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Does aperture "rule" in AP?

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#1 JADO

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Posted 15 April 2024 - 08:17 AM

Really debating another OTA for imaging, would like to stay around the 1000mm range......lots of 8" reflectors at 800mm.....would like at least 1000mm but with the exception of a GSO 8" (which I can't find), I would have to step up to a 10" reflector, and that means a lot of weight.......looked at a few 8" RC's, a 190MN, and even a Mewlon 180c......

 

This would be almost exclusively for small-mid DSO

8" Ritchey Chretien

10" f4 reflector (heavy)

190MN (heavy)

Askar 140 refractor

Mewlon 180c

 

I have tried an SCT and didn't like it much

 

I imaged with an 8" reflector the other day and really liked the brighter clearer image......but does aperture "rule" in AP like it does in visual?  This new OTA would be AP/EAA only.......I don't do visual at all

 

Just curious if aperture plays a major role in astrophotography like it does in visual.....or is correct gain/exposure/total acquisition time a more important factor?

 

 

Thanks!


Edited by JADO, 15 April 2024 - 08:19 AM.


#2 rj144

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Posted 15 April 2024 - 08:23 AM

It plays an important role for planet astrophotography, but not DSO AP.


Edited by rj144, 15 April 2024 - 08:31 AM.

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#3 sharkmelley

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Posted 15 April 2024 - 09:36 AM

The larger the aperture, the more photons you will collect from a given deep-sky object in a given time.  Generally speaking, a greater number of photons will give a better image.


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#4 Oort Cloud

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Posted 15 April 2024 - 09:46 AM

Larger aperture let's more light in, but tracking is king in the imaging world. So feel free to increase aperture until it has a negative effect on tracking. Beyond that point, it does more harm than good.

#5 idclimber

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Posted 15 April 2024 - 09:48 AM

The mount rules AP, not the scope. 


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#6 KBHornblower

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Posted 15 April 2024 - 09:52 AM

I have seen members of our club do stupendous imaging with small but high quality refractors on big, beefy mounts with good autoguiding.


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#7 JADO

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Posted 15 April 2024 - 09:56 AM

Thanks for the replies......the mount will be an iOptron HEM44EC........

 

 

Between these 5:

8" Ritchey Chretien

10" f4 reflector

190MN

120-130 refractor

Mewlon 180c

 

Bortle 5 sky

 

 

My ASI2600MC stays on my Esprit 80....I also have an ASI533MC.......camera choice for any of the above 5 is yet to be determined....


Edited by JADO, 15 April 2024 - 09:57 AM.


#8 jmillsbss

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Posted 15 April 2024 - 09:57 AM

I recently picked up the ExpSci 6" Mak/Newt and I'm really enjoying it.  At f/4.8, it's reasonably fast and the images are very well corrected. At 731mm, it does a very good job of drawing in smaller galaxies and frames very well with my 533MC-Pro.  I've read, and can attest, it's very similar to a 5" APO. Slightly somewhat heavy, 16 pounds bare, and 23.5 pounds fully kitted out, with the camera, filter drawer, OAG, dew controller and extended light shield/dew shroud.  I've moved all my computing and dew controllers to the tripod, and that helped with a little weight imbalance with the larger OTAs. The best money I've spent was to upgrade to a Losmandy/Wide dovetail.  That really help stabilize it laterally and gave me more dovetail to shift the whole payload in the saddle.  I had a really frustrating go at collimation but once explained and evaluated by Vic Menard, I got it dead on and images are brilliant.

 

10" F4 seems like a light bucket but the images require additional lenses to deal with coma and collimation can be very narrow with little room for mistakes at f/4.  That's with any scope as you go below f/5, is my understanding.  It's not that it's so difficult as it is unforgiving.  Also, that 10" OTA is HEA-VY at one end, and a windsail, if either of those are an issue for you.

 

Regarding the discouraged comment about your experience with an SCT, I can also testify to the difficulty, especially going from something akin to a 500mm refractor.  Even at f/6.3, the jump is precarious.  Going from 300mm to 731mm, my RedCat to the ES Mak/Newt wasn't so bad, but throw in 1400mm of focal length and slow it to f/6.3 with the C8, the "acceptable" zone narrows A LOT.  One factor I think I overlooked, the local seeing doesn't always allow such magnification/resolution, so I'm gonna start being much more diligent to identify the sky conditions when trying to decide which telescope to use from night to night.  I think I may get far more use out of my C8 when using the Hyperstar than at f/6.3.  Just what I need, another widefield telescope!

 

Unless you're just dying for a reflector, I would suggest considering the 115mm triplet from Astro-Tech.  They appear to be very capable and at a good focal length.  At f/7, they're a bit slow, but that can be worked around.  800mm is a good focal length for mid-sized galxies and smaller galxy clusters.  The reduction to 644mm at f/5.6 would be well corrected, but if you can deal with the mis-shapen stars at the edges, which is usually cropped for galaxy images, the images at native focal length that I've seen are still nice.  The Mak/Newt route is also a good route as well, but will require a very tight collimation and can be a significant wind sail, but if you get past that, the images are very good.



#9 Andros246

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Posted 15 April 2024 - 10:07 AM

Sure does! aperture is always king.....

 

Not slow aperture though dont get caught there.

 

You take a 8in F6 over a 10in F12


Edited by Andros246, 15 April 2024 - 10:08 AM.

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#10 kathyastro

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Posted 15 April 2024 - 10:19 AM

More aperture means more photons.  BUT!... More focal length means fewer photons per pixel.  So, as terrestrial photographers have known for 150 years, in photography, it is focal ratio that rules for light gathering power.

 

All f/4 scopes and camera lenses deliver the same number of photons per unit of area.  All f/10 scopes and lenses deliver the same number of photons per unit of area.  This is why terrestrial photographers don't even measure the aperture of their lenses in millimetres.  They measure "aperture" in focal ratios.


Edited by kathyastro, 15 April 2024 - 10:20 AM.

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#11 HenkSB

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Posted 15 April 2024 - 10:19 AM

For large targets aperture is not important and a small refractor will work fine.  For smaller targets large aperture is needed, though.  Many galaxies are small enough to require larger aperture.  Unfortunately, to be on par with tracking it also requires buying a disproportionally expensive mount, preferably in an observatory for multi-night exposures and shielding from the wind and light pollution.



#12 imtl

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Posted 15 April 2024 - 10:42 AM

More aperture means more photons. BUT!... More focal length means fewer photons per pixel. So, as terrestrial photographers have known for 150 years, in photography, it is focal ratio that rules for light gathering power.

All f/4 scopes and camera lenses deliver the same number of photons per unit of area. All f/10 scopes and lenses deliver the same number of photons per unit of area. This is why terrestrial photographers don't even measure the aperture of their lenses in millimetres. They measure "aperture" in focal ratios.

Yes and no.

First, for point sources, i.e. your stars, it is the aperture alone that rules.

For extended objects, focal ratio dictates the irradiance at the focal plane. But now, it will depend on the sensor(s) used.

Assuming the object is always contained in the FOV, if in two different systems, where one has a larger aperture than the other, while having the same image scale then the system with the larger aperture will prevail.

In regards to terrestrial photography, there are only extended objects there hence the use of ''focal ratio'' as units of aperture. They also use ''crop sensors'' to describe focal length so I would not go by their terminology whatsoever.

Edited by imtl, 15 April 2024 - 10:42 AM.

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#13 Beeham

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Posted 15 April 2024 - 10:59 AM

I like to think about this by comparing three pairs of hypothetical scopes, each aimed at a single astronomical target, and each with an identical camera sensor.

 

Pair I: two 100mm-aperture telescopes, one is f/4 (400mm focal length) and one is f/8 (800mm focal length)

 

Pair II: two 800mm focal length telescopes, one is f/4 (200mm aperture) and one is f/8 (100mm aperture)

 

Pair III: two f/4 telescopes, one is 100mm aperture/400mm focal length, one is 200mm aperture/800mm focal length

 

When comparing the first two, they clearly collect the same total number of photons from the target, because they have the same aperture.  The f/4 has a shorter focal length, so it produces a wider view.  The wider view concentrates the light of the target over a smaller number of photoreceptors, so it requires less exposure time.  If you do the math, the f/4 scope image covers 1/4 as many photoreceptors, so each photoreceptor gets 4 times as many photons per unit time, and the image requires one quarter of the exposure time.

 

In the second pair, the focal lengths are the same, so the incoming photos get spread across identical numbers of photoreceptors on the the camera sensor.  The 200mm aperture is 4x the area of the 100mm aperture, and admits 4x as many photons per unit time, so it exposes four times faster.

 

The third pair is where it all comes together: the 100mm admits 1/4 as many photons per unit time compared to the 200mm aperture, but concentrates them on 1/4 the number of photoreceptors due to it's wider field of view from it's shorter focal length.  The two factors end up cancelling each other out, and both scopes end up requiring exactly the same exposure time.  Focal ratio is the sole determinant of exposure speed.

 

Now - the obvious question is "why bother with a huge heavy scope if focal ratio is all that matters, I'll just buy a tiny refractor with a fast focal ratio!"

 

The answer is resolution; when you gain exposure speed by increasing field-of-view and concentrating the light over a smaller number of photoreceptors, you produce a lower-resolution image.  The only way to get both fast exposure speed and high resolution is to combine long focal length and fast focal ratio...which gives you a resulting big aperture, and something like a big, heavy 10" f/4 reflector.

 

I hope that's helpful.  The relationship between speed, resolution, focal ratio, aperture, and focal length is, in my experience, not very intuitive.


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#14 jml79

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Posted 15 April 2024 - 11:08 AM

Yes, no, maybe. It's complicated. Aperture matters if you can use it and everything else is already spot on. I can ALMOST compensate for a lack of aperture with enough imaging time but no software can fix time wasted with bad guiding and only so much can be done to fix bad optics. 

 

Using a 1000mm scope and the most common pixel size of 3.76 you are at 0.78 arcseconds of sampling. This is pretty good for most seeing and is a happy place to be but your guiding has to be that much better (sub arc second) and the optics have to support the sampling and seeing and only if you can guide the light bucket around 0.6 arcseconds will you get to see the benefit of the setup. So yes more aperture means faster and technically finer detail but you are better with a slightly smaller, slower scope if it means better optics or guiding.

 

One of my rigs lives in the 0.67-0.84 sampling range and I love it but it is unforgiving to bad seeing and guiding and to much wind. And that's a 102mm on an EQ6R.


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#15 starslicer

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Posted 15 April 2024 - 11:08 AM

More light tends to equal detail but only to a point when compared against the focal length. Like was said above your mount is key. If I'm you, I get that Askar 140. Sure my C6 gets just as much light but I have to deal with mirrors, a small baffle and all kinds of other garbage. It creates a nice image though when everything is working.

 

I got different fish to fry right now though before I replace my C6. I'm currently fighting with the city about a small deck and a skyshed pod. They don't want me to put it anywhere on my lot.


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#16 smiller

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Posted 15 April 2024 - 11:47 AM

You see several factors mentioned here and here is my take on a few of them:

 

1) Focal ratio and the resulting concentration of light gathering power on each sensor pixel.  A lower f-ratio concentrates more light per pixel.   My take:  I don’t know why this is still considered a significant factor in determining system image quality.  This used to be critical with very high noise CCD cameras but with low read noise CMOS coupled with reasonably long exposures, it’s still only a factor in a few extreme corner cases.  For folks that still champion this as an important factor in common use cases, I would like to see the mathematical reasoning behind this.   Otherwise, I suggest we stop touting this as an important factor.   (Curiously enough, I happen to be one of these corner cases:  Imaging with my Dob using very short exposures!   And I can show the math of why it matters with 4-8 second exposures, especially with narrowband)

 

2) Focal length.  Obviously, all else being equal, a larger scope generally has a longer focal length.   So you just need to ask yourself:  With the camera you are going to use, do the targets of interest fit in your field of view?  This dictates the longest focal length you can live with.  This is of course assuming you have a mount and guiding system that can handle the larger scope at longer focal length.

 

3) So this leaves aperture.  With light gathering per pixel not really important and assuming the resulting focal length gives you enough FOV, the answer is simple:   The larger the aperture the more light you gather per second of your intended target.  A 12” aperture will have 4x the productivity of a 6”.  

 

 

So in summary: I would say if maximizing imaging productivity is your goal, then you generally want the largest aperture that still supports the desired FOV (i.e. lowest focal ratio) that your wallet and mount/guiding capabilities can handle.

 

 

I’m ignoring all the other zillion considerations: Ease of setup/weight, potential for more image resolution, ease of maintenance and quality of the optics of the particular choices (ex: giant refractor versus Newtonian),  etc…


Edited by smiller, 15 April 2024 - 11:55 AM.


#17 Mike7Mak

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Posted 15 April 2024 - 11:49 AM

I would scratch the Mewlon from the list, unless you're prepared to spend the cost of the scope on additional accessories to make it AP worthy and still end up with a rig that has a high focal ratio and small corrected field. A 6 or 8 inch RC is a much better choice for AP than the 7 inch Mewlon. Been there, done that.



#18 dswtan

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Posted 15 April 2024 - 11:49 AM

+1 starslicer. After imaging extensively on 8" SCTs and an 8" RC, my best DSO images to date have been on my largest refractor, the frequently well-regarded and cost-effective Astro-Tech AT130 EDT at 910mm f/7 (from our hosts Astronomics). The "fire and forget" simplicity of a decent quality refractor is hard to beat, vs. fighting collimation and/or reducers on all my reflectors.

 

That's for the stated goals of OP. In full disclosure, I'm keen to return to longer f/l for the smaller but more common galaxies and planetaries. Now I have my CEM70, I'll return again to an SCT but move to my reduced EdgeHD 11 (normally my planetary scope). Refractors over ~1000mm f/l are out of my league. But UNDER 1000mm, I'm 100% for refractors after years of experience.

 

Example quick first draft of M51 from my weekend processing. 9hrs RGB only. Cropped ASI294MM on the AT130.

 

M51 RGB120s CC BXTco GC SPCC BXT0306 NXT08 EZsoft EZhdr SXTlo CTsatcon NXT08 SCNR4 LHE64202mask CTsat NXT08 SCNR4 USM155 ComHa PM120starsCT 3

 


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#19 CrazyPanda

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Posted 15 April 2024 - 11:54 AM

1) Focal ratio and the resulting concentration of light gathering power on each sensor pixel.  A lower f-ratio concentrates more light per pixel.   My take:  I don’t know why this is still considered a significant factor in determining system image quality.  This used to be critical with very high noise CCD cameras but with low read noise CMOS coupled with reasonably long exposures, it’s still only a factor in a few extreme corner cases.  For folks that still champion this as an important factor in common use cases, I would like to see the mathematical reasoning behind this.   Otherwise, I suggest we stop touting this as an important factor.   (Curiously enough, I happen to be one of these corner cases:  Imaging with my Dob using very short exposures!   And I can show the math of why it matters with 4-8 second exposures, especially with narrowband)

 

I don't do DSO imaging, but in my neck of the woods, if you're lucky there are probably a grand total of 15-20 clear, moonless, dark hours per month to do imaging in. Imaging time is at an ultra-premium. You could very easily spend an entire season capturing data for just one target. And in fact it might take a few years to produce a good enough SNR for a single target if your light pollution levels aren't great. You literally have to look at how much more time you will be alive and prioritize what targets you plan on imaging before you die because of how rare good conditions are.......

 

In such conditions, IMO the only factor that matters is focal ratio. Everything else is a very, very, very distant secondary consideration. If target type and image scale is your primary consideration where you want to image small targets.... good luck to you. Between the bad seeing and long integration time needed from long focal ratios of affordable apertures, you're going to have a bad time imaging small targets in good clarity.

 

Yes, prioritizing short focal ratios for faster integration time will mean avoiding small targets. That's just how it is. You can either choose to spend ages capturing data for fewer small targets using long focal ratios, or gather more data more quickly for more big targets using short focal ratios (or spend 10s of thousands of dollars getting short focal ratios in big apertures so you can have the best of both worlds).

 

EDIT: For example, people who image with f/9 Ritchey-Chretien telescopes... you must have the patience of saints and the surplus of clear, dark skies to complement it.


Edited by CrazyPanda, 15 April 2024 - 12:07 PM.


#20 smiller

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Posted 15 April 2024 - 01:33 PM

I don't do DSO imaging, but in my neck of the woods, if you're lucky there are probably a grand total of 15-20 clear, moonless, dark hours per month to do imaging in. Imaging time is at an ultra-premium.

….

 

In such conditions, IMO the only factor that matters is focal ratio. Everything else is a very, very, very distant secondary consideration.
 

Let’s get specific.  You’re imaging M101 and you have two scopes:

 

An 6” SCT with hyperstar at f2, 300mm fl.   ASI2600MC camera

A 8” SCT with a f6.3 reducer, 1260 fl.  ASI2600MC Camera

 

Both fully frame M101.  Are you saying the 6” scope will capture M101 more quickly due to the smaller f-ratio?

 

If so, please explain your reasoning.


Edited by smiller, 15 April 2024 - 04:01 PM.


#21 kathyastro

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Posted 15 April 2024 - 02:07 PM

First, for point sources, i.e. your stars, it is the aperture alone that rules.

...

where one has a larger aperture than the other, while having the same image scale then the system with the larger aperture will prevail.

Re point sources, I agree.  With the qualification that even well-focused stars are not quite point sources.

 

Larger aperture while having the same image scale is a wordy and indirect way of saying lower focal ratio.


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#22 bbasiaga

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Posted 15 April 2024 - 02:22 PM

As others say...only indirectly does aperture rule.


Select the focal length you need to frame your target. Let's say that's about 550mm. Now you want as fast an f ratio as you can get. that 550mm can be an F/6 90mm objective,or an f/2 11" (sct with hyperstar).

Focal length drives field of view. Aperture then drives f/ratio (or vice versa).

Brian

#23 jml79

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Posted 15 April 2024 - 02:32 PM

Let’s get specific.  You’re imaging M101 and you have two scopes:

 

An 6” SCT with hyperstar at f2, 300mm fl.   ASI2600MC camera

A 10” SCT with a f6.3 reducer, 1260 fl.  ASI2600MC Camera

 

Both fully frame M101.  Are you saying the 6” scope will capture M101 more quickly due to the smaller f-ratio?

 

If so, please explain your reasoning.

The f/2 is over 10 times faster than f/6.3 but the difference between 6" and 10" is 2.7 times bigger. So my head may be in the wrong but the 6" f/2 would capture the image much faster but with a ridiculous difference in angular resolution and fov which would make the comparison not terribly useful.

 

Now Bin the camera on the 10" by 4 so we have the same blurry mess of angular resolution and we can actually learn something useful.


Edited by jml79, 15 April 2024 - 02:36 PM.

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#24 CrazyPanda

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Posted 15 April 2024 - 02:39 PM



Let’s get specific.  You’re imaging M101 and you have two scopes:

 

An 6” SCT with hyperstar at f2, 300mm fl.   ASI2600MC camera

A 10” SCT with a f6.3 reducer, 1260 fl.  ASI2600MC Camera

 

Both fully frame M101.  Are you saying the 6” scope will capture M101 more quickly due to the smaller f-ratio?

 

If so, please explain your reasoning.

Aperture is irrelevant to focal ratio for extended objects (which M101 is)

 

Could be 6" vs 10 meters, doesn't matter. F/2 is ALWAYS going to be (6.3/2)^2 = 10x faster. That is physics. Light concentration per unit area at F2 is 10x greater than F/6.3.

 

If you take any two different apertures, both at F/5, they will image at exactly the same speed. Their light will have spread out the same amount relative to their apertures. 2x the aperture does not mean 4x the speed if the focal ratio is the same. Same focal ratio = same speed.

 

But given the focal length differences, M101 is going to be less than 1/4th the size in the 6" @ 300mm vs the 10" @ 1260mm. The 10" will show considerably more detail assuming you have the patience to get the SNR to a reasonable point when imaging at F/6.3.

 

So you trade image scale for speed between those two setups. Personally I would not image M101 in the 6" @ F2. I would stick to imaging bigger targets more appropriate for a 300mm focal length.

 

At F/2, I'll get a good SNR of its appropriate targets faster than the F/6.3 setup will of its appropriate targets.

 

EDIT: and since I know it will come up - yes, I know you can just reduce the image size in the 10" @ F/6.3 to improve SNR. Yes, I know you can do binning. My answer to those things is "anything you can do, I can do better". Whatever you do to the images or camera in the F/6.3 setup, you could do in the F/2 setup. There's no inherent advantage there that can't also be translated to the F/2 setup. The F/2 setup is ALWAYS going to start you off with an advantage over F/6.3 in terms of speed of integration time.


Edited by CrazyPanda, 15 April 2024 - 03:05 PM.

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#25 bbasiaga

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Posted 15 April 2024 - 03:01 PM

The f/2 is over 10 times faster than f/6.3 but the difference between 6" and 10" is 2.7 times bigger. So my head may be in the wrong but the 6" f/2 would capture the image much faster but with a ridiculous difference in angular resolution and fov which would make the comparison not terribly useful.

Now Bin the camera on the 10" by 4 so we have the same blurry mess of angular resolution and we can actually learn something useful.

Focal length determines FOV. F ratio determines how fast the blight I gathered - we can think of this as per pixel SNR per unit time.

So you're right...the f/2 6" captures it 10x faster than the 6.3 10". But a much wider FOV.

Now let's look at f/2 system with the same focal length as that 10". Same zoomed in fov, but the same speed as the 6". MUCH bigger objective...on the order of 650mm!

Brian

Edited by bbasiaga, 15 April 2024 - 03:02 PM.



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