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Old 02-03-2021, 07:17 PM
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Originally Posted by Ribodab View Post
From my understanding with astrophotography many use small apertures compared to visual astronomy. They use refractors, easier to hand, dont need as heavy duty mounts, no collimation and better sharpness and no centre obstruction. If aperture was the main issue then all astrophotographers would have hugh apertures, but simply don't.

The aperture comes into play with planetary, with astrophotography you put your money into the mount first, and aperture less concerning.
Hi R,

Aperture, focal ratio and other related measures like entendue are all important if you want to strive for the highest possible optical resolution, high signal to noise ratio possibly with reduced imaging time. Your point about all astrophotograhers not having large apertures can also be guided by other factors they choose to trade off, such as cost, portability, ease of use ....&c

Best
JA
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Old 02-03-2021, 07:25 PM
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Possibly the nearest perfect all around telescope would have thd be the Evolution series as it can converted to f2, f10 for planets, focal reducer for wider fields so a jack of all trades, but it does require a heavy duty more expensive mount

Maybe you should look to see why they use refractors, so many more advantages and in astrophotography unless planetary its not often about aperture, an f5 4 inch refractor will still have a shorter wider field, often Newtonians, unless modified may not be able to get enough focus. The defraction spikes can be bothersome. One of the best astrophotographers on You tube, Trevor Jones often uses a small 4" apo and even a 60mm and would not even know were done on a small aperture. With astrophotography light continues to gather and build up, unlike the eye. Also a bigger scope can give poor results due to atmosphere contains and a refractor often gives better steadier conditions. Beginners don't often grasp that in astrophotography its not always about aperture less important than for planets and visual, both totally different.

Also the bigger the scope the more beefy it needs to be and most costly. Also more likely effected by wind and collimation issues

One of the best most famous astrophotographera on you tube and why he says a refractor is better for astrophotography


https://astrobackyard.com/refractor-...rophotography/

Also if you have poor atmosphere conditions which is harder to achieve using a larger scope may have to bin your camera to get good results, so will cause you to have lower res and resolution so taking away the benefit of having a larger scope

You may also lose the benefit of a larger aperture as you get less good contains to use them, wereas a refractor is steadier and can be used more often than a reflector on poorer nights. If you use a larger aperture you may have to go to binning 2 or 3x so has an effect of dropping resoultion and sharpness so lose the advantage of using a bigger scope

Last edited by RB; 03-03-2021 at 06:08 PM.
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  #23  
Old 02-03-2021, 08:05 PM
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Originally Posted by Ribodab View Post
Maybe you should look to see why they use refractors, so many more advantages and in astrophotography unless planetary its not often about aperture, an f5 4 inch refractor will still have a shorter wider field, often Newtonians, unless modified may not be able to get enough focus. The defraction spikes can be bothersome. One of the best astrophotographers on You tube, Trevor Jones often uses a small 4" apo and even a 60mm and would not even know were done on a small aperture. With astrophotography light continues to gather and build up, unlike the eye. Also a bigger scope can give poor results due to atmosphere contains and a refractor often gives better steadier conditions. Beginners don't often grasp that in astrophotography its not always about aperture less important than for planets and visual, both totally different.

Also the bigger the scope the more beefy it needs to be and most costly. Also more likely effected by wind and collimation issues
Hi R,

I don't need to see why others use refractors: You are preaching to the choir as that's all* I have: refractors. I understand and have espoused their potential advantages (no central obstruction, contrast , etc....) a few times. My point was about aperture, focal ratio and entendue. I did not say that needed to be achieved by using a reflector as excellent as some are.

Best
JA

*Full disclosure - sorry I did forget one (unused) 10inch Newtonian

Last edited by JA; 05-03-2021 at 07:04 PM.
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Old 03-03-2021, 11:11 AM
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My take on this old chestnut is simply: aperture rules.

The notion that larger apertures require better seeing to work as well as a smaller scope is just plain wrong. The reality is with a larger scope you can actually detect the difference in the seeing, which small aperture users are blissfully unaware of most nights. There is a secondary benefit to a larger aperture, as while the airy disks may well turning in to "fuzz balls", they stay put on the focal plane. Whereas with a small scope they are bouncing all over the place. Think of a container ship that just sits there in a light chop, then look at a tinny on the same seas...it will be bouncing all over the place.

Careful what you wish for however. To get a reasonable field of view, larger scopes also require larger sensors, larger pixels, larger filters, larger diameter field correctors and much beefier focusers etc. Then there is the mount.....and the fact you need to permanently house all of this big stuff somewhere. Things get very expensive very quickly.

That said, I can think of no better example of the aperture rules mantra other than to point to the Chart32 team's results. Simply awesome imagery that you just can't get from an 71mm refractor

P.S. I did this roll-over a while back..while the image brightness looks similar, the larger scope shows more fine detail.

Last edited by Peter Ward; 03-03-2021 at 04:15 PM.
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Old 03-03-2021, 10:22 PM
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roll-over details

That's a great demo of resolution, Peter. What are the 2 scopes in the images?
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  #26  
Old 04-03-2021, 10:45 AM
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Quote:
Originally Posted by Ribodab View Post
From my understanding with astrophotography many use small apertures compared to visual astronomy. They use refractors, easier to hand, dont need as heavy duty mounts, no collimation and better sharpness and no centre obstruction. If aperture was the main issue then all astrophotographers would have hugh apertures, but simply don't.

The aperture comes into play with planetary, with astrophotography you put your money into the mount first, and aperture less concerning.
Respectfully, I would suggest that your statement is a bit generalised.

Whilst it might be true that many astrophotographers use refractors and those have small apertures, it is equally true that mount and guiding errors with wide field imaging hides a lot of errors. Obtaining more defined resolution of targets requires better mounts and better guiding performance.

Collimation is not an impediment to use of a telescopes. It is a skill all users of telescopes should learn and be skilled at to perform. If you know how to collimate your scope it will take no time at all to perform this function.

Whilst larger apertures are employed for planetary imaging, they are also employed for imaging objects in greater detail such as planetary nebulae and distant galaxies. That is why large telescopes are built and continue to be built larger and larger; to see detail in distant and smaller objects. The angular resolving power of a larger scope will always out perform a smaller scope.

Finally, you'll find that many people start with smaller apertures simply because of budget constraints or not wanting to spend too much initially.
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  #27  
Old 04-03-2021, 11:46 AM
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That's a great demo of resolution, Peter. What are the 2 scopes in the images?
From memory the ED refractor was a GT81 William optics. The larger scope is my AP305mm Riccardi Honders.
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  #28  
Old 04-03-2021, 01:58 PM
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Aperture

https://www.youtube.com/watch?v=Jjmipn3yeIA
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  #29  
Old 04-03-2021, 08:54 PM
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Quote:
Originally Posted by Peter Ward View Post
My take on this old chestnut is simply: aperture rules.

The notion that larger apertures require better seeing to work as well as a smaller scope is just plain wrong. The reality is with a larger scope you can actually detect the difference in the seeing, which small aperture users are blissfully unaware of most nights. There is a secondary benefit to a larger aperture, as while the airy disks may well turning in to "fuzz balls", they stay put on the focal plane. Whereas with a small scope they are bouncing all over the place. Think of a container ship that just sits there in a light chop, then look at a tinny on the same seas...it will be bouncing all over the place.

Careful what you wish for however. To get a reasonable field of view, larger scopes also require larger sensors, larger pixels, larger filters, larger diameter field correctors and much beefier focusers etc. Then there is the mount.....and the fact you need to permanently house all of this big stuff somewhere. Things get very expensive very quickly.

That said, I can think of no better example of the aperture rules mantra other than to point to the Chart32 team's results. Simply awesome imagery that you just can't get from an 71mm refractor

P.S. I did this roll-over a while back..while the image brightness looks similar, the larger scope shows more fine detail.
Peter's on point here.

Something that took me a little while to understand in one of Texereau's book was the difference between the physical size of a diffraction spot (airy disc) and its angular size.
My thanks to Mark S. for his patience in explaining this to me and for giving me a good analogy: newsprint halftone dots. It then made a lot of sense.

The first thing that sounds a little odd and counter-intuitive when you first read about it is that the physical radius of an airy disc for example an f/6 objective at a wavelength of let's say 560nm (green) is ~4.1 microns whether it's a 130mm refractor, a 16" RC or a 39m diameter telescope in Chile (assuming the same focal ratio for the sake of the exercise). That's that little disc of light at the focal plane where all the rays converge. Its size is only a linear function of the focal ratio and the radiation wavelength. Nothing to do with the aperture. So it's a little smaller for blue, bigger for green and even bigger for red, etc...

But the angular size of this disc is inversely proportional to the aperture. So as the scope diameter increases you can pack more of these little discs in the focal plane surface. Keep in mind we're talking about light here coming to focus. There is no CCD chip, no pixel size, no eye pupil, no eyepiece magnification of the image plane.

I did a quick illustration to visualise how it works. The top is a 4" f/6 objective and the bottom double the aperture 8" f/6. Then replicated for a 16" f/6 objective. This was done in PS. I pixelated a photo then applied a circular pattern with a radius matching the box sizes. In reality all the diffraction discs are much much smaller, overlap each others and are fuzzy with a primary concentric ring and other rings of various luminosity if there is a central obstruction and/or spherical aberrations, then on top of that the seeing will blur all this in one blob. It is not to scale but for the illustration this is fine and I respected the proportions between the different aperture panels which is a factor of 2 each time.

All the diffraction spots have the same physical radius in all cases (4.1 microns). So you see a star at the top that covers about 4 dots in the 4" f/6 focal plane. The dot size is the size of the smallest detail that your objective can resolve. In the next panel you see the same star that covers maybe 25 dots in the 8" f/6 focal plane. So if it was a double star the 8" f/6 would have resolved it because there are enough dots in this area to potentially cover the gap if any in between the two stars. The 4" f/6 would see only one star. Also note the 8" f/6 resolves two little stars at 9 o'clock and 5 o'clock in the insert. The 4" f/6 barely shows a hint of the one at 5 o'clock. So it doesn't matter what camera pixel size you fit to the 4" f/6 objective. You still won't resolve these stars because the light is not there. It's too spread out. Similarly you can see the potential resolution in the 16" f/6 objective vs. the 8" f/6. And so on as aperture doubles while keeping the same focal ratio.

The argument that the image in a larger aperture (let's say a 16" RC) is greatly degraded by seeing is flawed. It is equally degraded in a smaller aperture (let's say a 130mm refractor) but it's not as obvious because you can't easily see it.

Now consider deepsky imaging if we used the same CCD chip on the two rigs. The 16" RC might give you slightly larger stellar profiles but there is something called a PSF that any software doing decon can use. One or many non saturated stars in your field will give the algorithm a path to reverse the effects of the atmosphere and partly restore the details in your image. The 16" RC airy disc angular size is smaller than the 130mm refractor and much finer data will be captured by this objective. With the 130mm refractor similar data was never there in the first place because it's simply beyond the reach of the instrument aperture and no amount of processing can make up for it. The limitation is the wave nature of light. Light is not infinitesimal. This is a physical fact.

As a side note, in light of recent trends, use the telescope you have but be aware of its limitations and when processing, if it looks too good to be true, then it usually is. Use common sense. There are a few basic rules and some method to astrophotography in extracting an image from the data your objective is capable of capturing. No more, no less. It's not wedding photography with a free for all you can eat buffet. But if you choose to do so don't mention science. It has nothing to do with it anymore. It's art, compositing, painting, whatever you want to call it and open to interpretation.
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  #30  
Old 05-03-2021, 12:22 AM
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Next scope...

So after reading this, I would appreciate some practical thoughts...
Given that I have a fairly good 115mm refractor (sky rover triplet) should I see an improvement in my imaging if I move to an 8 RC + reducer?
Btw, I have an asi294mc pro.
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  #31  
Old 05-03-2021, 09:43 AM
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Originally Posted by MarkInSpace View Post
So after reading this, I would appreciate some practical thoughts...
Given that I have a fairly good 115mm refractor (sky rover triplet) should I see an improvement in my imaging if I move to an 8 RC + reducer?
Btw, I have an asi294mc pro.

100%. Better resolution and because of the field size less affected by light pollution ( gradients)
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  #32  
Old 05-03-2021, 10:11 AM
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Kudos to Marc for such an excellent write-up and graphic method of showing what's happening at the focal plane.
(Marc: an extended version would be worthy of a piece in Aust Sky & Tel)

One thing that was not touched on was the quality of the optic. Many instruments are built to a price, rather than specification.

While good value for money optics are out there, I'd pick a smaller high quality instrument over a rubbish light bucket any day...as even the ancient Romans would say: Caveat Emptor !
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Old 05-03-2021, 04:04 PM
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Nice article there Marc - thanks.

It becomes obvious as you progress in astrophotography that larger apertures display more detail and more highly magnified views.

So no one telescope does it all. Some come closer than others though.

Greg.

Quote:
Originally Posted by multiweb View Post
Peter's on point here.

Something that took me a little while to understand in one of Texereau's book was the difference between the physical size of a diffraction spot (airy disc) and its angular size.
My thanks to Mark S. for his patience in explaining this to me and for giving me a good analogy: newsprint halftone dots. It then made a lot of sense.

The first thing that sounds a little odd and counter-intuitive when you first read about it is that the physical radius of an airy disc for example an f/6 objective at a wavelength of let's say 560nm (green) is ~4.1 microns whether it's a 130mm refractor, a 16" RC or a 39m diameter telescope in Chile (assuming the same focal ratio for the sake of the exercise). That's that little disc of light at the focal plane where all the rays converge. Its size is only a linear function of the focal ratio and the radiation wavelength. Nothing to do with the aperture. So it's a little smaller for blue, bigger for green and even bigger for red, etc...

But the angular size of this disc is inversely proportional to the aperture. So as the scope diameter increases you can pack more of these little discs in the focal plane surface. Keep in mind we're talking about light here coming to focus. There is no CCD chip, no pixel size, no eye pupil, no eyepiece magnification of the image plane.

I did a quick illustration to visualise how it works. The top is a 4" f/6 objective and the bottom double the aperture 8" f/6. Then replicated for a 16" f/6 objective. This was done in PS. I pixelated a photo then applied a circular pattern with a radius matching the box sizes. In reality all the diffraction discs are much much smaller, overlap each others and are fuzzy with a primary concentric ring and other rings of various luminosity if there is a central obstruction and/or spherical aberrations, then on top of that the seeing will blur all this in one blob. It is not to scale but for the illustration this is fine and I respected the proportions between the different aperture panels which is a factor of 2 each time.

All the diffraction spots have the same physical radius in all cases (4.1 microns). So you see a star at the top that covers about 4 dots in the 4" f/6 focal plane. The dot size is the size of the smallest detail that your objective can resolve. In the next panel you see the same star that covers maybe 25 dots in the 8" f/6 focal plane. So if it was a double star the 8" f/6 would have resolved it because there are enough dots in this area to potentially cover the gap if any in between the two stars. The 4" f/6 would see only one star. Also note the 8" f/6 resolves two little stars at 9 o'clock and 5 o'clock in the insert. The 4" f/6 barely shows a hint of the one at 5 o'clock. So it doesn't matter what camera pixel size you fit to the 4" f/6 objective. You still won't resolve these stars because the light is not there. It's too spread out. Similarly you can see the potential resolution in the 16" f/6 objective vs. the 8" f/6. And so on as aperture doubles while keeping the same focal ratio.

The argument that the image in a larger aperture (let's say a 16" RC) is greatly degraded by seeing is flawed. It is equally degraded in a smaller aperture (let's say a 130mm refractor) but it's not as obvious because you can't easily see it.

Now consider deepsky imaging if we used the same CCD chip on the two rigs. The 16" RC might give you slightly larger stellar profiles but there is something called a PSF that any software doing decon can use. One or many non saturated stars in your field will give the algorithm a path to reverse the effects of the atmosphere and partly restore the details in your image. The 16" RC airy disc angular size is smaller than the 130mm refractor and much finer data will be captured by this objective. With the 130mm refractor similar data was never there in the first place because it's simply beyond the reach of the instrument aperture and no amount of processing can make up for it. The limitation is the wave nature of light. Light is not infinitesimal. This is a physical fact.

As a side note, in light of recent trends, use the telescope you have but be aware of its limitations and when processing, if it looks too good to be true, then it usually is. Use common sense. There are a few basic rules and some method to astrophotography in extracting an image from the data your objective is capable of capturing. No more, no less. It's not wedding photography with a free for all you can eat buffet. But if you choose to do so don't mention science. It has nothing to do with it anymore. It's art, compositing, painting, whatever you want to call it and open to interpretation.
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Old 05-03-2021, 06:18 PM
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Quote:
Originally Posted by Peter Ward View Post
Kudos to Marc for such an excellent write-up and graphic method of showing what's happening at the focal plane.
(Marc: an extended version would be worthy of a piece in Aust Sky & Tel)

One thing that was not touched on was the quality of the optic. Many instruments are built to a price, rather than specification.

While good value for money optics are out there, I'd pick a smaller high quality instrument over a rubbish light bucket any day...as even the ancient Romans would say: Caveat Emptor !
Of course optics and mechanical quality will make a diffrerence. In the illustration I assumed "perfect" optics to compare on the same grounds. Having said that an average 16" RC with a large central obstruction will still out perform a 130mm pristine TOA.


Quote:
Originally Posted by gregbradley View Post
Nice article there Marc - thanks.

It becomes obvious as you progress in astrophotography that larger apertures display more detail and more highly magnified views.

So no one telescope does it all. Some come closer than others though.

Greg.
With experience in long focal imaging as a result of larger apertures it also becomes blatantly obvious that smaller apertures just cannot reach that level of details in the raw data. Fishy processing is just a perception.
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Old 05-03-2021, 08:05 PM
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With experience in long focal imaging as a result of larger apertures it also becomes blatantly obvious that smaller apertures just cannot reach that level of details in the raw data. Fishy processing is just a perception.
Indeed Damien Peach posted a video that highlighted how false features can magically appear due sloppy processing.

It highlighted how images can seem to have amazing filigree details, but in fact were nothing more than processing artefacts.

One of the aspects of the CWAS Malin awards I totally respected was DM's vast knowledge of what objects should look like (due their physical make-up) as opposed to some "interpretation" based on a whim at best.

The latter was rarely viewed favourably. I never had a problem with that.
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Old 05-03-2021, 08:30 PM
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Indeed Damien Peach posted a video that highlighted how false features can magically appear due sloppy processing.

It highlighted how images can seem to have amazing filigree details, but in fact were nothing more than processing artefacts.
I know exactly the video you are talking about and the software involved as I posted a link to it a few months ago.

Quote:
Originally Posted by Peter Ward View Post
One of the aspects of the CWAS Malin awards I totally respected was DM's vast knowledge of what objects should look like (due their physical make-up) as opposed to some "interpretation" based on a whim at best.

The latter was rarely viewed favourably. I never had a problem with that.
Me neither. And I find there is enough natural beauty in what we image without the "lipstick". There is nothing more satisfying than to capture data right at the limit of a well tuned, collimated, balanced rig on a night of good seeing. This is what I strive for and I get a kick out of it when it happens. Because it almost always require very little processing and it looks fantastic.
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Old 14-03-2021, 03:36 PM
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Mark, I'm on the verge of dipping my toes into some imaging, and I'm a bit frightened by a lot of the responses and advice given on this thread!
My learning curve is going to be very steep and the age of my brain is not optimal.

I have a Meade LX200 8" ACF and a Skywatcher ED80 Pro, and I have spent a lot of time pondering which scope to use for imaging.
Then I got a tip to visit the Bintel website and use their Astronomy Setup Calculator to help me compare scopes and cameras.
I think the tip came from a Dylan O'Donnell video on Youtube, he created the Bintel tools.

The attached screen grabs show the different results, on a view of the Pleiades, using the ASI 294MC camera on both of my scopes.

My LX200 would only capture a small centre portion of the Pleiades.
My ED80 will capture the whole group, decision made!

Hope this helps with your research.

Chris
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Old 15-03-2021, 08:16 AM
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The 294m is a bit more versatile in that it can be 47mp or 11mp.

The 47mp would suit the ED80 and the 11mp would suit the Meade 8 inch SCT.

But it requires filters, filter wheel.

Colour cameras are these days very sensitive and have good Ha response for nebulas. Their images are easier to process.

The downside is whilst they are a lot more sensitive to narrowband than they used to be they are not ideal if you want to be imaging narrowband to get over light pollution.

Greg.
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