A Cultural Exchange
Posted 16 July 2014 - 01:59 PM
I am not as well versed in CAT and Cassegrain culture as you are. My cultural background hails from the achromatic refractor. I have owned a few CATS in the past- mostly Meade LX series – and stupidly let go a SkyWatcher 180mm f/15 Maksutov which I never properly tested. That said, I recently acquired another sample of the same Maksutov; a 180mm f/15 Orion branded instrument and while I have not had it for long, I have found it most enlightening. Under good seeing conditions and when fully acclimated, it serves up superb images that have easily outperformed what I consider to be a luxury ‘scope – my 5” f/12 achromatic refractor. The images were brilliant, almost overwhelming, in the larger Maksutov and require quite a bit of getting used to( I’m still on a learning curve). But I am now a keen student of the design.
It struck me that those who like refractors quite often lean towards catadioptrics. I can see the appeal; - one sits in a comfortable chair looking through the eyepiece at one end of the tube and at the other end, a large ‘lens’ that collects light. To me, it seems like a natural cross fertilization of telescopic genres and I would like to explore that very idea; call it a ‘cultural exchange’ if you will between my achromatic heritage and your catadioptric culture.
Like achromatic culture, you seem to have been persecuted by individuals who seem unable or unwilling to accept that our individual choices make us content.
I would be grateful if you could tell me more about your catadioptric culture and why you like to look through them. I understand that these telescopes have some issues which seem to come down to collimation and acclimation. How have you got around those problems and once there, what makes you come back again and again to your CAT?
Thank you for your attention,
Posted 16 July 2014 - 02:51 PM
Well long ago I was part of the working class who longed for the mythical Questar or C8, but entered through the clan of the Newtonian (RV-6 and Meade 8" model). Yes, a refractor was initially given to me from that culture, but at a youthful age I longed for the Questar which rivaled views of the Hale Telescope and C8 which belonged to the elite of the world, the clan of the SCTs. Why? It looked cool! It was powerful! It was portable! Easy viewing! Heck, Spock even had a C8 - it had to be good!
I finally paid my union dues and acquired one of the first LX200s and joined the culture of SCT & CATs. It was truly everything I dreamed of easy gotos (amazing for 1992), great views of targets (except for the faint fuzzies), and easy viewing angles.
Yes, I saw the refractors out there, but they did not impress me (views were bad) or they were way out of my price range. The culture of the refractor seemed for the Sears/Walmart crowd or the eclectic old timers of a by-gone era. The culture of the SCT was the perfect fit….
…except over time it was becoming painful to setup and use in the often cloudy and soggy midwest. My SCT became rarely used. Then came "Mini-Me", my ETX-125. Nice, portable and fun to use. I remained in the culture. I moved to New Mexico and then expanded further into the clan of the Cat & Casses.
With both my babies collimation is a non-issue (I rarely seem to need to) and I have adapted to cool down accordingly. It is just fun to use them.
Over time though, I kept hearing the rumblings of how good APOs were. Not only visually, but also for ap and in the quality of the build across the board. I ended up purchasing the Meade 80mm ED APO…and then my eyes were opened to this "new and improved" (to me) culture. Portability…wide field views…little or no cool downs…tack sharp views. I upped the game into the "refractor" culture to the 130mm and I love it!
Do I feel I have forsaken the clan of "Cat and Casses"? No, but some would call me a traitor to my clan. I am finding, like any tool both refractors and SCTs and the like have their uses and pluses and minuses. This may rock the boat, but I do think almost all of today's APO refractors offer very similar performance and quality metrics for every given size range at a fairly low cost allowing more to join the culture of the refractors and have a great experience without selling your first born. We are in the Golden Age of refractors.
Yes, I do agree with you, people do seem to be "persecuted by individuals who seem unable or unwilling to accept that our individual choices make us content". I am cool with our choices (unless you use Windows and/or root for Ohio State football) and hearing about the experience of others.
Either way, I straddle a couple cultures and love it!
Though, once I get that 3.5" Questar with similar views of the Hale Telescope that might change everything. They are a tight knit clan you know...
Posted 16 July 2014 - 04:24 PM
The main SCT issues AFAIC, are narrow FOV due to the large F/L, and diminished contrast due to the central obstruction. I am happy to accept those as trade-offs for being able to track and observe seated with a large aperture instrument.
The SCT is not an ultimate enthusiast scope. If you want 'pure' views an APO refractor will do better. If you want 'traditional looks' a long achro will do better. If you want huge aperture, a truss DOB will do better. All of those scopes though have disadvantages for the run-of-the-mill amateur. They either require a lot of effort in setup, transport and observing (large achros, apos and DOBS) are very expensive relative to performance (medium size APO's), or under-powered (small APO's).
So I come back to the SCT because it's by far the optimal combination of practicality and affordability. They are nice scopes. They are not the best in any particular measure, but sometimes the best is the enemy of the good. I would love to have a TEC 180FL or a 22" Starmaster--till I had to sign the check, and/or transport and set up at a dark site.
If I had a permanent observing location that was dark enough, I might change my tune, but the wife loves culture and big cities, so that's not really in the cards. Again: Most of us won't ever have the "dream" amateur environment or resources. But because of SCT's, we can get to enjoy our hobby with few regrets.
Posted 16 July 2014 - 04:45 PM
Don't worry, I think you'll soon fall in well enough over here in C&C. Originally, I didn't give cats much attention, until my curiosity was piqued by an irresistible "buy it now" offer. Viola, I was the proud owner of an Orion 5" Mak, now much tuned, studied and written about here.
Schmidt Cassegrains interested me not the least, after all, I'd read the list of complaints, plus everyone seemed to have a C8, which put me off even more. Inevitably of course, the irresistible "buy it now" struck me during a weak moment, and again I found myself with a bargain inoperable and even larger waste paper basket. Lots of time, and help from CN folks later, a C8 emerged that had been hiding its excellent optics under a bushel perhaps since leaving the factory! A superb planetary instrument against all previous expectations.
Cats have offered me good optics AND the chance to indulge my impulses to repair, modify, improve and otherwise get practically involved with the working of things.
Oh yes, and I share your appreciation for achromats
Posted 16 July 2014 - 09:31 PM
Posted 17 July 2014 - 06:36 AM
Thank you for your comments. I like the 180mm Maksutov very much. Last night I was shown yet more evidence of its superiority in resolution again over my 5” classical refractor when I set her up in my back garden. I turned the telescope on Lambda Cygni and was greeted with a wonderful split. I have never seen the companion so well! Aperture rules when conditions are favourable.
Thus far I have not noticed mis-collimation (it was perfect right out of the box) and its acclimation was not a problem either. It’s superiority in light gathering power was self-evident when I compared the views in my 5 inch glass to the larger compound. Thus the deficiencies of the refractor (namely limited aperture) were all too plain to see.
Like Andrew, I do like refractors and appreciate them in the smaller apertures. Like Stelios said, I believe the thermal management of these compound ‘scopes is somewhat exaggerated. Like Ed suggested, it is ‘scope that can be tinkered with to improve its performance even more.
The stellar images in the Maksutov are mid-way between that of a fine refractor and an SCT. It also shows a flat, fully illuminated field that is free of aberrations, just like my classical refractor. And yet it is not a refractor! My classical achromat produces a very modest amount of secondary spectrum; the Maksutov much, much less, the latter being more reminiscent of a fine apochromat of slightly smaller aperture. And yet it is far less expensive than the self same apochromat.
My Maksutov is a breath of fresh air. It provides a great way forward, appealing equally to the head and the heart.
Posted 17 July 2014 - 09:54 AM
The TEC 202 is, perhaps, the perfect scope for everything visual except wide field viewing. It is not fast enough for reasonable use as a Deep Sky photography instrument. (I wonder if I can get a focal reducer that will bring it to better than f/10.....) It can do wonderful high res lucky imaging of lunar, planetary and double stars.
As for the major 'knocks' against the Maksutov design: 1. the central obstruction is 25% which has only a very small effect on the diffraction pattern. I really don't see it as a problem. 2. the 'cool down' problem is NOT a problem if I can get the scope out about 1 hour before sunset. The scope performs beautifully from late dusk and seems to 'keep up' with a night of rapid cooling. I would add that nights of rapid cooling almost invariably have bad to terrible seeing which overpowers any cooling problems anyway...
I will add that the TEC has the best focuser in the world. Yes, the best. Despite being the 'moving main mirror' type it has absolutely zero image shift. It holds position perfectly and, with its micrometer dial, can actually be pre-set accurately by reading the dial.
Posted 17 July 2014 - 10:46 AM
I am not surprised that the OP took a liking to a 7" Mak over his 5" achro. Not really a fair comparison. If we set aside logistics of long/heavy tubes, cool-down times, viewing conveniences and cost, I find that inch-for-inch the humble colorful 6" achro with a CA ratio of around 1.5 offers better views at any magnification than any of the obstructed designs. Conrady 'fracs are even better. And this is coming from a Newt-guy even after I discovered the Paracorr.
Seriously, debate this all you want. Ask yourself how many times have you viewed through similar aperture commercial Maks, 'fracs, Newts and Schmidt-Newts in the same evening at the same objects. These are my credentials.
Where 'fracs and obstructed mirrors really diverge in image quality is contrast. I would much rather use my C6R than any other telescope in my collection. However, reality dictates that some nights I have neither the time nor the energy to set up or take-down the 'frac, and so I inevitably reach for .... a smaller ED 'frac.
When comparing designs of disparate apertures, comparisons will most likely favor obstructed mirrors as aperture increases. But such comparisons are hardly democratic and may not adequately represent reaching across the cultural divide.
Posted 17 July 2014 - 11:02 AM
My 200mm APO cost me about $400, oh wait, its a C8 Still, I could do a lot with the $29,600 I "saved" (if I had it...)
Confused myself one night with the 5" achro.... viewing too soon revealed that -shock, horror- refractors are not immune from cool-down requirements.
Like you say, this issue is sometimes overplayed, and a little forward planning in setting up ahead of time can really help, if one is able to do it.
Central obstruction? It's a rare thing that I can get 100% out of the achromat due to local seeing (largely I think, prevailing winds from the Pacific over the nearby mountains).
Bottom line: I like my array of different telescopes and their different viewing qualities. I also like that they were inexpensive purchases. The APOs may have a following, but I do feel some of what Neil says. If one is being cheeky, the adage "Less is more" could be applied. Not for me, I want a good focal length for magnification of the wee gems out there in the sky. Comfortable viewing with longer focus EPs that don't have to struggle with f/5 optics.
It's nice to be happy with what one has
Posted 17 July 2014 - 11:20 AM
Posted 18 July 2014 - 08:01 AM
The frequency with which the 180mm aperture delivered at my location leaves me in no doubt that a 7 or 8-inch achromat would be a better choice than a 6 inch, as I previously surmised. Achromats give dimmer images which are easier on my eye and I enjoy seeing some secondary spectrum. I believe this takes some of the energy away from the image making it less shimmery than a mirror or a triplet apochromatic objective. For now, the jury is still out on the relative aperture of my chosen retirement telescope. I do know that it need not be an unwieldy f/15 or some such. Faster achromats seem to do just fine. Indeed, even the fastest achromats can resolve very tight pairs. I had to ascertain that myself because no one else seemed willing to carry out those observations. So f/10 might be nice or even f/8
I hope to spend a season or two studying the planetary images through the Maksutov. In particular, I wish to employ filters that will match the image brightness to its contrast transfer. As the other thread I started shows, this telescope has the potential to provide fine planetary images.
I don’t understand the f ratio thing in Cassegrains though. I mean an f/10 achromat provides a flat field with little or no field curvature. But my old notes show the f/10 Meade SCTS have noticeable filed curvature when pushed off axis. Yet my Maksutov ( f/15?) does not display such curvature – at least from what I’ve observed. Why is that?
Posted 18 July 2014 - 08:21 AM
I find the brightness of the images a bit distracting; when the seeing doesn’t cooperate they swell too much. I take it it’s the same way with a large apochromat?
First time that's ever been said by an amateur astronomer.
Posted 18 July 2014 - 09:36 AM
I don’t understand the f ratio thing in Cassegrains though. I mean an f/10 achromat provides a flat field with little or no field curvature. But my old notes show the f/10 Meade SCTS have noticeable filed curvature when pushed off axis.
I suspect the field curvature is related to the fact that SCTs usually have a pretty fast primary mirror, somewhere around f/2-3 if I recall correctly. The entire system is usually f/10, however, once the contributions of the corrector plate and secondary mirror are factored in.
Posted 18 July 2014 - 09:48 AM
Exit for three decades, and coming back in to the game I still thought in terms of the 70s - smallish refractors and Newtonians. My first scope after returning was a splendid, and surprisingly well figured, Skywatcher 100ED Pro on an EQ5P. I thought this was Nirvana (and this is still my most used scope today), but this was before aperture fever set in. My next purchase was a used C8N f/4.9 Newtonian OTA with, again, excellent "consumer" optics. In short order I came to loath using this otherwise fine scope on the Atlas GEM I had acquired due to the contortions required to observe with it, even with Wilcox rings.
So what was I to do? Enter my first SCT: a very fine Celestron 9.25 XLT SCT. It was compact, powerful, a versatile imaging and visual tool that was easy to mount and use on my GEM. In short it was fantastic. I had great fun (and occasionally still do) with this scope, but it was acquired as an interim solution because I already had my sights set on the C11 Edge HD. Together the C925 and C11 Edge provided me with cosmic revelations about the effectiveness and versatility of the modern SCT design. They do everything I want them to do in a convenient, comfortable to use package, and at a reasonable price. But what about the central obstruction and loss of contrast? Much overrated in my book, primarily because the larger SCTs can largely compensate for the CO by sheer size and light gathering power – they provide brighter images with substantially increased color saturation. Jupiter, for instance, always shows more color for my C11 than with my splendid TEC 140 apochromatic triplet. How about cool down time? For the C925 I have a Lymax CAT Cooler, for the C11 Edge it is TEMP-est vent fans from Deep Sky Products; cooling is basically a non-issue if one takes active measures. Collimation? A piece of cake once you develop the feel for it, a task best learned by doing…and my scopes keep collimation very well. I am all around pleased with the performance of my SCTs and find them very comfortable to use. And the C11 Edge gives me pinpoint stars from edge-to-edge regardless of what eye piece or camera I use.
On a final note, my SCTs live side-by-side with a small herd of refractors: a TEC 140, SW100ED Pro, ES ED80, William Optics Zenithstar 66SD, and an AT65EDQ…and they all get along just fine. Here at Beaglehaven Observatory we are living a very peaceful multi-cultural existence
Posted 18 July 2014 - 10:48 AM
Posted 18 July 2014 - 11:00 AM
Thanks for your testimony. I can see the great attraction of the larger compound telescope. They are undoubtedly a winning design as evidenced by the vibrant culture they generate.
But if folding the light path works so well with the Cassegrain, it ought to do well with a large aperture achromatic objective too. You could get a nice f/15 system into a more much convenient package. Why doesn't the market offer these?
Posted 18 July 2014 - 11:02 AM
Thanks Scott. I still don't understand. So is an f/10 Cassegrain a true f/10 system? There is no field curvature in a f/10 achromat.
Yes, but its made of up F/2 optics. The design of the system is different than a refractor so an F/10 in one is not going to behave the same as an F/10 in the other, although an eyepiece or camera will function similarly in both. Bottom line though, a catadioptric is not a pure reflector and not a pure refractor, it has aspects of both; thus, its performance characteristics aren't exactly comparable.
Posted 18 July 2014 - 11:25 AM
I think many folks balk at the proposition of commercial folded achromats because they introduce a mirror(s?) into the light path. But others do purchase or make these folded scopes. One of the big attractions of the refractor is the lack of mirrors "in the tube", and I make this distinction because very few people think twice about the mirror or prism diagonal in their optical path! Folded refractors are not as long, but they are bulky...this solution compromises the elegance and eye appeal of a classical refractor, no doubt ruining the aesthetic for some.
Unquestionably, a folded design can help counter chromatic aberration, and maybe at a lower cost than an apochromat. But does it have the appeal of the traditional form? That is a personal decision. For my part, I prefer a more compact, albeit expensive, and traditional looking solution to dealing with unwanted color and other issues in my refractors, and that is why I don't own a long achromat today. Besides, I have my very affordable SCT big guns to provide all the focal length and color correction I want for visual and imaging work. I use them for what they do best, and the refractors for the tasks at which they excel.
Posted 18 July 2014 - 11:49 AM
Good points indeed. I understand. My Maksutov uses lenses and mirrors to fold the light and create images that are incredibly refractor like. On fine nights, the only difference I can discern between her and my 5" f/12 achromat is the intensity of the diffraction ring, which is brighter than those delivered by the refractor.
Looks count for something I suppose and in that regard the classical refractor owns the field. But appearance is a trivial thing and in the end what counts is what the telescope delivers.
The people lust over large refractors but soon discover they are made from unobtanium. They are simply beyond the means of the vast majority. The large colour free refractors are ridiculously expensive for what they deliver and require if not equally expensive mounts.
Designing a nice folded refractor with the same attention to detail as a cassegrain would be a game changer; the people would soon forget about how they look and delight in the beautiful images they could serve up. These could be offered at a much lower price than the current options.
In my opinion, the 'apochromat revolution' never happened unlike the Dobsonian revolution.
Bring the people a large, nicely made folded refractor and a true revolution can begin.
Posted 18 July 2014 - 12:43 PM
Don't forget, also, some of the wonderful off-axis reflecting designs that offer very good correction, such as the chief/scheif - spiegler (apologies if I miss-spelled). Some of these deserve to be "game changers", but remain a niche interest.
Of course, we ought to put some numbers into this, just to see what can be achieved. A folded refractor with 2 mirrors has about 1/3 the tube length. for example 4" f/15 is reduced from ~60" to ~20", ignoring dewshields, diagonals etc.
Celestron, of course have thought about this in a different way with the Edge range - include field flattening and correction in the SCT design. Admittedly, this is largely aimed at the imaging market, but can also benefit the visual observer, who - in terms of aberrations at least - gets a more refractor-like telescope.
Posted 18 July 2014 - 01:13 PM
I have no experience with these other designs but no doubt they do have their merits.
The technology is there; all that's required is the will to execute it. A big company like Synta would have the resources to design something interesting. Refractor aficionados seem to be fixated on 'iconic' designs. But as my Maksutov is teaching me; you'll soon get over what a 'scope looks like when the other scope delivers the readies.
I remember reading an article in Sky & Tel some years ago where some guy hobbled together a folded refractor with a 5 inch D&G lens on a shoestring budget. It didn't look nice, admittedly, but it was very portable and he wasn't disappointed with the views.
Posted 18 July 2014 - 03:17 PM
Similarly inexperienced myself..... and haven't looked at or through the Edge scopes, or an apochromat for that matter. Thank goodness - I'd hate to get expensive tastes
What I must do, is put my 5" f/12 Mak (~118mm actual aperture plus obstruction) up against my 102mm (really ~97mm) f/10 achromat I really think this would be close, and interesting - both are very good to excellent optically.
(As a side note, why do I always get less aperture than it said on the instrument?)
Posted 18 July 2014 - 03:40 PM
But if you want to know, I put my 127Mak up against my 1/10th wave 6"F5 Newt during some very good seeing last week and the difference was so slight you really had to look for it. The Newt was a bit brighter of course and had a bit more resolution, but you really had to look hard. They both showed colour very well and the detail was pretty much the same. But I find that the Mak needs good seeing to perform well and falls a bit behind the Newt when the seeing is average (this was first pointed out to me by a fellow CN'er and I also noticed it). The Newt also has better thermal control. But the little Mak is an amazing performer and definitely a keeper. Amazing optics in these little packages.