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Imaging instrument choice?

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#1 Auburn80

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Posted 19 March 2018 - 06:34 PM

Which would you choose for solar system imaging - 130mm APO or 180mm Mak? Is there enough aperture in the 180 to overcome its obstruction and probable lower quality of optics?

Thanks!

#2 MalVeauX

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Posted 19 March 2018 - 07:16 PM

Which would you choose for solar system imaging - 130mm APO or 180mm Mak? Is there enough aperture in the 180 to overcome its obstruction and probable lower quality of optics?

Thanks!

That's going to be a big preference game.

 

But, the winner is the 180mm Mak. The C.O. is smaller in this design and so the Mak still has more aperture advantage.

 

But why are you not considering something like a C9.25 in this weight class?

 

Very best,



#3 Auburn80

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Posted 19 March 2018 - 08:15 PM

That's going to be a big preference game.

But, the winner is the 180mm Mak. The C.O. is smaller in this design and so the Mak still has more aperture advantage.

But why are you not considering something like a C9.25 in this weight class?

Very best,


Thanks for the input. Maybe I've not given my SCTs enough of a chance but I just can't get the sharpness I want.
Seeing? It's possible even though there have been times when it should have been steady enough but the image was just muddy. Collimation? I've done the best I know how. Thermals? Check. My optics? Dunno.
I've seen really great results from others so I know the SCT should work.

#4 yock1960

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Posted 20 March 2018 - 12:12 PM

Thanks for the input. Maybe I've not given my SCTs enough of a chance but I just can't get the sharpness I want.
Seeing? It's possible even though there have been times when it should have been steady enough but the image was just muddy. Collimation? I've done the best I know how. Thermals? Check. My optics? Dunno.
I've seen really great results from others so I know the SCT should work.

A smaller aperture may yield a sharper image on planets, with them being so low currently, but aperture rules...assuming non-defective optics and good collimation.  I also recommend some type of active cooling for an SCT, especially if storing scope indoors! I think (and I include myself), that we often blame the scope, when it's not necessarily the issue.

 

Steve


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#5 Auburn80

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Posted 20 March 2018 - 12:26 PM

A smaller aperture may yield a sharper image on planets, with them being so low currently, but aperture rules...assuming non-defective optics and good collimation. I also recommend some type of active cooling for an SCT, especially if storing scope indoors! I think (and I include myself), that we often blame the scope, when it's not necessarily the issue.

Steve


I try to tell myself that. But when everything is variable; from optics to seeing to equilibrium to collimation, I feel like I'm just chasing my tail. Maybe the main culprit is seeing. Conditions here have been crappy for months. Which makes collimation a challenge too.

#6 ToxMan

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Posted 20 March 2018 - 01:28 PM

Well, variables do exist...many I can control, and a couple important ones that I cannot. If I don't "religiously" take care of those things that I can control, I will not be prepared when things I can't control finally shift into a most favorable situation bringing all the crucial elements together at once for the very best images. And, if I fail in spite of the uncontrollable being totally favorable, that's on me and my ability and attention to detail. A high resolution planetary imaging setup takes time to dial in and fine tune. Mistakes have a domino effect. Some are hard to identify. It's already been demonstrated here that one can image with all the telescopes mentioned in this thread. In my personal opinion, SCT is a very good instrument for planetary...and needs a lot of attention to detail.


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#7 Tom Glenn

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Posted 20 March 2018 - 07:37 PM

Depends on what you want to image.  When you say solar system imaging, do you mean mostly planets, or do you mean lunar and solar too?  And if so, do you want high resolution shots of small regions or do you want to fit the entire sun or moon into one frame?  130mm aperture is not going to resolve nearly as much as 180, so for planets you might be disappointed depending on what your expectations are.  Although 180mm isn’t even that large of an aperture for planets.  For lunar or solar, depending on what camera you use, a 130mm APO might fit the entire lunar disk in one frame, so if that is what you are going for then it would be excellent.

 

Assuming collimation is perfect in both, the largest factors that lead people to believe SCTs and similar designs give “mushy” views compared to APOs are thermal equilibrium (or lack thereof), and the increased focal length which means you are comparing apples to oranges when it comes to image scale.  A 130mm APO at F/7 will have a 910mm focal length, whereas a 180mm Mak at F/15 has a 2700mm focal length, and a similar aperture SCT at F/10 would have 1800mm focal length.  So you are dealing with at least 2-3x the focal length, which will exaggerate any thermal equilibrium issues, as well as seeing conditions. 

 

Just as an example, let’s say you had a camera with 3.75 micron pixels.  On a 910mm focal length scope, this would yield a sampling of 0.85 arcsec/pixel, which would be suitable to resolve details of about 2.5 arcsec.  The same camera on a scope with 2700mm focal length would sample at 0.29 arcsec/pixel, which would be suitable to resolve details to about 0.9 arcsec.  Now suppose you are encountering bad seeing conditions, of let’s say 2 arcsec (any worse than this and you should probably just go inside).  Which scope will give a sharper image?  Well, the overall resolution on the shorter length scope may not sound impressive, but since the resolving limit is higher than the seeing limit at least you could produce an image scale that looks good given the conditions, and have a larger field of view.  The Mak or SCT would give a reduced field of view with an image that looks mushy, because you are significantly oversampling the seeing.  It’s not that the scope is any worse of a design, it’s just that it produced an image scale that was not suitable for the conditions.


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#8 Auburn80

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Posted 20 March 2018 - 10:24 PM

Tom, thanks for posting such a thorough response.
What set me off was my experience a few mornings ago. I had checked collimation and it looked good. The scope had spent the night in my truck with Windows down and temps were dropping slowly. I set up for visual just to see how my rig would perform. Most of my time was spent at about 200x. The image would fuzz out on occasion but be stable for several seconds. Even during the stable periods, detail was practically non existent. The moon's looked like a child's drawing of a star. As implied, this was sort of a trial run for imaging Jupiter and Saturn.
It's possible the seeing was as described in a S&T article where the image would look stable but have no detail. I forget what term they used . . .
At any rate, I haven't given up on the C8 yet. Oh, and I do understand I think about sampling and seeing effects. My camera theoretically works best at f10-15, so a multiplier would be in order for a refractor.
Thanks again!

P.S. I do have one example in my gallery made with this scope - Saturn from last year. Not great but a start.

Edited by Auburn80, 20 March 2018 - 10:32 PM.


#9 Tom Glenn

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Posted 21 March 2018 - 01:15 AM

It's always tricky to try and diagnose these issues because so many things can conspire to degrade an image that it's difficult to know which were most responsible.  In general, anytime you see motion in the image or anything changing quickly with time, it's the seeing or thermals in the scope.  If you say that the scope was equilibrated with ambient temps and the temps were not dropping, that leaves seeing.  If collimation was off, it would stay off and the image would always look consistently bad.  If the image ever snaps into focus and then goes back to looking fuzzy, that's seeing.  Nothing else would cause a brief period of sharp focus and then gone.  

 

When you say the moons of Jupiter looked like a drawing of a star, this makes me think it was moving around rapidly, which would also point to seeing.  Also, what altitude was the planet above the horizon?  Jupiter and Saturn are extremely low in the Northern hemisphere this year.  At 30 degrees altitude you are looking through 2x the atmosphere as at zenith, but in practice it's even worse than that depending on what type of terrain you are viewing across or over.  

 

If we are talking about observing visually, 200x can very easily be much too high for Jupiter depending on conditions.  Jupiter is very sensitive to magnification and can fade away very quickly.  I don't spend much time visually observing anymore, but when I do I have eyepiece combinations that yield 94x, 188x, and 261x with my C9.25 Edge HD.  On most nights, Jupiter looks sharpest at 188x.  Going to 261x almost always causes it to fade and look bad, except on nights of excellent seeing.  Interestingly, Saturn tolerates the higher magnification better, likely due to fewer surface details and high contrast in the rings.



#10 TareqPhoto

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Posted 21 March 2018 - 06:15 AM

When you buy a Mak 180 please do a comparison with C6 and C8 so would like see the difference, convince me that SCT is a good option too beside a Mak, for now i am only saving for Tak refr and 14" SCT [i may ask about 16" if i put my Mak at many use with great results], then i won't look back or for more. 




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