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Hi Bob,
I will be away for a few days so any further reply from me will have to wait until after the weekend.
Regarding OTA (Optical Tube Assembly) and other acronyms, I know this can be a bloody nuisance. You are reading an article and come across one and then have to sift back through it to see the first time it was used!
Anyway concerning your question. Bresser is a distinct and different manufacturer from the people (Synta company) who make ‘Skywatcher’ and ‘Celestron’. (Bresser may be Taiwanese, but I am not sure – You could google it.) The problem comes in that brackets and fittings are probably not interchangable. You see for the 150mm Celestron, I attached a ‘Skywatcher’ mounting bar (plus rings), to mount the 80mm guide/finder. Parts are identical.
The Chinese moderately priced stuff is quite good.
Now for my SCT’s I was at first worried about the 30% plus obstruction by the secondary. Theoretically it does drop some contrast. In a practical sense for the F10 or F11 units, you would notice very little (if any difference), from a refractor. Of course out of focus images are a donut. Also, your SCT would be 127mm, 150mm or even (I would recommend) 200mm, which still weights in at around 5kg just like a 120mm refractor. At the loss of a small amount of image quality you have extra lightgrasp and resolution and more convenience in use – and no spurious colour.
Still the refractors, being traditional, have allure, and the big ones in my early days were very expensive – so I wanted one. (The dreaded ‘I wannas’.) I have one and it is nice but….
Late last year I bought a 120mm F1000 mm Skywatcher OTA and matched it to an EQ5 mount and added an RA drive. (I had always wanted a big refractor.) then, lo and behold, within months the local agent was selling 150mm 1200mm Celestron OTA.s for half price. I mated this with an old superseded Celestron DX mount which I got for 43% of the price that the same mount cost four years ago for my C14.
I managed to sell the 120mm for only a modest loss.
Now, my assessment.: Chromatic aberration is a problem with achromats. The way to largely avoid it is either have a focal ratio with a CA ratio greater than 3.0 or go to an ED glass version and pay more than twice the price. The CA ratio is determined by dividing in the focal ration by the aperture in inches. Thus for a 102mm (4 inches) to reach 3.0 it would have to be F12.
Okay, so I have had a 120mm at F8.3 and own a 150mm at F8. What have I found when using them?
Colour fringes resulted, bordered by a lovely pastel yellow green shade to the limbs of bright objects like the Moon and Jupiter not to forget a violet haze on these bright objects. The purists and ‘pretty picture’ imagers, of course, decamped to purchase ED glass versions, even triplet lenses, at costs starting several times greater for the equivalent sized optical tube assembly. A further factor for the 6” or 150mm (5.9”) refractors is that with the heavy objective, the tube becomes top heavy, so there is considerably more of it is below the point of balance than above. This means an extra high mount for comfortable viewing unless you propose to observe higher objects from a sitting or lying on the floor position. In my view the mount must be high enough to allow a minimum viewing height at vertical of (say) 80cm, namely around 10cm above standard table/desk height. It must also be sturdy enough to support the ~10kg weight and any mechanical issues resulting from a longer tube.
Okay, a 150mm F8 lens (which should be at least F18 by the formula), is an extreme example. A minus violet filter will clean up the image, taking a loss of light and in my case I have a yellow filter that does much the same thing and improves contrast but gives this a dirty yellow caste. What I am saying is to use these filters on bright objects when seeking contrast and removal of spurious blue/violet, but remove the filters when observing deep sky.
For many years I have had reflectors (up to 16″), then Schmidt Cassegrains (to 14″) and have added the large 150mm refractor. My assessment of the visual tightness of the star image (admiitedly possibly unconsciously influenced by ‘seeing’) is that if reflector images are a 6, then SCT’s are an 8.5, Maksutovs a 9, and refractors 9.5. (No optics are perfect.)
Then we are looking at the weight. In Skywatcher terms, a 100mm OTA will perhaps weigh 3kg, and be okay on an EQ3, a 120mm (5kg+) will need an EQ5 (not much heavier mount), but a 150mm is around 10kg and needs much more.
I sold my 120mm to an older fellow and the whole setup probably was not more than about 15 to 16kg.
Now, though refractors look nice and classical and are wonderful conversation pieces standing in the corner of the lounge room, they are awkward and bulky, particularly to negotiate up stairs. How about a nice short SCT or Maksutov, which is much more convenient to use?
From a quick search on the internet, probably the cheapest reasonable unit (around USD$80), is a Meade 56mm Plossl with 52deg field in a 2″barrel unit. So if you crave a long focal length eyepiece for your F10 SCT, then this should do it with an exit pupil of 5.6mm.
Alternatively you could purchase a screw on focal reducer and field flattener. (I have one that lives on the C11 for photography.) This reduces the focal ratio from F10 to F6.3. and means you would get the same magnification and exit pupil with a 35mm eyepiece and so the standard 32mm Plossl in a 1.25″ tube would still provide a 50 degree field of view and give a large 5.08mm exit pupil. It is swings and roundabouts.
Personally I have 50mm, 60mm and 70mm eyepieces. All are 2 inch size because otherwise the field of view would be very restricted. Very simply for standard SCT’s at F10, the 50mm, 60mm and 70mm translate to 5,6,and 7mm exit pupil, the last mentioned being as large as the (young) human eye can accommodate. Now, for the simple reason that most eyepieces tubes are 1.25″, a 40mm is still capable of producing a field width in the low 40’s (degrees) and any longer focal lengths need the 2″ tube.
Other factors are the cost and weight of these larger, longer focal length units, though some might be cheaper (say) if they are simple 3 element Kellners.
The old visual professional refractors ( say think of the 24″ at Lowell Observatory and its ilk), had some quite long focal length eyepieces in regular use – longer than I have quoted above. But they were special.
For myself, and perhaps I am lazy, I usually just use the standard 1.25″ eyepieces and a 32mm focal length is about as far as you can go with a 50 degree field. On the C14 when I observe my lunar occultations, I use a 25mm with a very pleasant 60 degree wide field.
