Reflector Vs. Refractor Vs. SCT

Hi from the not-quite-so-cloudy UK!

Limiting mag depends on so many variables (sorry!). I use a 14" dob, and the limiting mag relies heavily on sky conditions. I am in a small village with no light pollution, and my faintest mag so far is 16.5, but usually I get to about 14.5-15. I also used to have a 6" refractor and got to about 13.5 with that, so in terms of faintness, go for the dob, despite the ancillary problems like realuminising every 5 years or so. You do need to know the sky very well for this, since you will be finding everything 'manually'. But getting to know the sky is a profound pleasure, not a job, so good luck with that!

Also, certain parts of the sky seem to give better results, probably due to the background. For example, the fields around EM Cyg and TZ Per look much darker (and thus better contrast) than, say, R Leo which is a long way from the galactic plane where the dark nebulae are. Add to that the possibility of background illumination (faint, but 'there' nonetheless) from the highly-plentiful external galaxies in these areas

I'd say that, unless you are doing CCD stuff, anything near 8" aperture is out. Simply not enough light-grasp. I constantly dream of a 20" scope!

...

Hello Ivan,

I'm pleased to learn that you consider yourself ready to move up in instrumentation in your pursuit of variable star observing. Certainly more visual observers are badly needed these days. I started out more than 50 years ago myself and my interest has never waned one iota. Over the course of those decades I have owned and employed just about every aperture of instrument in my VSO, ranging from wide field 10x50 binoculars to a 20" f/5 Dobsonian. In doing so, I've learned a great deal about what works best for observing convenience as well as VSO productivity.

First off, I think that it might be beneficial to you to point out that the step up from an 80mm refractor to a 12.5" reflector is truly an enormous one. Such a sudden increase in aperture and its resulting magnitude threshold gain can easily prove frustrating when it comes to even locating VS fields that you are familiar with from working with the 80mm. So many fainter new field stars will be included in the view, particularly anywhere near the Milky Way where most of the more interesting variables are located, that you may quickly become utterly lost in the star field while attempting to identify the variable and its comparison stars. This is particularly true if you're away from home observing site happens to enjoy relatively dark skies. A very good knowledge of heavens, one more often available to the traditional observer of long experience, is a virtual necessity to quickly finding your way from field to field with a big scope.

A very important second point to consider is how easily your instrument can be handled. One's ability to cover more than a few variable stars per session is inversely proportional to the aperture of your telescope. When I was heavily engage in observing binocular variables decades ago I had no difficulty estimating 100 different stars with accuracy in the course of just an hour of 2. My very portable 8" RFT Newtonian could still manage nearly 50 estimates per hour, But once I had my 12.5" in a permanent backyard observatory - even with all my charts and sequences memorized - I rarely could cover more than perhaps 35-40 stars in the course of an hour. By the time I up-graded to my 20" Dobsonian, although I could go very faint indeed with it, I had difficulty observing more than a dozen stars during a one hour observing session in spite of have a great familiarity with the fields. In short order I pretty much returned to observing with the old workhorse 12.5".

Now I would also make the point that observing from one's own backyard is far far better than hauling out your scope, stuffing it into the car, driving out to a better site to set up and after the session doing it all in reverse. This inconvenience becomes progressively worse as the size of the telescope increases and eventually leads to one re-considering about doing it on a clear night. I've personally found that, as a young man, an 8" Newtonian is very easy to manage for away from home observing sessions, while a 10" often tends to grow annoying to man-handle on any regular basis. A 12.5", unless it's one of those of modern light weight skeletal designs, is just a bear to manage and its remote used after the initial joy of it wanes quickly goes to making a remote observing session a task rather than enjoyable.

Just some observations from one of today's AAVSO Old Guard. Good luck whatever your ultimate choice may be.

J.Bortle   (BRJ)     

Hello Ivan,

I'm pleased to learn that you consider yourself ready to move up in instrumentation in your pursuit of variable star observing. Certainly more visual observers are badly needed these days. I started out more than 50 years ago myself and my interest has never waned one iota. Over the course of those decades I have owned and employed just about every aperture of instrument in my VSO, ranging from wide field 10x50 binoculars to a 20" f/5 Dobsonian. In doing so, I've learned a great deal about what works best for observing convenience as well as VSO productivity.

First off, I think that it might be beneficial to you to point out that the step up from an 80mm refractor to a 12.5" reflector is truly an enormous one. Such a sudden increase in aperture and its resulting magnitude threshold gain can easily prove frustrating when it comes to even locating VS fields that you are familiar with from working with the 80mm. So many fainter new field stars will be included in the view, particularly anywhere near the Milky Way where most of the more interesting variables are located, that you may quickly become utterly lost in the star field while attempting to identify the variable and its comparison stars. This is particularly true if you're away from home observing site happens to enjoy relatively dark skies. A very good knowledge of heavens, one more often available to the traditional observer of long experience, is a virtual necessity to quickly finding your way from field to field with a big scope.

A very important second point to consider is how easily your instrument can be handled. One's ability to cover more than a few variable stars per session is inversely proportional to the aperture of your telescope. When I was heavily engage in observing binocular variables decades ago I had no difficulty estimating 100 different stars with accuracy in the course of just an hour of 2. My very portable 8" RFT Newtonian could still manage nearly 50 estimates per hour, But once I had my 12.5" in a permanent backyard observatory - even with all my charts and sequences memorized - I rarely could cover more than perhaps 35-40 stars in the course of an hour. By the time I up-graded to my 20" Dobsonian, although I could go very faint indeed with it, I had difficulty observing more than a dozen stars during a one hour observing session in spite of have a great familiarity with the fields. In short order I pretty much returned to observing with the old workhorse 12.5".

Now I would also make the point that observing from one's own backyard is far far better than hauling out your scope, stuffing it into the car, driving out to a better site to set up and after the session doing it all in reverse. This inconvenience becomes progressively worse as the size of the telescope increases and eventually leads to one re-considering about doing it on a clear night. I've personally found that, as a young man, an 8" Newtonian is very easy to manage for away from home observing sessions, while a 10" often tends to grow annoying to man-handle on any regular basis. A 12.5", unless it's one of those of modern light weight skeletal designs, is just a bear to manage and its remote used after the initial joy of it wanes quickly goes to making a remote observing session a task rather than enjoyable.

Just some observations from one of today's AAVSO Old Guard. Good luck whatever your ultimate choice may be.

J.Bortle   (BRJ)     

I have made many VS estimates with a wide variety of instruments, from 6x30 binoculars to a 36-inch Cassegrain. Each scope was selected at the time for the star to be observed. Good work can be done with any aperture, as long as you select your target stars appropriately.

John Bortle is absolutely right, productivity is inversely proportional to aperture - the bigger the scope, the fewer stars per hour. Also, portability is another drag on frequency and productivity. I solved that problem by gaining use of two observatories in addition to my convenient home instruments. I can simply drive to either (fairly dark) site, hauling nothing larger or heavier than a key. Then I can go to work with 20x100 mounted binoculars, 7-inch refractor, 12-in Dob, 14-in and 16-in SC, or 36-in Cass (there's also a 28-in Dob, but I have never used it because it is very difficult for one person to start it up and requires a lot of climbing). I have also occasionally taken home two 10-inch Dobs that are loaner scopes from one of the clubs, and other apertures are available. You may not be so fortunate.

At any rate, I would strongly recommend a 12-in Dob over a 6-in refractor. I have reached as faint as 13.6 with a 5-in f/12 refractor, and I have done a lot of PEP with the 7-in refractor, but both are permanently mounted in a roll-off shed and a dome. (The 7-in even has a motorized pier, just push a button and the scope rises or descends to bring the eyepience to a convenient level). Except for very short-focus refractors, larger apertures and heavy, cumbersome to set up, and the eyepiece height to reach a particular field may be anywhere from knee-level to over your head.

My experience with 12-in and 13-in f/5 Dobs has been very satisfactory. The eyepiece may be close to the ground when you are observing down at 10-degree altitudes, but otherwide they are easy to point around the sky and don't require a ladder or step stool near the zenith.  An 8-in Dob is even easier to move and set up, but in my experience it is almost too small - The eyepiece is seldom as high as you waist, so buy a set of knee pads if you choose to acquire an 8-in.

As for limiting magnitude, that's a question of magnification and therefore eyepieces. A high-quality eyepiece is always worth some additional investment. The only problem with very high magnification is the narrow field of view and the Earth's rotation. The narrow field becomes a problem without a clock drive, since you have to constantly nudge the scope along to keep the field in view. Except for special occasions when you are greedy to observe a very faint star, I would plan to use a Dob at low to medium power; even then, there are more variables available than you could ever observe.

Clint Ford once told me that "Your telescope should be located no further than 50 feet (15 meters) from your bed." I'm sure he had one there, too. Of course, he told me this when he was out at his "cabin" in Wrightwood, California - on the west coast of the US when I visited him. Chas. Scovil was also there. They both lived on the east coast of the continent!.

We were there to use his 18 inch telescope he had in a dome about 4 miles out of town. We started with a short, windy night. The next day there was snow - a lot of snow. That kept us from even leaving his house. I had brought a portable 17 inch f/4.5 Newtonian reflector that I set up in his driveway and used for the night. Clint looked thru it and approved. The road never was drivable to his observatory, and the rest of the time, it was cloudy, as best as I recall.

Like you, Ivan, I have significant light pollution. I combat this by building large diameter telescopes. When I was doing visual observing, I'd use higher power eyepieces. Now, I do only CCD observing and get down to 19th magnitude stars with my 74 cm (29 inch) reflector which I have changed to a prime focus instrument.

Lew

In the backyard I use a 80mm refractor or 4.5" reflector on a good mount. But nothing is better than putting them all ( including a 12" dob and binoculars) into the back of the 67 El Camino and going out to the woods!

Duane

Well, even if I too own a 8 f/5 refractor, it's more for Sun monitoring and my favourite workhorse is a venerable 23 years old binoculars 15 x 70. If you want go deeper than a 80 f/5 refractor, you have to evaluate what is your available storage, what telescope size your observing site allows, what variable stars are your favourite target.

I would drop the SC option. Compact, portable, but expensive and with a mere 0.8 degree field of view, unless you introduce another expensive focal reducer. But it's an excellent, overall general purpose telescope, so if you want have if for observing targets different from variable stars only, that's ok. 12" GSO dobson it's fine to follow Mira full cycle, even their faintest minima, monitor quiescent former or dwarf novae and a good share of cataclismic variables. But it's big and heavy and if you don't have a good dark sky nearby it's not easy to carry around:  a good 200 mm f/6 Dob is a compromise betewen limit magnitude and portability and its shorter 1200 mm focal length gives you a larger and well definied field of view nearly to the edge, compared with 300 mm f/5 Dob: coma correctors and variable star estimations does not work very well along together. Gso, Orion XT8, Skywatcher, it does not matter, I had XT8 and GSO and both of them have good and weak spots, but overall they are all fine. A 6" refractor ? Yes, but why ? Modern acromats do not suffer too much coma and false color troubles, but a humble 150 mm f/5 refractor is in the 5.5 - 6.1 kg range weight, compared with a 9 - 10 kg for a 150 mm refractor and even with a 32 mm / 1.25" eyepiece gets a respectable 2 1/3 field of view, with no false colors and not too much coma. And it costs 1/3 of a 150 mm refractor, asking only a decent alt-azimuth mount or a small equatorial to work. Moreover, GSO has a 150 mm f/6 Newton too ( see European distribution ) with a 2" focuser and a perfectly matched 50 mm obstruction. Eager to swap my used XT6 f/5 for this GSO f/6, as soon it worns out definitively.

First of all I would like to thank everyone for the time you took and shared on this topic. For me as a mere beginner in VSO this is a great and valuable source of knowledge and enthusiasm. Special thanks to the most senior VS observers John Bortle and Lew Cook for their advice. Over 200,000 reported observations, that is breathtaking, legendary. It got me thinking and reevaluating my goals in VSO. 

To be honest, I did not think about quantity and as John described, productivity drops inversely proportional to the aperture of the telescope. How is it even possible to do approx. 50 estimates per hour with an 8" Dobsonian? That's like one VS per minute. I mean I am a rookie, no doubt, but I can't even theoretically imagine it. Just to look at the next map, point the scope, look through the eyepiece, focus, find the VS, find the comp. stars, make the estimate and write it down, just in 60 seconds?! How? I can imagine that long year practice and memorizing the star fields will do the trick, but still it's super fast. 

Then I had to ask myself, what would be the goal I should set for my observing sessions? Productivity, because there is a lack of VS observers? On the other hand, doesn't the accuracy of the estimates suffer if I push myself to observe more and more VS per hour? Or is there maybe a lack of reported observations for the fainter VS? Then one needs a bigger scope, but the productivity drops. 

So there is naturally my new question. If there is a lack in VS observers, there has to be also a lack of reported observations. Where would you engage new observers if you could decide?  

Hello John, maybe it's time to tell which variables you wish that should be observed more. I'm have done now more than 30.000 visual estimations of mostly mirastars. I stopped observing a few years ago but I want to start again. I'm just looking for a scope again and a new program. Maybe you can send me on the right track.

I am using a CCD on my 11" SCT. Even with the light pollution from being in the center of a town with 25,000 people I get 17th magnitude stars and can do photometry on stars around 15-16 magnitude. My AR102 does 16th magnitude stars. So to defeat light pollution and to keep from having to keep buying bigger and bigger telescopes you can go to CCD imaging. However, there is something still magical about doing visual estimates and it is much cheaper! 

Greetings from once again rainy Croatia!

I am waiting eagerly to test my newly acquired 10" GSO f/5 dobson and want to use the time to thank you all for the input and advice. It has been a great help and encouragement. Initially I was thinking of buying a 12" dobson, but in the end decided that a 10" would be somewhat better for a new VS observer like myself.  It's much more easier to pack it in the back of the car and haul it to the dark site. Also the shorter focal length (1250 mm) allows for a decent FOV.

Attached are a few pics of the new scope. The only upgrade I've done so far was a new 8x50 right-angled finderscope, so I can still observe VS from the binocular list.