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Post by Viktor Z. on Nov 30, 2013 22:22:23 GMT -5
I have a beautiful 6" Istar anastagmatic refractor that is custom built into a Meade tube. Now, I am already thinking about my next ptoject to build a larger refractor. My dilemmas are the length of the scope and the objective diameter chioces, where I would like to get your advise.
My considerations:
- a couple of tenth of limiting magnitude difference between the diameters of 204, 228mm and 250 mm's are significant or not when observing deep sky objects?
- i am considering a focal length that still allow a limited portability of the scope, but at the same time the refractor allows enough magnification without significant colouring on bright objects? I am not sure how f5, f6, f7.2 or f7.7 compares regarding Istar lenses? I have no complains about my 6" f8 R30, it is an awesome optics, but what about shorter focal ratios?
Thank you in advance for any advise.
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Post by mikey cee on Dec 1, 2013 2:19:32 GMT -5
Hi Viktor. Well I built a 250mm f/11 R30. It weighs a whopping 32 pounds or 14.5 kg's. That includes about a 1.5 pound countrcell that I had to have made. They can't make them in the "slim" glass formulas.  Mike |
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Post by Viktor Z. on Dec 1, 2013 17:10:41 GMT -5
Mikey, I completely forgot about the weight factor. Indeed yours is a heavy duty killer optics and has a considerably heavy weight, as well...I am jealous. But seriously, compared to your previous 8" Refractor, if I remenber you had one, what is the biggest difference you notice? Magnitude? Brightness? Details? Resolution?
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Post by mikey cee on Dec 1, 2013 18:58:14 GMT -5
Viktor the biggest surprize was how small the airy discs seemed versus the 8 incher. I live in a city of around 600,000 locals and the sky is a mag 5 area. It's the globulars and nebulae that stand out better. Can I see more stars in clusters? I mean I can see some fainter ones easier but really not that many more individual stars if you get my drift. On a 9+ night with low humidity Jupiter looks exactly like a "faded out" processed image thru a C14. Of course imagery doesn't mean you can see the same stuff visually with a C14 either.  Mike |
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Post by Mike on Dec 2, 2013 14:44:38 GMT -5
Viktor,
The big surprise for me was the increased brightness of DSOs. After testing so many 150mm scopes I sold a 204mm and when I tested it I was amazed at the greater detail. I have a list of objects that I regularly look at for testing and they all looked much brighter. I know what you may be thinking. Duh of course 50 more mm is going to be a big improvement. But it was still a delightful surprise at what an 8 inch piece of glass will do. I wish I could look through Mikey's sometime.
Mike
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Post by Viktor Z. on Dec 3, 2013 2:46:00 GMT -5
Very interesting surprises. Both the increased brightness and the better controlled optics are great features in means of quality and aperture. I made a simple calculation and as per surface area: the 204 mm lens has 32685 mm2 and the 152 mm lens (that I currently have), 18145 mm2, so the difference is about 44%, if my poor math is right. In case of the 228mm lens, this difference is 55%, and for the 250 mm lens it is 63%. It means that a 204 mm lens almost doubles up my 152 mm refractor. Dawes limits (resolving power): 152 mm: 0.76", 204 mm: 0.56", 228 mm: 0.5", 250 mm: 0.46", of course if sky conditions permit. No wonder why the DSO's are brighter and the details are greater. I indeed find that the 204 or 228 mm optics would be a great choice! I would still prefer the R30/35 version for better light control. Please let me know if I am wrong or if I should take other things to consideration, too?
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Post by Viktor Z. on Dec 3, 2013 17:44:35 GMT -5
I am loudly thinking:
1. TCR 204 f8 R35: pros: great optics, light weight, no need expensive heavy mount, easy handling
Portable
cons:
length
Less FOV than an f5 or f6 scope
getting use to unconventional design?
2. Phoenix WFT 204 f6 :
Pros:
wider FOV,
Shorter, easier to handle
Cons: not an all-around scope
Not sure about Weight? Mount capacity required?
3. 228 f7.7 objective
Pros: aperture advantage
Cons: Less likely. need to build the whole scope from scratch: budget restrictions
Big and heavy scope, less portable.
Requires a large mount
4. 250mm f7.2
Same as the 227 mm and even if a Dream++scope, need a permanent setup most likely.
Please share if you would like to add anything to this list?
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Post by mikey cee on Dec 3, 2013 19:00:51 GMT -5
Vik......35 years ago I purchased a Jaegers 6" F/8 and have never regretted it. Their F/5 would have been limited to a more colorful low powered view.....very restrictive no options with double stars or planets etc. Their F/10 and F/15 no real portabilty and no real low wide power capabilities. Regardless of the aperture don't hem yourself in with no options. Go with the F/8. What's portability going to be worth when you'll be constantly wondering would that 9" have been all that more restrictive. Your buying this thing to view with not practice lugging something in and out, back and forth with the least amount of effort. Everything has a downside but I'd err on the side of larger rather than smaller. No pain no gain. Addressing the cost factor that's easy. You'll only get to save a buck(euro) just once but the mistake will last your viewing lifetime. Me I'd go with a 9" F/8 or F/9 seeing how you're not interested in a permanent set up. If you were then a 9" F/11 minimum. I love the R30 flavoring.....it sure does taste sweet! Mike |
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Post by Viktor Z. on Dec 4, 2013 15:32:10 GMT -5
Absolutely, the. R30/35 lens has some very sweet spots, literally:) in focus and out... I am almost convinced to aim for a little more saving and beg from Istar for a 227 mm R35/R30 f8 or so TCR;) indeed, that would be the greatest equipment in my life but I would not be disappointed with a slightly smaller lens neither. The inner image of a refractor starts to form shape...
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jpb30
Full Member
Posts: 84
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Post by jpb30 on Dec 23, 2013 1:52:57 GMT -5
Hello Viktor
An element very important to consider with the big refractors it's the equatorial Mount!!! For my 288mm F/9, the Astro-Physics AP1200 GTO is really just!!not for the weight ( 25 kg), but for the length of the tube, if you want a good stability for visual or astrophotography you must have a heavy équatorial mount!!! they are very expensive
JP
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Post by Viktor Z. on Dec 26, 2013 1:24:10 GMT -5
Hi JP,
Thank you for the great advise! Indeed, I have been thinking a lot about that I really need a reliable and great mount first so it is no longer an issue. The AP1200 is wonderful, and you have the point that a smaller. mount may limit my usable focal length of my next big refractor. However, I found that an AP1200 is a bit heavy for me if I want a portable setup. Forr this reason, I did a big step forward and I secured an AP 900 mount. I am pretty sure that its load capacity is more than enough to support a modarately heavy and parge diameter lens Istar refractor and will easily handle my TMB 4" for photographic purposes. The inner picture start to form reality, thanks to your helps. The final result surely be a satisfaction, even if it is different from a big picture dream. Anybody has experience what is the biggest Istar refractor that an AP900 can handle? In this regard I have two options in mind:
1. What is the biggest lens diameter / focal length for visual?
2. Same question for photographic use?
Thank you again your inputs, in advance.
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Post by Viktor Z. on Dec 26, 2013 1:25:22 GMT -5
Sorry for the typo errors...
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jpb30
Full Member
Posts: 84
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Post by jpb30 on Dec 26, 2013 1:51:59 GMT -5
Hello Viktor
The AP900 is a very good Mount!!! Easily transportable
As I said it, it is not the weight the problem, but the length of the optical tube and its imbalance!!!!
For the AP900 I think that you should not exceed 150-160 cms in length of tube Max
Asteria TCR 204-8 R35 seems a good choice, but can be limited maybe in astrophotography by the short F/D and chromatic aberations!!!
JP
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Post by Viktor Z. on Jan 28, 2014 7:33:56 GMT -5
Woww, my 150 mm Istar refractor just looks great on my AP900 mount! As if they were made for each other! I have never experienced such a smoothness and stability of a mount and my refractor was ease and fun to use from the light pollured balcony. Closing in on Jupiter and bright DSO's such as Orion nebulae produced usual high quality images of an Istar refractor! Can't wait to drive out to a dark site! The mount was indeed very expensive, but the refractor building project of a 6" is now complete and will take time and use it and having fun observing with it until I am ready for a "big brother light bucket refractor" project. Oops, sorry: now I need a cool observing chair:)
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Post by Mike on Jan 28, 2014 12:30:03 GMT -5
Viktor, after I send you the Istar logo decals would you take a good picture that shows the Istar name and a good angle that shows the AP900 mount please? Then email it to me.
Mike
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Mike
Mike
