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81mm Triplet vs. 103mm Boublet for Astrophotography

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#1 Clete

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Posted 19 February 2019 - 06:21 PM

I'm getting very close to pulling the trigger on the purchase of a new scope but I'm really torn between two options, both of which are exactly the same price.

 

Here's a link to the two scopes I'm looking at...

 

Option 1: William Optics Gran Torismo 81

 

It has an f/5.9, 81mm Synthetic Flourite FPL 53 Triplet, Air Spaced, APO

Focal length: 478mm

 

Option 2: William Optics Zenithstar 103

 

It has a f/6.9, 103mm Synthetic Flourite FPL 53 Doublet, Air Spaced, APO

Focal length: 711mm

 

If I did just observational astronomy, the 103mm scope is what I'd buy but I mostly do astrophotography and so that makes it harder to decide. Do I want more aperture and a longer focal length, or should I go for a wider field of view with higher quality and faster optics but less aperture.

 

By the way, the scope will be mounted on a Celestron AVX which should be able to handle either scope without a problem.

 

Which would you guys buy and why?

 

Thanks in advance for your advice!,

Clete


Edited by Clete, 19 February 2019 - 06:22 PM.


#2 bobzeq25

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Posted 19 February 2019 - 06:34 PM

I'm getting very close to pulling the trigger on the purchase of a new scope but I'm really torn between two options, both of which are exactly the same price.

 

Here's a link to the two scopes I'm looking at...

 

Option 1: William Optics Gran Torismo 81

 

It has an f/5.9, 81mm Synthetic Flourite FPL 53 Triplet, Air Spaced, APO

Focal length: 478mm

 

Option 2: William Optics Zenithstar 103

 

It has a f/6.9, 103mm Synthetic Flourite FPL 53 Doublet, Air Spaced, APO

Focal length: 711mm

 

If I did just observational astronomy, the 103mm scope is what I'd buy but I mostly do astrophotography and so that makes it harder to decide. Do I want more aperture and a longer focal length, or should I go for a wider field of view with higher quality and faster optics but less aperture.

 

By the way, the scope will be mounted on a Celestron AVX which should be able to handle either scope without a problem.

 

Which would you guys buy and why?

 

Thanks in advance for your advice!,

Clete

I recommend the 81 for a few reasons.

 

478 is much easier to learn AP with than 711.  Difficulty goes up very fast with focal length, it's not a linear relationship.  You _want_ shorter.

 

The AVX is a marginal mount.  Theoretically it can handle either, in practice it will do better with the 81.

 

Faster is also more useful than is obvious.

 

The increased aperture, so important for visual, is much less important for AP.

 

In my opinion, this is not a close call.  <smile>


Edited by bobzeq25, 19 February 2019 - 06:35 PM.

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#3 OldManSky

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Posted 19 February 2019 - 06:36 PM

Don't forget that for AP, you'll want the flattener/reducer to use with either scope (and they both use the same one, the Flat 6AII), which is a 0.8x reducer as well.

 

So your 478mm f/5.9 GT81 becomes a 382mm f/4.72 scope.  And your 711mm f/6.9 Z103 becomes a 568mm f/5.5 scope.

 

Do those focal lengths work for you?

 

I'm planning on getting a Z103, by the way.  Saving for it now.  For a bit more aperture and focal length than my current Z61 (568mm for AP over 360mm).  Not that I don't like the GT81, but at only 22mm more focal length than I already have, not enough of a difference to bother with :)

Good luck deciding.


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#4 ngc7319_20

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Posted 19 February 2019 - 06:48 PM

Get the triplet.  Otherwise you will be fighting red or violet halos, etc., on bright stars in your images.


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#5 pinzmann

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Posted 19 February 2019 - 06:49 PM

Bob is correct. I started out with an 80mm for almost a year then transitioned to a 102mm. Made the learning curve much more gradual. Not familiar with that mount but I started with the Orion Sirius, which works fine with the 80 but struggles with the 102 and assorted add-ons.
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#6 gnowellsct

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Posted 19 February 2019 - 11:28 PM

I would get the SW Esprit 80 mm at f/5.  It does cost more but the focuser is said to be very solid, a major consideration, and the short focal ratio will bring in the images a lot more quickly than even an f/5.9.  

 

Greg N


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#7 Clete

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Posted 20 February 2019 - 08:41 AM

Wow, thanks for the advice you guys. I didn't say so in the opening post so as not to bias the advice one way or the other, but I have been leaning toward the GT81 for all the reasons you all have indicated.

 

I've mostly been a little worried about the AVX mount. It just doesn't seem to track real super smoothly and I was worried that it's lack of performance would be evident with the added focal length. But every time I go to the WO website, I see that Z103 for the exact same price and I just keep hearing "Bigger is better! Bigger is better!" pounding away in my head like the latest Armin Van Buuren dance music track.

 

I'm going to definitely be buying the GT81. If I want more focal length I can add a barlow or something.

 

So I have one last question for those of you in the know about optics...

 

APO refractors come in doublets, triplets and quads. I've heard that a quad APO is just the same thing as a triplet but with the field flattener built in. My question is whether that is actually true or not?



#8 Clete

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Posted 20 February 2019 - 09:02 AM

I would get the SW Esprit 80 mm at f/5.  It does cost more but the focuser is said to be very solid, a major consideration, and the short focal ratio will bring in the images a lot more quickly than even an f/5.9.  

 

Greg N

I've looked at that scope. I can't figure out how they're getting the extra speed out of it. They're using Schott/Ohara ED glass which I've had a hard time getting any detailed information about but I can't imagine that it out performs FPL53 to that extent.

 

As for their price, it isn't really any more expensive because theirs comes with a field flattener and a canon t-adaptor and some other things that the WO doesn't include in thier base price. When you add all that up, it's actually cheaper and I think its the glass that accounts for it. I only wish I could get a good explanation for their claimed f/5 ratio.



#9 bobzeq25

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Posted 20 February 2019 - 09:29 AM

Wow, thanks for the advice you guys. I didn't say so in the opening post so as not to bias the advice one way or the other, but I have been leaning toward the GT81 for all the reasons you all have indicated.

 

I've mostly been a little worried about the AVX mount. It just doesn't seem to track real super smoothly and I was worried that it's lack of performance would be evident with the added focal length. But every time I go to the WO website, I see that Z103 for the exact same price and I just keep hearing "Bigger is better! Bigger is better!" pounding away in my head like the latest Armin Van Buuren dance music track.

 

I'm going to definitely be buying the GT81. If I want more focal length I can add a barlow or something.

 

So I have one last question for those of you in the know about optics...

 

APO refractors come in doublets, triplets and quads. I've heard that a quad APO is just the same thing as a triplet but with the field flattener built in. My question is whether that is actually true or not?

Bigger is better is a visual thing.  It can be true for an imager well down the road, learning will go better if you start small.  If your ultimate goal is to image small galaxies with a big scope, you'll get there faster/better/cheaper if you start small.

 

Quads could be built for a variety of purposes, but getting the field flat is usual.  I don't know of a quad that doesn't do that.  It's a nice feature, I have a Stellarvue SV100Q.  More than a little above your budget.

 

 

I've looked at that scope. I can't figure out how they're getting the extra speed out of it. They're using Schott/Ohara ED glass which I've had a hard time getting any detailed information about but I can't imagine that it out performs FPL53 to that extent.

 

As for their price, it isn't really any more expensive because theirs comes with a field flattener and a canon t-adaptor and some other things that the WO doesn't include in thier base price. When you add all that up, it's actually cheaper and I think its the glass that accounts for it. I only wish I could get a good explanation for their claimed f/5 ratio.

Speed is a matter of design.  Faster is harder to build well, and if not done well, you wind up with various optical aberrations.  The Esprit has a good reputation.

 

Get the specs somewhere near where you want them.  Short, light, and fast are good, in that order.  Set a budget, research, choose a scope.  But...

 

If saving some money on the scope will get you a better mount sooner, that's a serious consideration.  AVXs have a very mixed reputation, and there can be disputes.   People who got a good one can't fathom why people who got a bad one dislike the mount so much.  I have no idea of the relative proportions, there are significant numbers of both.

 

This is another visual deal.  Visual people getting into imaging have a hard time wrapping their brain around the fact that the mount is more important than the mount, deserves more of their budget.  Putting a $1650 Esprit on an $800 AVX is a strange idea, you're unlikely to get the value out of the scope.  On the other hand, experienced imagers will put a $1000 scope on a $5000+ mount, and it's not a silly thing to do.

 

It's just not intuitive.  Your gut will lead you badly astray on this one.  <smile>

 

The above is all solid information.  This is an opinion.  Get the cheapest scope you're comfortable with, it will do fine for learning on.  This would be a good choice.  $469.

 

https://www.astronom...fpl-53-f-6.html

 

Know this.  If you were buying everything new I'd recommend one of the $1200 mounts and that 72, over an Esprit on an AVX.

 

Scroll down to the picture of this very experienced author of this book.  That's a $500 refractor (70mm doublet, no longer made) on a $1200 Sirius.  He did not make a mistake, using that as an example of a good beginner setup.

 

http://www.astropix....bgda/index.html


Edited by bobzeq25, 20 February 2019 - 09:50 AM.

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#10 gnowellsct

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Posted 20 February 2019 - 09:33 AM

I've looked at that scope. I can't figure out how they're getting the extra speed out of it. They're using Schott/Ohara ED glass which I've had a hard time getting any detailed information about but I can't imagine that it out performs FPL53 to that extent.

As for their price, it isn't really any more expensive because theirs comes with a field flattener and a canon t-adaptor and some other things that the WO doesn't include in thier base price. When you add all that up, it's actually cheaper and I think its the glass that accounts for it. I only wish I could get a good explanation for their claimed f/5 ratio.


You might be over thinking it. It is the go to telescope for novices at the north east astro imaging conference. If color were an issue it would be known.
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#11 Hesiod

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Posted 20 February 2019 - 09:35 AM

Design...glass mating it is not the only option available to optical designers.
Anyway, in the first telescope for AP I rate easiness of use and mechanical features higher than chromatic correction
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#12 gnowellsct

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Posted 20 February 2019 - 09:36 AM

think its the glass that accounts for it. I only wish I could get a good explanation for their claimed f/5 ratio.


The explanation is it was designed for the imaging market.

#13 gnowellsct

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Posted 20 February 2019 - 10:00 AM

If I want more focal length I can add a barlow or something.


You can add a barlow but you can't change the laws of physics. You'll have a tiny exit pupil so even at equivalent magnification to a longer focal ratio instrument your eye will notice you're doing something weird. This is why eyepieces below 3 mm are infrequently found.

But there are so many frustrations in imaging it is wise not to prioritize visual observing from the same scope.

I would prioritize a used premium mount. Even for visual. Greg N

#14 bobzeq25

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Posted 20 February 2019 - 01:36 PM


I'm going to definitely be buying the GT81. If I want more focal length I can add a barlow or something.

Oops.  Yet another unintuitive thing, sigh.

 

Almost nobody (there are possible exceptions, but we're in expert territory) uses a Barlow on DSOs.  The reason is that the loss in speed (higher F number) far outweighs any theoretical gain in resolution.  DSOs (even so called "bright" DSOs) are so dim, getting a good signal to noise ratio with a 2X Barlow would involve 4X the total imaging time.  Nobody wants to do that.  At 1X my rule of thumb is one hour minimum, 2 better, 4 good.  You do not want to make that 4 hours minimum, 8 better, 16 good.  At least not when you're starting out.

 

This business involves teasing a tiny signal from a sea of noise.  There are _many_ consequences of that (for example, it's a bad idea to omit the camera calibration frames; bias, flats, darks, even on your first image).  It's completely unlike terrestrial photography or lunar/planetary imaging.

 

I regret having not said this before.  Get this superb book.  That's more factual than opinion.  <smile>

 

https://www.amazon.c...d/dp/0999470906


Edited by bobzeq25, 20 February 2019 - 01:39 PM.


#15 RandallK

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Posted 20 February 2019 - 02:14 PM

I just got the Z103 along with the flattener in October last year. I have it mounted on my trusted HEQ5-Pro (Sirius), with absolutely NO issues. In fact I can load my Meade OTA 8" SCT with NO issues as well. I am using my one shot colour ZWO ASI 1600MCS Pro 3 mainly. On my wide field images using the matching flattener for the Z103, there is no elongation at any of the 4 corners of my images! I have also not seen any noticeable aberration or distortion of colour. If you need proof I will post some images on request.


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#16 Clete

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Posted 20 February 2019 - 03:23 PM

I just got the Z103 along with the flattener in October last year. I have it mounted on my trusted HEQ5-Pro (Sirius), with absolutely NO issues. In fact I can load my Meade OTA 8" SCT with NO issues as well. I am using my one shot colour ZWO ASI 1600MCS Pro 3 mainly. On my wide field images using the matching flattener for the Z103, there is no elongation at any of the 4 corners of my images! I have also not seen any noticeable aberration or distortion of colour. If you need proof I will post some images on request.

Please, by all means, post some of your photos.

 

I'd be very interested to see just how much field of view you get with that scope.



#17 Clete

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Posted 20 February 2019 - 03:37 PM

You can add a barlow but you can't change the laws of physics. You'll have a tiny exit pupil so even at equivalent magnification to a longer focal ratio instrument your eye will notice you're doing something weird. This is why eyepieces below 3 mm are infrequently found.

But there are so many frustrations in imaging it is wise not to prioritize visual observing from the same scope.

I would prioritize a used premium mount. Even for visual. Greg N

A new and better mount is next on my list for sure. I had thought about getting that first but decided to go with the better scope. What sucks is that I thought I was getting a good mount when I bought this one. I was too new to this stuff to know any better at the time. Not that it's a terrible mount. Its actually pretty good considering the price, but I'd have waited and spent more money had I known the quantum leap you get in quality by spending 50% more money.



#18 Clete

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Posted 20 February 2019 - 03:44 PM

The explanation is it was designed for the imaging market.

So is virtually every APO refractor in this size class and price range.

 

You could get a faster scope by using oil spaced objective lenses because you have less reflectivity to deal with but even that is marginal now with the high quality non-reflective coatings they are using. But even at that, the SW Esprit 80 uses air spaced objectives.

 

Don't get me wrong here, I'm not criticizing the scope and I don't have good reason to doubt that they have achieved f/5. I'd just like to know what they did different to WO, Celestron, Stellaview, Orion, Explore Scientific and every other 80mm ED scope I've looked at.


Edited by Clete, 20 February 2019 - 03:45 PM.


#19 bobzeq25

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Posted 20 February 2019 - 05:34 PM

So is virtually every APO refractor in this size class and price range.

 

You could get a faster scope by using oil spaced objective lenses because you have less reflectivity to deal with but even that is marginal now with the high quality non-reflective coatings they are using. But even at that, the SW Esprit 80 uses air spaced objectives.

 

Don't get me wrong here, I'm not criticizing the scope and I don't have good reason to doubt that they have achieved f/5. I'd just like to know what they did different to WO, Celestron, Stellaview, Orion, Explore Scientific and every other 80mm ED scope I've looked at.

What they did.  Designed it differently.  Perhaps manufactured it more carefully.  Set the price at $1650.  <grin>

 

It's entirely possible the stars from this would look better.

 

https://www.stellarv...plet-refractor/

 

Not that you'd be likely to notice the difference on an AVX (ducks and runs for cover <grin>).


Edited by bobzeq25, 20 February 2019 - 05:35 PM.


#20 Clete

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Posted 20 February 2019 - 06:20 PM

What they did.  Designed it differently.  Perhaps manufactured it more carefully.  Set the price at $1650.  <grin>

 

It's entirely possible the stars from this would look better.

 

https://www.stellarv...plet-refractor/

 

Not that you'd be likely to notice the difference on an AVX (ducks and runs for cover <grin>).

Well that one you linked to is at F/6. It's the Espirit that claims an f/5 that is in question.

 

In any case, I had pretty much settled on the William Optics scope because I've been completely unable to find anyone who didn't totally love it, the price is reasonable, and it looks nice to boot.

 

I had been looking at the carbon fiber version of the Explore Scientific ED80 but I read a review that said the focuser has an imprecise, spongy feel which didn't sound good and the WO has all three objective elements made from FPL53 glass which accounts for the slightly faster optics (and higher price). Also, the WO has double the warranty.

 

Do you have an opinion on WO scopes in general?



#21 gnowellsct

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Posted 20 February 2019 - 06:33 PM

So is virtually every APO refractor in this size class and price range.

 

 

?  There are f/7s, f/8s, that are for visual.  The ED81s, which has been re-named, goes for over a grand.  Orion has an f/7.5 80 mm.  Skywatcher 80 mm ED pro.  f/7.5.  CFF has an 80 mm f/6.9 for $2k.  I don't consider that photographic, but it *could* be on a good enough mount.  The longer the FR the better you have to track for your longer exposures.  But CFF is also selling an 80 mm f/6.

 

If you mean in the $1500 range, you might be right.  I don't know the 80mm market well enough to know what's out there in the f/7 ish and up range.  I consider f/7 and above to be leaning more towards visual use (given the alternatives). 

 

Nonetheless I think your statement is "broadly true" if we mean >$1000 f/7 ish and higher 80mm class instruments to be hard to find, and the field seems to be dominated by Asian imports that are looking to get into the photographic market.  Still the Esprit hits a sweet spot in terms of a good focuser and fast focal length--and the approval of a fair number of imagers.  

 

Greg N

 

This Esprit dropped by my house to visit my f/7.7 81 mm--it was impressive upon inspection and the imaging results excellent (but not done by me).

 

vixen and skywatcher- cn size.jpg


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#22 gnowellsct

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Posted 20 February 2019 - 06:48 PM

A new and better mount is next on my list for sure. I had thought about getting that first but decided to go with the better scope. What sucks is that I thought I was getting a good mount when I bought this one. I was too new to this stuff to know any better at the time. Not that it's a terrible mount. Its actually pretty good considering the price, but I'd have waited and spent more money had I known the quantum leap you get in quality by spending 50% more money.

Well remember there is a relationship between the mount and the optic.  It's not just a matter of stability.  The more you compromise on the mount, the faster you want to  image so as to minimize the impact of tracking errors on the final image.

 

Chuck has some nice videos on using the AVX including some glitches.   He's got more than one video so you might as well settle in and see whether that's the mount for you.  Read his comments at one point he gets sent a new mount.

 

All I can say is that I was greatly reluctant to drop serious coin on a mount way back in 01 or 02 and someone said I needed to get a G11 for $2,000.  I laughed at the idea of spending that much.  A year later I had one.

 

Now on the positive side, if you amortize $2,000 over 17 years you get $117 a year....and that mount still has resale value.  I have a couple other mounts as well.  A Vixen Super Polaris and an AP900.  I'm not sure what I would get if I were buying today but likely something upper tier from these shores.   

 

Greg N


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#23 RandallK

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Posted 20 February 2019 - 11:08 PM

As requested by OldManSky here are two images (the weather over the last 5 months has not cooperated) of the Pacman Nebula and the Lagoon Nebula (M20) . They were both taken from my back and front yards. The Lagoon was very low on my front yard, taken amid 3 sodium vapour street lights with cars continuously coming down the road past my place! But look at the image!  The Pacman was taken with a near 3/4 moon on the rise. Each final comprised about 35- 5 minute RAW colour images from my ZWO ASI1600MC-S Pro 3 (discontinued). My scope was the William Optics Z103 with WO Flattener attached. Stars are sharp right out to the edges. There was no cropping in either final image.

 

M8 - Final Integration3_CN.jpg

 

 

1 more to follow....

 

 

 


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#24 RandallK

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Posted 20 February 2019 - 11:10 PM

And the Pacman Nebula NGC 281...

 

NGC281_FINAL_MedianCN.jpg

 

Lots of stars in this one and pretty round out to all four corners...very happy.

 

 

Randall

 


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#25 gnowellsct

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Posted 20 February 2019 - 11:22 PM

Very nice work!




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