I understand that the usual suggestion for a beginner is the biggest Dobsonian that he/she can afford/store/transport, and I can understand why. Certainly in terms of all round bang for your buck it's hard to ignore the 8 or 10 inch GSO and Skywatcher Dobs at $450/$650 from Andrews, complete with (almost) everything you need to get started. But...
I live in a unit just 13 km from the Sydney CBD so observing from home means peering at light polluted skies from my balcony. The confined space doesn't seem suitable for a Dob, almost all of the sky would be inaccessible for one reason or another. A short tube Cassegrain configuration reflector or refractor mounted on a tripod just above railings height would fare much better in this regard and enable me to access most of the western half of the sky. That's still not great, but short of moving home that's the best I can do.
I'm sure there are people thinking that even trying to observe from such a terrible location is a waste of time, and I should just forget about my balcony, buy a Dob and take it somewhere more sensible when I want to observe. The thing is, I really don't want to buy a telescope that I have to take for a drive somewhere in order to use (at least, not until I already have another telescope for home use...) because if I do that it's going to end up being used for just a handful of nights a year. Something that I can use for a bit of casual stargazing from the comfort of my own home whenever I want is going to get a lot more use than something that requires an expedition.
Portability is still an important factor for me though, when I do get the opportunity I want to be able to take the telescope with me to better sites during weekends away, holidays, etc. This means it needs to be easy to fit into a (small) car, and if it can be made to fit within airline baggage limits (OTA as hand luggage, presumably) that'd be even better.
Given the light pollution at my home site I'm expecting to do more lunar and planetary observations than DSO stuff, so detail and contrast at high magnifications will be more important to me than wide field capabilities.
Because of all the above I've found myself looking longingly at 127/150 mm aperture Maksutov-Cassegrains, such as the Sky Watcher Black Diamond Mak or maybe the Bosma Omega MC. These seems to tick all my boxes:
1. Compact configuration compatible with balcony observing.
2. Long focal lengths suitable for high magnifications without requiring extremely short focal length eyepieces or high magnification Barlows.
3. Small central obstructions for good contrast and almost no chromatic aberration, all good for lunar/planetary views.
4. Compact size and reasonably low weight for their apertures, so fairly portable.
5. Reasonable aperture, small compared to most Dobs but bigger than all but the most extravagant refractors.
Of course all this works out quite a bit more expensive (and somewhat less versatile) than the standard beginner's Dob. It looks like $700-$800 for the OTA, and then of course I'd need a suitable mount too. I'm thinking alt-az for the sake of simplicity, but it'd need to be fairly heavy duty to support these 6-7kg telescopes, especially with high magnifications. Maybe an Skywatcher AZ4 or SkyTee, or a GSO/Bintel Sky View II, another $400 or so.
So, am I being foolish in considering a 150 mm Mak instead of a 250 mm Dob? Is there something else that would suit my situation better, for example a small ED refractor?
Sounds like you're being a realist to me. Nice to want to view whenever you can even under adverse conditions. I'm not sure how the Mak will perform under those conditions not using one myself but I know my 250mm DOB would not work well from a balcony. You seem to have some experience with scopes, your evaluation of your options makes sense to me. I believe aperture rules in LP skies so the 150 Mak would have an advantage over most (affordable) refractors.
And it's portable so anytime you do manage to take it away it's big enough to see something worthwhile.
Good luck with whatever you decide, we'll expect a first light report.
Thanks Brent, good to know that I seem to be making sense.
As for me having experience with telescopes, well, sort of. My day job is actually astronomy related, I do R&D on astronomical instrumentation. As a result I have a reasonable understanding of the principles of telescopes and associated optics, and have practical experience with professional gear, but I have next to no practical experience with visual observing. I've never had a proper telescope of my own, that's why I'm appealing here for the advice of those with direct hands-on experience of the real world capabilities and limitations of the various types of telescopes.
I do have a set of cheap 20x80 binoculars that I've used for occasional casual exploration of the sky, and about a year back I impulse bought a Celestron Firstscope 76 mm tabletop mini-Dob (same as the Skywatcher Heritage 76 but without the finderscope) but that performs about as well as you might expect an f/4 spherical mirror with very basic simple eyepieces to perform. A couple of weeks ago I did have an opportunity to look through a colleague's Dob (10", I think) at a very dark site under good conditions, and the views through that have reignited my interest in visual telescopes.
Rule #1 is apeture rules. But rule #2 is pick the telescope you will use most. No point choosing a big DOB if you can't set it up where you plan to observe from most.
The style of scope ticks box #2, so just get the biggest one you can afford.
I have no experience with MAKs so others can comment on your particular choice.
There are plenty of bright objects to be found that will keep you busy for a while.
When you talk about using a manual alt/az mount with a long focal length, narrow field of view Mak, aren't you overlooking how difficult it will be to keep a target in view, given the magnification these produce?
I'd imagine the combination would quickly prove frustrating, unless you're prepared to also spring for some expensive TV Ethos or Explore Scientific, 100 degree eyepieces.
Personally, I'd pick a goto SCT like the Celestron Nexstar SE series - prices on the 8" are awfully tempting at Andrews and Bintel at present and the extra diameter will give you a much better view of the objects that it's tracking.
When you talk about using a manual alt/az mount with a long focal length, narrow field of view Mak, aren't you overlooking how difficult it will be to keep a target in view, given the magnification these produce?
A fair point. I was assuming that I'd at least need slow motion controls on the mount to make manual tracking at high magnifications feasible, but I don't have a good feel for how difficult it would still be. A computerised mount would of course made the whole thing easier, but my idea was to go with a manual mount to begin with on the basis that a computerised mount would inevitably be a least a little bit heavier (thus making the setup as a whole a bit less portable) and of course more expensive. I did have in my mind the idea that I could always upgrade the mount later.
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Personally, I'd pick a goto SCT like the Celestron Nexstar SE series - prices on the 8" are awfully tempting at Andrews and Bintel at present and the extra diameter will give you a much better view of the objects that it's tracking.
Thanks for the suggestion, I'll look into those in a bit more detail. There are certainly some very hefty discounts from the RRP, though the extra aperture and computerised mount of the 8SE still comes out about twice the total cost of the 6" Mak manual setup I talked about in my initial post. The 6" is pretty competitive though.
The main thing with Maks is they have very long focal ratios, typically f/13. Sure, they give high magnifications very easily, however, they are limited to how low in magnification they can go. This restricts their ability to give bright images of DSO's (most DSO are seen at their brightest/easiest at low power). As consquence, unless you know what you are looking for, you just won't see many of them, especially from urban areas.
They also have a restricted capability to how wide a field of view they can give due to the size of the hole in the primary mirror. SCT's are also limited this way. It is more of a concern with low powers. And you can't use 2" eyepieces with either.
It's true that the Maks have long focal lengths (both of the ones I mentioned are f/12 focal ratio so have a focal length of 1800mm) and I understand that they won't be able to achieve very wide fields of view. Note though that the current generation of 6" and 8" Skywatcher Maks and the Bosma Mak I mentioned have 2" focusers (and presumably primary mirror holes sized accordingly) so they won't be quite a limited in this respect as the 1.25" Maks. I don't have the specs for exactly what the maximum true field of view really is, but if the maximum illuminated image circle is somewhere between 1.25 and 2" (which seems a fair assumption) then it'll be between 1 and 1.6 degrees.
Given the light pollution I will usually be contending with I figure that no telescope is going to give me particularly good views of DSOs, hence my focus on lunar/planetary viewing in my initial post. Lower magnifications do increase the apparent surface brightness of extended objects but they also increase the apparent surface brightness of the sky by the same factor, the contrast stays the same so while low magnifications can help to some extent the only way to really win is to find darker skies...
For when low magnification, wide field views are what's really needed then I do have the 20x80 astro-binos. I figure they're a good complement to the telescope so I'm covered for either extreme without requiring very low magnification from the telescope itself.
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However, SCT's have an advantage over Maks. SCT's for one are a little faster optically, f/10, but you can also use focal reducers on SCT's that further reduce the focal ratio of f/6.3, making faint DSO's a distinct possability. Many folks use these focal reducers in imaging, but this one is very useful visually too. These focal reducers don't work with Maks.
That's a good point, the availability of focal reducers makes SCTs a little more versatile. Odd in a way that there don't seem to be any available specifically for Maks, fundamentally they're not such different telescopes (both long focal length Cassegrain configuration catadioptrics) so I would think it would be possible.
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I've got a Celestron C5, a 5" SCT. As you're in Sydney, I'd be happy to show it to you some evening. You can compare it to a dob too.
Thanks, that's a very kind offer, I may well take you up on that at some point.
I was going to make an addition to my post, and, WHAMO, I deleted the bloody thing!
I got really, really peeved I came within a whisker of tossing the damned thing! I've only calmed down enough now this morning to dare have another look.
Hotdog, I see you where replying to my now deleted post. Amazing 'backup' of my post you've got there, .
The new maks come with 2" focusers? Interesting. I'd be suspicious that this mod is only to accomodate the larger focuser, rather than an actual mod to the primary too. I'd talk to Bintel and Andrews about that one. Making a larger hole in the primary really impedes its performance. Doesn't make sense to me. I can see the advantage in larger Maks and SCTs, as this hole won't be larger than the secondary obstruction. But in a 6"?
Yes both Maks and SCTs are Cat's, but thier optical technicalities are distinct. You will see that a Maks tube is longer than a SCTs for the same size apeture. This will tell you that the primary and secondary are performing differently between the two styles. The focal reducers for SCTs are designed FOR SCTs. The light path coming from a Maks secondary is a different shape to that from an SCT.
"Just because" you live in a light polluted sky enviroment doesn't mean you need to sell yourself short on performance. It is easier to go "up" in f/ratio (with eyepieces and barlows), much harder to go "down" (reducers can introduce unwanted aberrations). It would be a shame to find your f/ratio is too slow in those chances you do get to go to a dark sky.
I'll give you an example. My f/4.5 17.5" dob is out performed here at home (eastern suburbs Sydney) by my f/4 8" dob, and a mate's f/4.5 13.1" dob. It may not seem like much in f/ratio terms, But in a light polluted sky, it made all the difference in seeing DSO's and not! Some may argue that the large apeture of the 17.5" and 13.1" also introduced more light pollution. I beg to differ.
20X80 binos are fantastic! I'd love to get my mits on a pair. They are also useful from Town too! Very, very useful. I've got a 10X50 pair, and I've been able to see the Sombrero galaxy and M83 galaxy through them, again here from home. Your 20X80 will give amazing views of the sky. Do you have them set up on a tripod?
Hotdog, I've got a standing invitation to IIS members to join me at my place to sketch the Moon this weekend, starting tonight (weather permitting). You'd be welcome. See this thread for details:
_____
PS You might like to drop a PM to Wavytone on the subject. I don't know of many others who have the optical experience he does. His current "love" is a 7" skywatcher mak. This is Wavytone's member's page:
Of course all this works out quite a bit more expensive (and somewhat less versatile) than the standard beginner's Dob. It looks like $700-$800 for the OTA, and then of course I'd need a suitable mount too. I'm thinking alt-az for the sake of simplicity, but it'd need to be fairly heavy duty to support these 6-7kg telescopes, especially with high magnifications. Maybe an Skywatcher AZ4 or SkyTee, or a GSO/Bintel Sky View II, another $400 or so.
So, am I being foolish in considering a 150 mm Mak instead of a 250 mm Dob? Is there something else that would suit my situation better, for example a small ED refractor?
Seems like you have thought this through and a 150mm Mak should provide years of enjoyment but i will add my 2c on these to queries.
As a Mak has a rather narrow FOV I would suggest going for slo-mo controls. The Sky View mounts appear quite good but you could also look at the Vixen Portamount or Saxon AZ4 which is a Portamount clone. I own the Skywatcher AZ4 and while it is very solid (mine has a 1.75" steel tripod) i find myself occasionally thinking slo-mo would make observing the planets easier. This is with a Megrez 90 that has a very generous FOV. For DSO's and panning the Milky Way it is superb and very smooth but balance is critical at high mags with planets.
Small ED refractors are also very nice but can be very expensive. If i were in your shoes i would buy one of the Synta 100ED's. Why? I love refractors. Pin sharp stars, wider FOV, better contrast, will reach ambient temp much faster etc. For $1099 with case, diagonal and an eyepice the Skywatcher may be worth a look. Sure you won't see as deep as the Mak will collect more light but this is lessened slightly due to the Mak's central obstruction. I was in a similar situation as you when i bought my Megrez 90 but i have a 12" dob to seriously observe DSO's so aperture was not as critical. Due to size restrictions my 12" lives out in the country at a dark site and the Megrez with me for grab n go stuff in the city. A 150mm Mak will likely be my third (and last) scope to squeeze that extra detail out of the planets. That would not be the case though if only i could afford that 5" refractor
The new maks come with 2" focusers? Interesting. I'd be suspicious that this mod is only to accomodate the larger focuser, rather than an actual mod to the primary too. I'd talk to Bintel and Andrews about that one. Making a larger hole in the primary really impedes its performance. Doesn't make sense to me. I can see the advantage in larger Maks and SCTs, as this hole won't be larger than the secondary obstruction. But in a 6"?
Yeah, it would be surprising for a 6" Mak to manage an image circle covering the full 2" that the focuser allows, but I figured the image circle is probably at least 1.25" otherwise the 2" focuser is kind of pointless. So, unless it is just a gimmick then a 1 degree true field of view should be possible. I need to find out though.
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Yes both Maks and SCTs are Cat's, but thier optical technicalities are distinct. You will see that a Maks tube is longer than a SCTs for the same size apeture. This will tell you that the primary and secondary are performing differently between the two styles. The focal reducers for SCTs are designed FOR SCTs. The light path coming from a Maks secondary is a different shape to that from an SCT.
Sure, I realised that SCTs and Maks would need different focal reducer optimised for their respective characteristics. I just couldn't see a reason why it would be much harder to produce one for a Mak than it is for an SCT, which makes the apparent lack of focal reducers designed for Maks a little surprising to me. Unless it's an inverse-telephoto design then a focal reducer will inevitably reduce back focal distance, but that issue applies to any telescope.
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"Just because" you live in a light polluted sky enviroment doesn't mean you need to sell yourself short on performance. It is easier to go "up" in f/ratio (with eyepieces and barlows), much harder to go "down" (reducers can introduce unwanted aberrations). It would be a shame to find your f/ratio is too slow in those chances you do get to go to a dark sky.
I'll give you an example. My f/4.5 17.5" dob is out performed here at home (eastern suburbs Sydney) by my f/4 8" dob, and a mate's f/4.5 13.1" dob. It may not seem like much in f/ratio terms, But in a light polluted sky, it made all the difference in seeing DSO's and not! Some may argue that the large apeture of the 17.5" and 13.1" also introduced more light pollution. I beg to differ.
I'm a little confused by this. When you're looking through the eyepiece the apparent surface brightness of an extended object (or the sky...) will depend only on the ratio of the aperture to the magnification, so for a given magnification focal ratio makes no difference and aperture always wins.
Now, for a given maximum image circle size (set by a primary mirror hole, or focuser size, or eyepiece aperture, or whatever) there will be a maximum usable true field of view which will be proportional to the ratio of the image circle size to the focal length of the telescope (to within small angle approximations). Then if you divide your eyepiece apparent field of view by the maximum true field of view you end up with a minimum magnification for the telescope which is essentially just proportional to the telescope focal length. If instead you're limited in the minimum magnification you can use by the maximum focal length of the eyepieces you've got then the minimum magnification would still be proportional to the telescope focal length.
OK, so if you are using the telescope at the minimum magnification then the apparent surface brightness of extended objects will end being proportional to the ratio of the aperture to the focal length, which is indeed the inverse of the focal ratio so the same object will appear brighter in a faster focal length telescope.
This only applies if you're pushing the limits of low magnification though, if you're operating in the range where a pair of telescopes of different focal ratios can match magnifications then all that should matter is aperture.
There are downsides to a faster telescope focal ratio, too. The faster the focal ratio then the worse the aberrations afflicting both telescope and eyepieces, i.e. in order to get the same image quality out of a faster telescope of otherwise equivalent design you'll need better corrected (and therefore more expensive) eyepieces and telescope optics. A relatively simple eyepiece may well be sufficient in an f/12 telescope while the same eyepiece would look awful when fed with an f/4 beam.
I guess I'm struggling to understand how a 17.5" f/4.5 telescope could be out-performed by an 8" f/4 telescope, even in the situation where both are up against their low magnification limit the difference in apparent surface brightness would only be 13% and the bigger telescope would produce that very similar surface brightness over a image over twice a large. Unless the targets you want to see are simply too extended to fit in the larger telescope's field of view I can't see how the small difference in focal ratio would be enough to swing things in favour of the much smaller telescope.
Am I missing something here?
Anyway, given that I've pretty much ruled out a Dob or other big Newtonian on physical size grounds I don't think focal ratio is really going to end up being a deciding factor. As it's going to come down to a choice between an f/12 Mak, a f/10 SCT or maybe an f/8-9 refractor the differences aren't going to be huge in any case. I need to weight up the other relative merits of these telescope types to make my decision.
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20X80 binos are fantastic! I'd love to get my mits on a pair. They are also useful from Town too! Very, very useful. I've got a 10X50 pair, and I've been able to see the Sombrero galaxy and M83 galaxy through them, again here from home. Your 20X80 will give amazing views of the sky. Do you have them set up on a tripod?
Yes, I've been really pleased with the 20x80s, especially since I picked them up for just a couple of hundred dollars. They're branded "Sakura" but I think they're just the Kunming United Optics ones you see re-branded under lots of different names, they're BA2 series I think. I found them for sale at a kiosk in a shopping centre mixed in with a load of complete garbage cheap and nasty binos and snapped them up. Optically they're pretty good, especially for the price, but mechanically they're a bit crap. The central focuser is too low geared and has developed a tendency to drift so that they now need almost continual refocusing, and the central bar has come loose so that the binos can rotate about their axis relative to the tripod mount. Despite all that I have had some good views, a few weeks back I got to use them at a really dark site (Siding Spring Observatory) and the Orion Nebula looked great, and scanning around the Milky Way spotting clusters was also amazing. I do generally mount them on a photographic tripod, though the one I have been using is a old cheapie that I've had for many years which really isn't up to the job, far too wobbly especially with the central column extended and not tall enough. In order to get the most I can out of the binos until they finally fall apart completely I've recently bought a much sturdier and taller tripod, a Manfrotto 055XPROB. When the mechanical deterioration of the 20x80s becomes too much I plan to replace them with some 25x100s for even better wide angle, deep sky viewing.
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Hotdog, I've got a standing invitation to IIS members to join me at my place to sketch the Moon this weekend, starting tonight (weather permitting). You'd be welcome. See this thread for details:
Thanks, that would be great but I can't make this weekend, I'm going up the coast for a weekend away. Another time maybe.
My (sons) first scope was an Meade ETX125 Mak. We live in light polluted Croydon Park. For planetary it gives outstanding views whether city or dark skies, and my father reckons that in a dark site it still gave better DSO views than his 12" LX200 in kellyville.
We often take it up to Macq Uni for the their open nights and often have people come up to us and say that viewing Jupiter through it was better then any of the other "bigger" scopes there.
I also have used it for some planetary video and have attached a pic of what was taken in our backyard in the city.
if you enjoy the 20x80's you will do well to avoid the 25x100 (a pain to mount) & get a pair of binoviewers for the mak - AEC a short time ago did have a 5" intes mak on sale at a killer price - one of the best mak's you can buy & it punches well above its weight & price. google intes mak & read some reviews -excel moon & planetary views in such a small pkg (i owned a 5" for a while)
My first scope was a 90mm ETX Mac. The views were good through it but dim on deep sky.
I think that a ~150mm Mac is a great compromise for city viewing as long as you can aim it and keep it tracking. I agree with the sentiment that slow mo controls would be very desirable.
The relatively long focal length is better for planets as you don't have to use short focal length eyepieces that are much more difficult to use.
Very wide field will be restricted by the light polution anyway so losing the ability to see "wide fields" is not much of a loss.
Hunting double stars, small planetaries and galaxies plus the planets is well within the capabilities of the scope.
Go for it!
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