Advantage of aperture (?)

[MarkInSpace]

I consider myself newish to astrophotography, I’ve been imaging for nearly a year, I started with a ZS71 and graduated quite a while ago to a 115 triplet refractor.
In spite of my online investigations, I have yet to fully understand the advantage of moving to a larger aperture telescope, particularly if I keep the f ratio the same.
As an example, I see that I can buy an 200mm f/8 RC with a reducer that gives me the same f/5 that I have with my current 115 refractor with a reducer flattener.
What practical advantage will this give me if I keep my same camera (asi294mc)?
Thoughts and images especially welcome.

[raymo]

The larger the scope, the smaller the detail that can be resolved. For instance
an 8" will show a lunar crater half the size that a 4" will. It is linear, so a 12"
will show one a third the size, and a 16" one a quarter the size.
Telescopes gather light according to the square of their diameter, so a 2"
gathers 4, a 4" gathers 16, a 6" gathers 36, and an 8" gathers 64, and so on.
that obviously means that an 8" [64] gathers light 4 times faster than a 4"[16], enabling four times as much data to be gathered in the same exposure time.
raymo

[billdan]

What Raymo said plus,

The Field of View of the RC will be almost half of the Triplet.

Your image scale in acrsec per pxel is approx = px size x 200 / focal length

For the RC its = 4.63 x 200 / 1000 = 0.926 arcsec per pxel
As the camera is approx 4000pixels wide thats a Field Width of 0.926 x 4000 = 3704arcsec ( 61.73 arcmins).

For the Triplet its = 4.63 x 200 / 575 = 1.61 arcsec per pixel
Field Width = 1.61 x 4000 = 6440 arcsecs ( 107.3 arcmins)


There is also telesope resolution to consider

Dawes Limit = 120 arcsec / Objective Diameter (mm)

For the RC its 120/200 = 0.6arcsec
For the Triplet its 120/115 = 1.04 arcsec

The other difference is Star Magnitudes, a larger aperture can see fainter stars than a smaller aperture.

[Startrek]

Mark,
I recommend you read an article by Craig Stark ( original developer of PHD guiding software) called “ Sampling Myths” circa 2010
Puts it all into perspective plus an attached image comparison

A bit technical but even an old dumbo like me can sort of understand it

Cheers
Martin
PS: I’ll take a 6” , 8” or 10” newt over an 80mm or 100mm refractor any day , no disrespect to anyone at all , it’s how deep you want to explore into an object and get the most resolution out of , that’s my challenge

[MarkInSpace]

Thanks for the comments so far - I will read the whole series of signal to noise articles.
Here’s my problem: if the F number doesn’t change, I believe my ability to speed up capture does not increase. If my camera does not change, at some point, I loose any resolution benefit from the wider aperture. Am I on the wrong track?

Is there a graph or table that relates cameras to ideal apertures and focal lengths?

[Rerouter]

The imaging time changes a little. but your right that its practically the same.

6" 762mm vs 8" 1016mm = about 1.788x times the light collecting area but the image is 33.33% more zoomed in, so ends up about 0.5% brighter / less imaging time needed

Edit: I fogot that zoom deltas are a square. e.g. 5x zoom is 25x smaller area.

[Atmos]

In its simplest, a larger aperture allows you to image at a finer resolution without losing SNR.

If you have a 4” F/5 and an 8” F/5 both of them put the same amount of photons per pixel (using your ASI294). The 8” captures 4x the amount of photons but it has twice the focal length; 4x less photons per pixel. So, you have the same amount of photons per pixel but at twice the resolution; twice the focal length.

[Startrek]

Mark
Here’s a few notes ( not in order ) from Craig Starks document

F ratio determines photon density which is explained as follows -
Aperture = total photons
Focal length = spread of photons
F Ratio = density of photons

Photon density per pixel well rather than total photons collected is more important
There is a balance between Aperture and focal length. As we go up in focal length for same Aperture past the point of over sampling, you will lose SNR without gaining any spatial information.
Scopes that have higher focal ratios of say f8 to f10 spread photons thinly which reduce the photon count per pixel and reduces SNR. Running a scope at a lower F ratio will make each pixel cover more Sky which provides better images.

Finally
Quote - Photon count for an extended object is driven by F ratio. Image scale is driven by focal length. Want more resolution at the same pixel wise SNR ?, boost the Aperture but keep the same F ratio. Want more SNR in your images and your willing to trade of some spatial information ( area ) or your already asking for more spatial information ( area ) than your conditions will allow ? Then drop your F ratio !!

Cheers
Martin

[Startrek]

Forgot to mention
Camera type , pixel size, well depth , Gain, sensor temperature, QE , colour or mono etc etc... is a separate issue not discussed but is directly associated with the optical physics above

[MarkInSpace]

You all are great! Thanks for the info - enough links to keep me reading for the rest of the weekend!

[Camelopardalis]

On top of all this good stuff, you need to consider seeing...

All the modern CMOS cameras with small pixels can make for some impractical resolutions (per pixel) with a larger scope when coupled with “average” seeing.

The limits at which you’re no longer capturing extra detail but just further characterising the blur can quite easily be reached with modest gear.

Of course, on a better than average nights, the results can be gobsmacking...it’s just that these nights are inversely proportional to your aperture :lol:

[gregbradley]

Its an interesting topic. Traditionally the answer would be aperture rules and its still true but perhaps to a lesser degree.

I think it comes down to what you want to image.

You can quote Craig Stark but try to image most spiral galaxies and you'll see the advantage of aperture and longer focal length.

With advances in sensitivity of cameras and higher quality modest aperture scopes I would say 70-130mm is good for widefield, 8-12 inches is probably a practical limit to galaxy and close in view imaging. There are quite few modest aperture APOs now that are not super expensive and are capable of a great deal.

I would class astro scopes into 4 categories:

Up to about 106mm aperture APO for widefield imaging.

Around 130mm APO for higher resolution widefield imaging.

Medium focal length wider aperture like up to about 12 inches and often F4 or faster. This for smaller objects but also can do fairly large objects. Its quite flexible. Focal length around 1260mm.

Longer focal length large aperture to get the small dim galaxies with decent detail.

There are tons of superb 10-12 inch Newts around F4 that show the power of aperture and a fast F ratio. A large aperture allows a faster F ratio whilst also maintaining a decent focal length needed for smaller dimmer objects.

As pointed out the latest CMOS sensors tend to have smaller pixels than the usual crop of CCD cameras which are typically 9 micron and as small as 4.54 micron. Whereas CMOS smallest is 2.3 micron and 3.76 is a common size now and 6 micron being one of the largest (also an older less sensitive sensor).

Plus the trend for these sensors is going to be smaller and smaller pixels so less and less suitable for astrophotography.
These later CMOS can be binned 2x2 but its not a hardware CCD-like binning but rather done after the fact of an image by software so the gains are less.

Greg.

[lazjen]

I thought some of the more recent CMOS cameras could offer hardware binning, e.g. ASI6200/QHY600 ?

[gregbradley]

No its software binning only.

The 294M bins 2x2 giving 11mp. It can be run 1x1 giving 47mp. its the only one I am aware of that you can bin.

Greg.

[lazjen]

According to the PDF manual for the ASI6200 (section 5.8), it supports hardware bin for bin2 and bin3 (software for bin2, bin3 and bin4).

I'd presume the QHY600 would be similar?

[gregbradley]

Oh OK.. The sensor is the same in the ASI and QHY except the QHY photographic version uses the industrial grade sensor whereas the ASI uses a lesser consumer grade sensor.

I see there is some reference to binning here on the QHY website but its not done the same way as CCDs to its detriment. I am under the understanding that 2x2 binning does not raise the signal to noise ratio much at all compared to CCD where it does. It does increase the full well depth though.
FAQs
1. Does QHY600 support hardware binning?
The CMOS sensor itself has some binning function but it should not be the hardware binning (FD binning). And also the binning in the sensor is based on the location of the bayer color . it means it will binning with the same position of the same color.And for monochrom QHY600 sensor, it is still use such a position to do binning. So we think it is not a good solution for the monochrom binning.
And since the very low readout noise of the QHY600, so the digital binning may bring more advantage. First , it can increase the fullwell. Binning at 2*2 will gives four times of the fullwell. Second, it will increase the AD sample depth. Binning at 2*2 will give 18bit data range. For readout noise, the N*N digital binning will cause the readout noise become SQR(N*N）= N times. For example, if the readout noise is 1.9e at 1*1 binning. The 2*2 digital binning readout noise will become 1.9*SQR(2*2)=3.8e.


Greg.

[xthestreams]

Assuming my ASI2600 is using a similar approach to the 6200, I think it’s all software binning. Even the so called increased FWCs are “virtual” (in that if one pixel overflows, you’re non-linear).

Turtles all the way down sadly.

[billdan]

Its a bit misleading when they state the Full Well Capacity increases 4 times when 2x2 binning.
The 2x2 binning also captures photons 4 times faster, in other words the stars will reach saturation at the same rate of time if it was not binned.

[gregbradley]

The ASI6200/QHY600 apparently have some hardware binning.

On the QHY600 there are different readout modes. One of these extends the full well depth. I don't think that is only virtual in that case. This is one of the main QHY advantages, these different readout modes.

But its something I will keep an eye on as I started using the extended full well depth and it seemed to hold highlights better and reduce background banding.

Greg.

[denodan]

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.

[JA]

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

[denodan]

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-telescope-astrophotography/

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

[JA]

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

[Peter Ward]

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 (http://www.atscope.com.au/BRO/roll/RefractorVSRha.html) a while back..while the image brightness looks similar, the larger scope shows more fine detail.

[MarkInSpace]

That's a great demo of resolution, Peter. What are the 2 scopes in the images?

[Paul Haese]

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.

[Peter Ward]

From memory the ED refractor was a GT81 William optics. The larger scope is my AP305mm Riccardi Honders.

[sn1987a]

Aperture :P

https://www.youtube.com/watch?v=Jjmipn3yeIA

[multiweb]

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 (http://www.astropic.net/astro/diffraction_spot.jpg) 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 (http://www.astropic.net/astro/diffraction_spot_b.jpg). 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.

[MarkInSpace]

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.

[multiweb]

100%. Better resolution and because of the field size less affected by light pollution ( gradients)

[Peter Ward]

Kudos to Marc for such an excellent write-up and graphic method of showing what's happening at the focal plane. :thumbsup::thumbsup:
(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 !

[gregbradley]

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.

[multiweb]

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.



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.

[Peter Ward]

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.

[multiweb]

I know exactly the video you are talking about and the software involved as I posted a link to it a few months ago.


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.

[chrisp9au]

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! :D

Hope this helps with your research.

Chris

[gregbradley]

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.