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Post by Deleted on Jan 2, 2014 23:51:17 GMT
Has anybody used either a Startravel 120mm or 150mm refractor for VA. They are both reasonably fast at f5 and can be made faster with a reducer. They are also pretty cheap although they will suffer from a bit of CA. I love refractors, and have various scopes from 150mm to 80mm, but they are not as fast as I would like. My ED80 is f6.25 and can be used with a reducer but then image scale gets quite small. What I want is more aperture, a bit more focal length and faster. The Star travel scopes have these attributes, but are they usable for VA or would they dissapoint.
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Post by Deleted on Jan 3, 2014 0:11:05 GMT
Has anybody used either a Startravel 120mm or 150mm refractor for VA. They are both reasonably fast at f5 and can be made faster with a reducer. They are also pretty cheap although they will suffer from a bit of CA. I love refractors, and have various scopes from 150mm to 80mm, but they are not as fast as I would like. My ED80 is f6.25 and can be used with a reducer but then image scale gets quite small. What I want is more aperture, a bit more focal length and faster. The Star travel scopes have these attributes, but are they usable for VA or would they dissapoint. This is an interesting question for me as I'm also looking a decent fast refractor. I did look at the Startravel scopes as the prices are very good but then moved onto the Equinox. I liked the idea of using the Equinox ED80 as a video finder to help when using the C11 but also to use on larger targets. For this reason I felt the Startravel may not be good enough when used as my main scope for larger targets. I can not offer advice but will watch this post with much interest Regards Karl
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Post by Deleted on Jan 3, 2014 4:49:59 GMT
Sorry not much help to you as ive never used/owed any of them. But im sure Davy had a 102mm startravel and said the CA was brutal. Im yet to hear a bad word about the ed80... so im guessing to improve what you have you will need to go for a ED 100mm+ scope costing a fortune!? As i wouldnt think a 120 normal v's a 80ed would even be a fair fight I only have a 70mm cheapo refractor and the CA is very noticeable on bright things! Like Karl i really want a wide field ED/APO refractor for the large objects that wont fit in the FOV of a SCT! Just un-decided on what one!! I do keep thinking tho that i would be buying a scope just for 2 objects... m31 and m42 lol
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Post by Dragon Man on Jan 3, 2014 8:34:43 GMT
Yes, I have the 150mm (6") f5 Startravel Skywatcher. And I have the rare 120mm f5 Saxon version. On the 150mm (6") I fitted an AstroPhysics 2.7 Focuser. Allan, what you want is actually a contradiction: "What I want is more aperture, a bit more focal length and faster". More focal length will make it slower. I have many types of scopes to use but I am mainly a refractor user for Video. In refractors I have an ED80, an ED100, a 120mm f5 Achromat, and a 6" f5 Achromat, and the ED80 kills the both Achro's and the ED100. Here is a sample image I made of the difference in FOV and quality of image between my ED80 and the 6" f5 Achromat. Notice how sharper the ED80 is, and the extra amount of detail visible (same camera and Focal reducer): Attachments:
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Post by davy on Jan 3, 2014 9:13:23 GMT
Allan. .if you have an ed80 dont waste your cash..on a startravel scope as you say they are cheap and for a reason, dont get me wrong it's a good grab n go scope for visual use but for ap on bright object like the moon ca is brutal on deepsky with va I don't know how it would go...what about yhe quattro series grom skywatcher its an astrograph shorter focal lenght but very fast..ok you will hear its for advanced users ect on sgl nut a review on flo says that's not quite accurate. If I had a bigger mount I would go for that...8" for £450 approx ..think you get a ca corrector for it too.
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Post by Deleted on Jan 3, 2014 13:14:02 GMT
Thanks for the guidance fellas. I really want to stay with fracs, I am just looking for companion scope for the Equinox 80. The ST150 would give more aperture, more FL, and be faster, but at a loss of image quality. I'll have to give this more thought.
The SW 120mm ED pro has always been on the wish list, but funds won't stretch to that and another cam for VA at the moment.
So many choices, so little money.
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Post by Dragon Man on Jan 3, 2014 13:24:58 GMT
Allan, to give you an idea of how hard it can be to find a scope that gives sharp quality views, fast, I tried many many scopes, and yet I always seem to come back to my ED80 because it gives such a better view in the camera than all the others. And believe me, I've tried a few Here's some of them: Attachments:
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Post by Deleted on Jan 3, 2014 23:05:46 GMT
Yes, I have the 150mm (6") f5 Startravel Skywatcher. And I have the rare 120mm f5 Saxon version. On the 150mm (6") I fitted an AstroPhysics 2.7 Focuser. Allan, what you want is actually a contradiction: "What I want is more aperture, a bit more focal length and faster". More focal length will make it slower. I have many types of scopes to use but I am mainly a refractor user for Video. In refractors I have an ED80, an ED100, a 120mm f5 Achromat, and a 6" f5 Achromat, and the ED80 kills the both Achro's and the ED100. Here is a sample image I made of the difference in FOV and quality of image between my ED80 and the 6" f5 Achromat. Notice how sharper the ED80 is, and the extra amount of detail visible (same camera and Focal reducer): Nice one Ken, great example and I think it's the ED80 for me! However, what I can't get me head around (numpty question time) is that you have you have used the same reducer on an F5 scope and an F7.5 but the F7.5 gives a wider FOV If I use my C11 with the MF5 I am assuming this will reduce my scope to F5 but would this give for example the same FOV as your 6" F5 achro? Cheers Karl
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Post by Dragon Man on Jan 4, 2014 12:16:11 GMT
Yes Karl, the f7.5 gives a wider FOV then the f5 because of Focal length. The shorter the focal length the wider the FOV. Even though the ED80 is a slower scope the difference in focal length is greater than the difference in focal ratio. The ED80 is only 600mm focal length whereas the 6" Achro is 750mm focal length. The difference in 150mm focal length is greater than the difference of 2.5 f stops. It does get confusing. Newbies to Video expect to get a lovely wide FOV with an 8" f5 reflector because it is f5. But the focal length of that scope is 1000mm. This is why we need to stress to people that Focal length is the most important thing in Video Astronomy. Even the guys using 11" SCT's use Faststar and Hyperstar to get their large scopes down to only 500mm focal length at f2. To give an example of how this works, here are some images of the same camera using the same focal reducer but at different focal lengths, You'll soon see the problem of focal length in larger aperture scopes. The setting I will use will be for a 1/2" video astronomy sensor and a 0.5x Focal Reducer. ED80 at 600mm focal length, 6" refractor at 750mm 8" Dob at 1000mmm. Take notice of the size of each FOV inside the rectangle being pointed out by the red arrow:
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Post by Dragon Man on Jan 4, 2014 12:22:01 GMT
and your C11 is 2800mm focal length, so using a 0.5x Focal Reducer it will have this FOV: Attachments:
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Post by Deleted on Jan 4, 2014 12:38:24 GMT
Good question Karl i wanted to know this too! And great answer Ken! Its all very confusing haha but the pics help Heres a question.... So my lx90 is f10 and so is karls C11. But my focal length is only 2000mm compared to his 2800mm. So would that mean that even tho he has a much bigger aperture with the same f ratio that i have a wider field of view just cause my focal length is lower? So much learning involved with this hobby lol!
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Post by Dragon Man on Jan 4, 2014 13:00:33 GMT
Yes Chris, your focal length is shorter, so your FOV will be wider. Here is an animation of the two to compare: Attachments:
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Post by Deleted on Jan 4, 2014 13:15:15 GMT
Thanks for that Ken! I had no idea that was how it worked. Very useful to know.
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Post by Deleted on Jan 4, 2014 13:23:58 GMT
Excellent info Ken!!
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Post by Dragon Man on Jan 4, 2014 13:30:10 GMT
Thanks for that Ken! I had no idea that was how it worked. Very useful to know. Yep, so the dream scope for Video would be a super fast short focal length Large aparture Apo Refractor and no such thing exists. The closest is the $12,000 'Officina Stellare Veloce RH 200' Ricardi Honders 8" f3 at 600mm www.optcorp.com/of-vrh200-fp-veloce-rh-200-telescope-full-package.htmlThrow a 0.5x focal reducer in that baby and you are really cooking! 8 inch f1.5 at 300mm focal length! DROOOOLLLLLL
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Post by Deleted on Jan 4, 2014 13:44:57 GMT
Jeez, thats a serious bit of kit. I'm off to buy a lottery ticket.
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Post by Dragon Man on Jan 4, 2014 13:48:36 GMT
Fastar and Hyperstar are alternatives when fitted to SCT's that accept them That will bring them down to amazing fast scopes with very short focal lengths. A Fastar will bring an 8" SCT f10 down to f/1.95 Focal Ratio at 390mm Focal Length. Nice In appearance the FOV is very similar to that of the 150mm f5 Refractor with a 0.7x FR. Attachments:
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Post by Deleted on Jan 4, 2014 20:52:48 GMT
Yes Karl, the f7.5 gives a wider FOV then the f5 because of Focal length. The shorter the focal length the wider the FOV. Even though the ED80 is a slower scope the difference in focal length is greater than the difference in focal ratio. The ED80 is only 600mm focal length whereas the 6" Achro is 750mm focal length. The difference in 150mm focal length is greater than the difference of 2.5 f stops. It does get confusing. Newbies to Video expect to get a lovely wide FOV with an 8" f5 reflector because it is f5. But the focal length of that scope is 1000mm. This is why we need to stress to people that Focal length is the most important thing in Video Astronomy. Even the guys using 11" SCT's use Faststar and Hyperstar to get their large scopes down to only 500mm focal length at f2. To give an example of how this works, here are some images of the same camera using the same focal reducer but at different focal lengths, You'll soon see the problem of focal length in larger aperture scopes. The setting I will use will be for a 1/2" video astronomy sensor and a 0.5x Focal Reducer. ED80 at 600mm focal length, 6" refractor at 750mm 8" Dob at 1000mmm. Take notice of the size of each FOV inside the rectangle being pointed out by the red arrow: Thank you Ken, you are a gentleman and scholar! This makes things much clearer and is making me realise just how accurate my Gotos have been! I think the ED80 is definitely on the cards and for the future a Fastar would also be a nice addition Thanks again Ken you are a fountain of knowledge! Regards Karl
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Post by Deleted on Jan 5, 2014 12:22:28 GMT
Great posts / information on focal reduction above, the free software / FOV calculator from Ron Wodaski is also very useful.
Regards
Pat
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Post by Deleted on Jan 11, 2014 15:39:44 GMT
Ken, you are tearing a hole in my fledgling VA program, not to mention badly bending my bitty brain. I think I'm hearing I'd do better to sell off my Dobs (except for visual) and turn my investment toward an ED80 and a mount like the ZEQ25 or similar? Actually, I have an Orion ST80 with f/5.0 400mm f/l which isn't so bad as for CA. Maybe could use it to see before popping for an ED80?
Bob
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