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Would going up from 90mm to 120~150mm refractor worth it? (in both visual and imaging)

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#1 givememul

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Posted 14 October 2020 - 03:16 AM

Hi, I've just started viewing the heavens for about a year and trying out some basic imaging using my Nikon D5500.

 

The scopes that I currently use are Sharpstar 90mm triplet apo (f/6.67) and GSO 200 mm classical cassegrain(f/12).

 

I currently don't have much time to spend all night imaging, so I'm doing visual observations mainly, but I'm very intent on imaging if I get enough time later on.

 

So I've upgraded my eq mount from CEM25P to GEM45 to support heavier loads and is now looking for a decent addition to my visual and imaging scopes.

 

I'm interested in refractors in 120~150mm range with focal lengths near 700~1000mm.

 

From the money per aperture stand point, these refractors do get very expensive pretty quickly, so I first wanted to thoroughly check if it's worth the investment.

 

So upgrading a 3.5" to 5~6" range apo refractors, do they offer drastic improvements in visual and/or imaging? What would they offer?



#2 PETER DREW

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Posted 14 October 2020 - 05:57 AM

From the visual perspective, I have a 4" flourite and a 6"ED,  there is not a staggering difference in lunar and planetary detail and the image quality is similar.  Where the 6" scores is a brighter image at the same high powers making the detail easier to see.  I'm not an imager so can't comment on this, imaging has quite different parameters.



#3 emilslomi

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Posted 14 October 2020 - 06:44 AM

I have an 80, 100 and 140. 80 to 100 is not that much of a gain - noticeable but not overwhelming. 80 to 140 is about 3 times more light gathered, i.e. a little more than a magnitude gained. I appreciate it less for star fields than for extended objects. The Orion nebula looks nice in almost all scopes, but you can start exploring its depth, nooks and crannies and outer reaches with the 140 while the 80, however beautiful the image may be, is somehow stuck in the overview mode. Aside from that, seeing has to be quite bad (by my local standards) for the e and f of the trapezium not to be visible in the 140 while it has to be very good for them to be visible in the 80. So, 80 to 140 is, to me, drastic visually.

 

Emil


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#4 givememul

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Posted 14 October 2020 - 08:04 PM

From the visual perspective, I have a 4" flourite and a 6"ED,  there is not a staggering difference in lunar and planetary detail and the image quality is similar.  Where the 6" scores is a brighter image at the same high powers making the detail easier to see.  I'm not an imager so can't comment on this, imaging has quite different parameters.

 

 

I have an 80, 100 and 140. 80 to 100 is not that much of a gain - noticeable but not overwhelming. 80 to 140 is about 3 times more light gathered, i.e. a little more than a magnitude gained. I appreciate it less for star fields than for extended objects. The Orion nebula looks nice in almost all scopes, but you can start exploring its depth, nooks and crannies and outer reaches with the 140 while the 80, however beautiful the image may be, is somehow stuck in the overview mode. Aside from that, seeing has to be quite bad (by my local standards) for the e and f of the trapezium not to be visible in the 140 while it has to be very good for them to be visible in the 80. So, 80 to 140 is, to me, drastic visually.

 

Emil

 

Peter and Emil, thanks for your insights.

 

Hmm.. Visually, I see that the bigger aperture do show some improvements over the smaller refractors, and that's promising!

 

I hope somebody could provide some info about the imaging.

 

I have 1x flattener for my 90mm with 600mm of focal length. 130mm f/7 (910mm) with 0.75x reducer would provide a little less than 700mm fl.

 

When imaging with the above two systems with the same camera, would there be some differences between them apart from the different fovs?



#5 RAKing

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Posted 15 October 2020 - 01:25 PM

For visual observing, you will definitely notice the improvement going from a 90 to a bigger scope.  Aperture has always ruled, especially for visual work.

 

For imaging, it gets more complicated.  Here, the mount becomes more critical, so you might see a nice improvement simply by keeping your current scope and using it on a better mount.  The increased aperture might hurt unless you go for a fast, flat-field imaging scope.

 

When you get serious about it, visual astronomy and photo-astronomy can seem like two different hobbies. cool.gif

 

Cheers,

 

Ron


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#6 Jon Isaacs

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Posted 15 October 2020 - 01:53 PM

90 mm to 120 mm is very significant visually. To 150 mm, it's huge. 

 

A 90 mm is fighting for both light and resolution. 

 

Jon


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#7 MalVeauX

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Posted 15 October 2020 - 01:58 PM

Hi, I've just started viewing the heavens for about a year and trying out some basic imaging using my Nikon D5500.

 

The scopes that I currently use are Sharpstar 90mm triplet apo (f/6.67) and GSO 200 mm classical cassegrain(f/12).

 

I currently don't have much time to spend all night imaging, so I'm doing visual observations mainly, but I'm very intent on imaging if I get enough time later on.

 

So I've upgraded my eq mount from CEM25P to GEM45 to support heavier loads and is now looking for a decent addition to my visual and imaging scopes.

 

I'm interested in refractors in 120~150mm range with focal lengths near 700~1000mm.

 

From the money per aperture stand point, these refractors do get very expensive pretty quickly, so I first wanted to thoroughly check if it's worth the investment.

 

So upgrading a 3.5" to 5~6" range apo refractors, do they offer drastic improvements in visual and/or imaging? What would they offer?

You get a lot of improvements visually, simply from having more aperture. Brightness goes up. Resolution goes up. This assumes they are high quality optics and not just big aperture but low quality optics.

 

Imaging wise, you potentially can get significant improvements, but it depends on the subject matter; for tiny objects that could benefit from a finer image scale, yes. And from solar system objects that can benefit from the increased resolution from aperture, yes. As long as you keep in mind that your image scale will be finer, with a more narrow FOV, so this can limit imaging some larger subjects that simply will not fit in that FOV.

 

Realistically it's ideal to simply have both... your 90mm and a 120mm or 150mm, they compliment each other for wide field and for higher resolution.

 

Very best,


Edited by MalVeauX, 15 October 2020 - 01:59 PM.

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#8 noisejammer

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Posted 15 October 2020 - 04:03 PM

This is very much a yes ... but question.

The scope will be a huge jump visually.

 

A 6" triplet needs a serious mount. This can be a huge jump financially - the mount could cost quite a bit more than the optics. To get the most out of it you will also need a field flattener or reducer & some means of guiding.

 

Provided you have a good polar scope, it's still fine for a one-nighter, albeit approaching the limit of what's reasonable.



#9 givememul

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Posted 15 October 2020 - 09:19 PM

Thanks all! Your comments were a huge help!

 

It's great to see that the increase in aperture will be a definite improvement visually, and a potential upgrade image wise. 

 

As I'm quite new to astrophotography, I'll have to practice my imaging skills and get some more experiences.

 

Then I'll try to look for something to compliment my current imaging system.

 

Clear skies and wish you all good health!

 

Dae-Woong



#10 Gary Riley

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Posted 16 October 2020 - 09:34 PM

Yes.


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