true. An EQ mounted SCT can be broken down and carried in small manageable components where as a Dob can't. Look at the size of a C6 or C8 SE and you will realise they are tiny compared to a 150p or 200p dob.

If portability or storage requirement is an issue then a SCT is better than a Dob. Otherwise, a dob will be a better choice due to greater FOV and lower cost.

PS. a 8" Newtonian does not have 1/3 more light gathering than a 8" SCT. The central obstruction in a C8N (newt) 28% by diameter and 7.6% by area, a C8 SCT is 31% by diameter and 9.8% by area. The difference is only 2.2%, which will be barely noticeable.

[scarp15]

and most importantly, don't get caught up in the hype about this coating vs any SCT even with a focal reducer AND anytime I have used and still USE one to gain more field they add additional lens elements which reduce the image brightness and over contrast quality, not good on deep space objects!

Possibly true to an extent, the focal reducer performs best for myself, when observing and framing selective open clusters under a dark sky, and switching between a Low medium 2" with a medium power 1.25" eyepiece.

[michael.h.f.wilkinson]

The main reason I prefer not to use the focal reducer for visual is simply the fact that since adding a 2" visual back and diagonal, the widest 2" EPs give me the max FOV the rear cell of the SCT allows, and with much less hassle. In the past I had to unscrew the visual back, screw in the reducer, screw back the visual back, and then view. Now I just swap EPs. I just checked the reducer I have, and the internal diameter is a bit smaller (42mm) so the 2" superwide or ultrawide EPs can give a larger max FOV compared to the reducer.

BTW, extra glass does not necessarily reduce contrast (especially with modern coatings). It may in fact increase contrast, if it provides better correction of residual aberrations (like field curvature).

[Ajohn]

Now things are getting a bit daft. Take F10 concentrate to say F6.2 and it gets darker due to tiny losses from multicoated optics?

Actually in real terms an SCT mirror is larger than it's aperture so it has more even illumination across the field of view. The enhance mirror coatings are usually over coated silver giving about 99% reflection from memory. Much more than aluminium anyway. If people do want make dubious comments about light loss it's worth making one about something that is at least basically factual.

John

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[michael.h.f.wilkinson]

Now things are getting a bit daft. Take F10 concentrate to say F6.2 and it gets darker due to tiny losses from multicoated optics?

Actually in real terms an SCT mirror is larger than it's aperture so it has more even illumination across the field of view. The enhance mirror coatings are usually over coated silver giving about 99% reflection from memory. Much more than aluminium anyway. If people do want make dubious comments about light loss it's worth making one about something that is at least basically factual.

John

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As I noted earlier, for visual, the focal ratio is meaningless, when it comes to the apparent brightness of the view. Only aperture and magnification determine this (or the ratio: the exit pupil. The wider the exit pupil, the brighter the image on the retina. The SCT with reducer at F/6.3 with a 6.3mm Plossl (there is one lying about here somewhere), gives the same image as a 10mm in the F/10 normal setup. However, inserting glass in the optical pathway does take a way a tiny bit of light (in the order of 0.02 magnitude) through internal reflection and absorption in the glass itself. Therefore, the view with reducer is slightly (but not noticably) fainter. Typically, you use the reducer at a high exit pupil (well, obviously), which you cannot achieve with your EPs alone, so there generally is no problem.

For imaging, the reducer makes sense, provided your camera has a comparatively small chip.

[E621Keith]

As I noted earlier, for visual, the focal ratio is meaningless, when it comes to the apparent brightness of the view. Only aperture and magnification determine this (or the ratio: the exit pupil. The wider the exit pupil, the brighter the image on the retina. The SCT with reducer at F/6.3 with a 6.3mm Plossl (there is one lying about here somewhere), gives the same image as a 10mm in the F/10 normal setup. However, inserting glass in the optical pathway does take a way a tiny bit of light (in the order of 0.02 magnitude) through internal reflection and absorption in the glass itself. Therefore, the view with reducer is slightly (but not noticably) fainter. Typically, you use the reducer at a high exit pupil (well, obviously), which you cannot achieve with your EPs alone, so there generally is no problem.

For imaging, the reducer makes sense, provided your camera has a comparatively small chip.

I totally agree with Michael here. Furthermore, the SCT focal reducer are designed to worked at a specific distance from the focal plane. If they are not put in the correct position they can introduce aberration of their own. (quick google: Reducer focal plane distance for various SCT reducers: Antares 82mm, Celestron, 105mm, Meade 92mm)

A 2" eyepieces with a maxed out field stop like a 40 Aero ED, 42 LVW or 31 Nagler will make a SCT focal reducer redundant for visual use. The 40 Aero cost about the same as a SCT 0.63x reducer.

[scarp15]

Whilst I haven't used mine as often as I thought I might thus far, my Celestron Reducer / Corrector does appear to produce a slightly flatter field of view, with no discernible light loss.

[Dunkster]

This should be true, but you also find a lot of SCT owners splashing out on Naglers, Ethoi etc as well.

Reckon we're all as daft as one another on this one.

We're just doing our part to keep TV in business so they can keep making fancy EPs for the dob/newt guys

Glad to see we all love our scopes though, warts and all! Getting quality time under the sky with our Naglers/Ethoi/Pentax/other is all that matters

[RikM]

We're just doing our part to keep TV in business so they can keep making fancy EPs for the dob/newt guys

And we thank you for it

[Ajohn]

No point in replying to the posts that quoted me earlier. I'm sure that had been covered earlier and I don't see how point source and aperture effects follow on from mirror reflectivity. Daren't go into extended sources or photographic limiting magnitudes or sky glow.

Actually I suspect a 2in eyepiece at max field stop wouldn't be making best use of C8 resolution and contrast wise but not as badly as using it on an 8in newtonian type. 2in is more in the realms of 10in and greater. On the other hand with suitable extension tubes an eyepiece could be used with a reducer but obviously if the scope is usable with 2in eyepieces there would be very little point unless it reduced aberrations and any residual petzval curvature. The latter is rather important for photography and a lot less so for visual use.

John

PS The best buy from TV is their Plossls assuming that they are still manufacturing to the same standard as they were when I bought mine. Pre China. You might say that as far as Plossls go they are a life time purchase and great for F6 or so and above. Some testers would have us believe that TV optimise their eyepieces for their own scopes. Pass but stranger things do happened.

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[E621Keith]

No point in replying to the posts that quoted me earlier. I'm sure that had been covered earlier and I don't see how point source and aperture effects follow on from mirror reflectivity. Daren't go into extended sources or photographic limiting magnitudes or sky glow.

Actually I suspect a 2in eyepiece at max field stop wouldn't be making best use of C8 resolution and contrast wise but not as badly as using it on an 8in newtonian type. 2in is more in the realms of 10in and greater. On the other hand with suitable extension tubes an eyepiece could be used with a reducer but obviously if the scope is usable with 2in eyepieces there would be very little point unless it reduced aberrations and any residual petzval curvature. The latter is rather important for photography and a lot less so for visual use.

John

PS The best buy from TV is their Plossls assuming that they are still manufacturing to the same standard as they were when I bought mine. Pre China. You might say that as far as Plossls go they are a life time purchase and great for F6 or so and above. Some testers would have us believe that TV optimise their eyepieces for their own scopes. Pass but stranger things do happened.

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Some current Televue eyepiece are made in Republic of China (aka Taiwan) but none as far as I'm aware are made in People's Republic of China (aka. Mainland China). Anyway, where it is made is becoming less and less relevant when all of the precision parts are all made by machines. It's a matter off putting the blank in a machine and press start. Whether a Chinese, an American, a Mexican or a chimp pushes the start button would have very little effect on the final quality, but it does have a significant affect on the final price. The biggest problem is quality control, but TV ship everything back to their factory is the States for QC, so it's not a problem for them.

[Carbon]

I like them both. I like the dob for its simplicity.

[Stu_2011]

Some current Televue eyepiece are made in Republic of China (aka Taiwan) but none as far as I'm aware are made in People's Republic of China (aka. Mainland China). Anyway, where it is made is becoming less and less relevant when all of the precision parts are all made by machines. It's a matter off putting the blank in a machine and press start. Whether a Chinese, an American, a Mexican or a chimp pushes the start button would have very little effect on the final quality, but it does have a significant affect on the final price. The biggest problem is quality control, but TV ship everything back to their factory is the States for QC, so it's not a problem for them.

I wish this true, it would make my working life soooooo much easier, I'm currently working with a well known Chinese company on optics and its can of worms, even with state of the art new machines. It's much better than it used to be and I'm a huge fan of Chinese working approach but Chinese companies have their challenges.

Guess where I'm going in the new year......

Thanks

Sent from my iPad using Tapatalk

[michael.h.f.wilkinson]

Actually I suspect a 2in eyepiece at max field stop wouldn't be making best use of C8 resolution and contrast wise but not as badly as using it on an 8in newtonian type. 2in is more in the realms of 10in and greater. On the other hand with suitable extension tubes an eyepiece could be used with a reducer but obviously if the scope is usable with 2in eyepieces there would be very little point unless it reduced aberrations and any residual petzval curvature. The latter is rather important for photography and a lot less so for visual use.

John

This I do not understand. I would say the long focal length of SCTs creates an extra need for 2" EPs, because 1.25" only capture a small FOV (unless the focal reducer is used). 2" EPs do not benefit from the field flattening of the reducer, so they might suffer there (although in practice, this effect is very small). I used a 2" EP with 45.6mm field stop, and the image was crisp with no discernible vignetting. In a 6" SCT such wide field stops make less sense, as the baffle tube of that SCT will limit the FOV. Regarding "making optimal use of resolution": at low magnification (as used for most DSOs) no scope shows all detail.

In Newtonians, there is of course the issue of coma, which is predominantly a problem n ultra-wide EPs (even short ones).

[E621Keith]

This I do not understand. I would say the long focal length of SCTs creates an extra need for 2" EPs, because 1.25" only capture a small FOV (unless the focal reducer is used). 2" EPs do not benefit from the field flattening of the reducer, so they might suffer there (although in practice, this effect is very small). I used a 2" EP with 45.6mm field stop, and the image was crisp with no discernible vignetting. In a 6" SCT such wide field stops make less sense, as the baffle tube of that SCT will limit the FOV. Regarding "making optimal use of resolution": at low magnification (as used for most DSOs) no scope shows all detail.

In Newtonians, there is of course the issue of coma, which is predominantly a problem n ultra-wide EPs (even short ones).

I tested my 42 LVW in a C6 with 2" diagonal and found the 1.25" baffle tube in the C6 doesn't cause significant vignetting in the 2" eyepieces. The image doesn't get any brighter, but I do get a wider field. 40mm eyepieces are really designed for SCTs and slow refractors. The exit pupil they produce is too large for anything faster than F6.

[Ajohn]

Thought I had replied to the comment about 2in eyepieces and C8's or smaller. Must have forgotten to post it or it's gone awol. Happens from time to time as I may be doing other things while posting including going out for an hour or more. It was a long post.

Basically I'm guessing and assuming that full use is being made of the field in a 2in eyepiece or at least significantly more than is available in a 1 1/4. All scopes of a particular design have problems eventually as the field size is increased. It's one of the design criteria as well eg the over sized mirror in a sct is partly sized to help fully illuminate a certain field size. I do have a book somewhere that ray traces the commercial designs but am truly not sure where it is.

I also added that in real terms an SCT is obviously an ideal scope for an amateur but it's a pity about the cost over newtonian types. It's fortunate that sct are relatively easy to make accurately otherwise they would cost a lot more. The pure Schmidt sct are based on are probably the most precise instruments available photographically speaking. There are aberrations of one sort or another introduced by changing the design but they are still pretty good scopes and also rather compact which helps with nice rigid stands and portability etc. Add a focal reducer and they are good photographic instruments too. This subject partly cropped up in another thread. I don't like scopes without a drive so why can't i buy a cheap newtonian on a decent rigid stand with a drive at a sensible price that can be set up quickly and apart from the normal fast F ratio newtonian problems be used for what ever I want to do. There was such a scope and it did show that rigid pier mounts don't have to cost a lot of money. Copied from another post -

For put it easily in the car and a decent equatorial drive plus mount there seems to be a hole in the market. There was an ideal scope in many way. The older Meade Starfinder range. There are a couple of pictures of one here http://www.astromart...fied_id=347395. As the add mentions a very rigid short pier. The only heavy bit is the counterweight. Broken down they literally take mins to set up. The drive runs on internal batteries for a long long time too. They did from 6 to 16in ones I believe and at F4.5 the 10in tube will fit across the back seat of an ordinary car with the stand broken down in the boot. They used the infamous sonotubes - rather thick cardboard tubes. Sounds daft but this works out rather well. In many ways it's an ideal material. I understand the whole idea was a good over all portable scope of moderate size.

For me no drive is the aspect of Dobs I like least. Looks like I wont be too keen on SW synscan goto either. I can't just set the scope up and have the drive running. Star alignment seems to be a must. I would much rather level the scope, point it via a compass and that's that if I don't want to use the goto.

John