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DaveS

Big Maks no good for DSO?

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Yeah, I was really making a point about the "received wisdom" that I'd, as it were, received.

Thinking about it magnification is just that, doesn't matter what focal ratio the 'scope, though getting those huge wide field views may be out, moderate fields are still very doable.

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I think it's fair to say we agree Dave :grin:

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I have a 4.3" modified cassegrain and 2 years ago it showed me many of the Messier list in wonderful clarity. There is nothing stopping a little scope seeing such wonderful objects.

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I don't own a mak, so can only give a theoretical answer to Qualia. The resolution limit of a telescope depends importantly on an atmospheric factor called the Fried parameter: if aperture is less than the Fried parameter then the telescope's own diffraction (Airy criterion) is the limit, but if aperture is larger than the Fried parameter then it's the atmosphere that is setting the limit. In UK skies this can mean an aperture of about 6 inches. In other words, when it comes to resolving planetary detail, a scope of around 6" may be all the aperture you need. But that is assuming that all aperture is equal. In practice if you put an astigmatic 6" objective up against a perfectly figured 10" mirror then the latter is obviously going to give a better view, while a perfectly figured 4" objective will give a better planetary view than a mis-collimated 16" newtonian.

Any telescope with a central obstruction (e.g. newt or mak) will have a diffraction effect (scattered light) not suffered by a refractor. This can result in loss of contrast. So when it came to planetary viewing, the instrument of choice in days of yore was a long-focus refractor. Proper light baffling etc is also important. My 8" dob gives better planetary views than my 12" dob. It's probably because of a combination of factors including relative size of central obstruction (smaller on the 8"), greater light scattering in the low-profile Crayford focuser on the 12", and maybe the 8" mirror is better figured.

For DSO viewing the only thing that matters (apart from the pre-requisite of a dark sky) is aperture, because you're interested in light grasp rather than resolution. A 16" lemon will beat a Questar if the task is looking at 15th mag galaxies at a dark site. If the task is looking at M31 at a light-polluted site then anything you use will show a featureless fuzzy blob.

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Acey thanks for the enlightenment.When choosing a telescope for high resolution planetary use then choosing an aperture just above the average Fried parameter would be appropriate?To accommodate any better seeing that may happen?I wonder in Qualia's case if he may not be as limited by the Fried parameter as those in the UK?Acey,what about the notion that a longer focal ratio "smooths" out the seeing somewhat,to give better views-your thoughts?Thank you

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Acey thanks for the enlightenment.When choosing a telescope for high resolution planetary use then choosing an aperture just above the average Fried parameter would be appropriate?To accommodate any better seeing that may happen?I wonder in Qualia's case if he may not be as limited by the Fried parameter as those in the UK?Acey,what about the notion that a longer focal ratio "smooths" out the seeing somewhat,to give better views-your thoughts?Thank you

The theoretical work on this is, to my knowledge, only concerned with the effect of aperture and central obstruction - focal ratio isn't relevant, as far as I'm aware, nor can I see why it might have an effect. Of course it does have a practical influence for all sorts of reasons, e.g. on the extent of coma or (in a refractor) chromatic aberration, the latter being why long-focus refractors were preferred in the days before apos etc (and before imaging created added pressure for wide fields). If I were choosing an instrument for visual planetary observation I'd probably go for the best 4" refractor I could afford. For DSOs, the biggest aperture I could easily move to a dark enough place (which currently is my 12" dob).

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A 8" aperture theoretically has a resolution of 0.5 seconds of arc, for most of us the seeing rarely approaches 1.0  so on paper it's not much point having anything larger in the UK  for planetary observation. However, there's more to it than that, a number of planetary details are observable because of their colour contrast and it is here where larger apertures, particularly reflectors, will have an advantage. I also think it is a mistake to think that any old "light bucket" will do for DSO's. Poor quality optics will not concentrate as much light into the Airy disc as those of high quality, this results in a similar effect to very slightly defocusing. If you defocus a faint star it disappears therefore a well focused set of quality optics will allow you to see deeper.  :smiley:

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Mr Drew wields his considerable experience and knowledge in a superb post! (I am being serious!) It's an irresistible combination :smiley:

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Mr Drew wields his considerable experience and knowledge in a superb post! (I am being serious!) It's an irresistible combination :smiley:

Some very useful nuggets of information contained within that post by Peter, explained simply and put very elegantly  :smiley: . The info I can appreciate/understand, Makes a  lot of sense, but the experience I have not got using the types of scopes  and eyepieces Peter will have looked through, which no doubt is countless and that counts for so much. 

It is stuff/posts like that get imprinted into my memory bank :grin:  , where you hear some of the finer points confirmed and suspect some of that stuff to be the case, but when you hear it straight from the horse's mouth it solidifies that stuff.  

Not that I am suggesting Peter is a horse btw  :-0)

Edited by AlexB67
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I have been following this thread with great interest, i really like Maksutov telescopes,  i own both a 6" Mak and a 6" newt, this thread leaves me thinking that at low mag, both scopes will show DSO at a similar quality

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As above, there's no argument over matters of taste :D I have really tried to like the little Skymax 127, really I have, but I just don't get on with it. A friend over in Cheltenham has a 180 Mak and it is very nice but still doesn't give as a good a view as my 250PX Dob.

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 i own both a 6" Mak and a 6" newt, this thread leaves me thinking that at low mag, both scopes will show DSO at a similar quality

I was just about to say that you really need to get them side by side to be able to judge but your reply is good enough for me.

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I was just about to say that you really need to get them side by side to be able to judge but your reply is good enough for me.

tell you what, i think i will do just that, just need some clear skies again

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tell you what, i think i will do just that, just need some clear skies again

Just out of curiosity when you do give it ago, do you have a focal reducer to try out on the Mak as well?

I know people say they are for AP but it would be worth a try as a comparison.

Edited by hobsey

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No focal reducer, i would have to do my sums and work out eyepiece combo`s to match magnification with the mak and newt

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As above, there's no argument over matters of taste :D I have really tried to like the little Skymax 127, really I have, but I just don't get on with it. A friend over in Cheltenham has a 180 Mak and it is very nice but still doesn't give as a good a view as my 250PX Dob.

I've been through that "journey" too. I've owned mak-cass's from 127mm to 180mm, SCT's from 5" to 8" and a 6" mak-newtonian. The last one I really liked and I'd strongly consider another mak-newt but I just could not warm to the other designs at all for some reason. 

While I still love refractors I've now become a big fan of slightly slower newts with high quality optics. An 8" F/8 newt with 1/10th wave optics and a minimal CO would be a great instrument to own  :smiley:

Back to the original post, surely the OP was not surprised that a 180mm scope showed a much better view of an extended DSO than a 90mm one ?. It collects 4x as much light !

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I have done most of my DSO-hunting (especially galaxies, globulars and planetary nebulae) with my C8, which has has a similar false reputation as a "planetary only scope". Only a handful of very wide objects do not fit into the FOV, and for these the 80mm F/6 triplet is better. On any other target, aperture rules for visual, and focal ratio is a secondary consideration. Your F/15 Mak gives the same image with a 30mm EP as my F/10 SCT at 20mm (or an F/5 scope with a 10mm). Thus, when galaxy hunting, I use a 22T4 where most Dob owners would grab an 11T6, 12T4, or 13T6. Big deal.

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I've been through that "journey" too. I've owned mak-cass's from 127mm to 180mm, SCT's from 5" to 8" and a 6" mak-newtonian. The last one I really liked and I'd strongly consider another mak-newt but I just could not warm to the other designs at all for some reason. 

While I still love refractors I've now become a big fan of slightly slower newts with high quality optics. An 8" F/8 newt with 1/10th wave optics and a minimal CO would be a great instrument to own  :smiley:

Back to the original post, surely the OP was not surprised that a 180mm scope showed a much better view of an extended DSO than a 90mm one ?. It collects 4x as much light !

No, not really surprised as such. I had, perhapas taken on board the idea that maks were "Lunar and Planetary" and not really suitable for DSO observing. Very happy to be proved wrong.

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This is really a great thread and some excellent posts have been made (thank you, Acey and Peter for your help).

I'm only thinking out loud but if central obstruction is pretty much a function of the focal ratio, could something like a barlow or powermate help reduce that on faster scopes? As Peter mentions, aperture gain will not only help in resolution but also in contrast. If the optics are good and well collimated, then, an 8" newt should be able to hold up well against a 7" Mak? It's also interesting that Michael mentions the simple fact of getting longer focal length eyepieces to view DSOs in slower scopes. That makes a lot of sense; f/5 10mm, f/10 20mm, f/15 30mm. So in light of the OP, would a longer focal length eyepiece help out, albeit the faint fuzzy will be limited only by the 7" aperture and the quality of the skies?

Final thought, do you think the average eye would be able to tell the difference between a standard Chinese 1/4th wave optic mirror and a OO 1/6th, 1/8th or 1/10th? 

I'm sorry if these questions have messed up the OP and if the mods think this post should be errased then that seems absolutely fine :grin:

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No, the OP is quite happy to have started a discusion that's gone beyond what he expected. :grin:

Although I'll be looking for a longer EP, I remember reading (On here, I think a long time ago) that because of the baffling system low power wide EPs show vignetting, limiting the field of view to less than 1 deg. But this is still wide enough for most DSOs, and in fact, what I was used to with my old newt, and the narrow field EPs of the time.

The central obstructon looks less in proportion than my old newt, but I haven't got round to measuring it yet.

What is a little dissapointing, from Astro-Baby's site, is that Skywatcher didn't put an oversized mirror in, so it's more like a 170mm rather than 180. How much more would a 200mm spherical mirror have cost? Peanuts by comparison.

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Qualia,you have most likely seen these before,but if not they are informative. http://legault.perso.sfr.fr/obstruction.html, http://legault.perso.sfr.fr/collim.html,I paticularly like the description of miscollimation in terms of equivalent spherical abberation,"effective" increase in the CO,effective aperture size etc.Intersting to note that a 20% obstruction only results in a 4% light loss,assuming perfect collimation and that too small of central obstruction can actually increase its effective central obstruction.

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Never seen those links before Gerry, very interesting and thanks for posting them. :)

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Not got a mak but I only use my sct 9.25 for planets and the moon every thing else it as to be the 16 " dob

Pat

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Can't see myself ever getting a dob, Perhaps if I moved to a *really* dark site, but then it would need to be big, fast, and on a tracking mount. I'd also need to budget for a field rotator for the camera. I suppose if it were big enough I could fit it at the prime focus without a diagonal.

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+ 1 for Gerry's link. Again, like Mike I have never seen it, so after supper will sit down and have a read. Thanks for posting it up, Gerry :grin:

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