Impact of aperture for astrophotography

[Lee_P]

Ever find that as you gain knowledge, you start to question your understanding of more fundamental topics? I'm getting that right now. I wonder if someone would be so kind as to answer this question: assume you've got two telescopes that have very similar specifications, but one has a larger aperture than the other. The bigger one has more light-gathering ability; but what does that actually translate to in real terms for astrophotography? Shorter total integration time needed to get the same signal-to-noise ratio as its smaller counterpart? Sharper images? Something else?

Thanks in advance!

[ONIKKINEN]

Im a bit of a beginner in most ways, but as i understand it both are true to a point. So sharper and better SNR. Sharper probably only with planetary as long exposure is seeing limited. Well maybe long exposure is affected with very small apertures in sharpness, but generally i think all other effects are greater.

 

Photons dont arrive as steady streams from dim targets, but as just a likelyhood of x per aperture area. As the area increases, photons hitting the aperture become more and more likely. If the camera and focal length stay the same SNR must improve.

[Stu]

You will likely have to specify if it’s the focal length or focal ratio which are remaining the same as the aperture increases in order to get valid answers.

[Lee_P]

46 minutes ago, Stu said:

You will likely have to specify if it’s the focal length or focal ratio which are remaining the same as the aperture increases in order to get valid answers.

Ok, good point. Let's say the focal length stays the same. The focal ratio will be lower for the larger aperture telescope, given that focal ratio = focal length / aperture, right? Makes sense, bigger aperture means you're collecting more light. But then if you have the smaller telescope, you could just aim for a longer total integration time to then match the bigger telescope. So what are the other benefits? Increased resolution, i.e. being able to separate closer objects? 

[vlaiv]

There is some increase in resolution for DSO imaging - but not as much as for planetary imaging.

For planetary / lucky type imaging, resolution grows linearly with aperture diameter.

For DSO / long exposure things are not quite so straight forward. Aperture combines with seeing and mount performance to produce final FWHM. This is for diffraction limited optics. Once you start adding reducers / flatteners / coma correctors and examine FWHM over larger field - things get even worse, as scopes tend not be diffraction limited in those cases.

As far as speed is concerned - that really depends.

I would put it like this - larger aperture scope has potential to be part of faster setup. We can define speed of setup by simple metric of "aperture at resolution". Resolution is determined by focal length and pixel size.

Speed grows with aperture size, and falls with increase in resolution - 200mm at 2"/px is faster than 100mm at 2"/px, and 100mm at 2"/px is faster than 100mm at 1"/px.

If you change both parameters - aperture and resolution - then you have to take their squares in order to compare them - so something like aperture^2 * sampling rate ^2.

Comparing 200mm at 1"/px vs 100mm at 2"/px would then give you 200^2 * 1^2 = 40000 versus 100^2 * 2^2 = 40000

So 200mm aperture at 1"/px is as fast as 100mm at 2"/px.

There is another very important aspect of speed - that favors big scopes.

Big scopes tend to illuminate big sensors - and that is advantage.

Say you compare 8" F/8 scope and ASI1600 to 16" F/8 scope and ASI6200. They will produce same FOV, can be binned to the same resolution and at same resolution 16" will clearly win as it has x4 aperture surface.

Thing is - you can't illuminate ASI6200 with 8" F/8 RC - at least not natively without flattener. And even with flattener - it is really question of how good corner stars will be at such large sensor.

 

[Lee_P]

10 hours ago, vlaiv said:

There is some increase in resolution for DSO imaging - but not as much as for planetary imaging.

For planetary / lucky type imaging, resolution grows linearly with aperture diameter.

For DSO / long exposure things are not quite so straight forward. Aperture combines with seeing and mount performance to produce final FWHM. This is for diffraction limited optics. Once you start adding reducers / flatteners / coma correctors and examine FWHM over larger field - things get even worse, as scopes tend not be diffraction limited in those cases.

As far as speed is concerned - that really depends.

I would put it like this - larger aperture scope has potential to be part of faster setup. We can define speed of setup by simple metric of "aperture at resolution". Resolution is determined by focal length and pixel size.

Speed grows with aperture size, and falls with increase in resolution - 200mm at 2"/px is faster than 100mm at 2"/px, and 100mm at 2"/px is faster than 100mm at 1"/px.

If you change both parameters - aperture and resolution - then you have to take their squares in order to compare them - so something like aperture^2 * sampling rate ^2.

Comparing 200mm at 1"/px vs 100mm at 2"/px would then give you 200^2 * 1^2 = 40000 versus 100^2 * 2^2 = 40000

So 200mm aperture at 1"/px is as fast as 100mm at 2"/px.

There is another very important aspect of speed - that favors big scopes.

Big scopes tend to illuminate big sensors - and that is advantage.

Say you compare 8" F/8 scope and ASI1600 to 16" F/8 scope and ASI6200. They will produce same FOV, can be binned to the same resolution and at same resolution 16" will clearly win as it has x4 aperture surface.

Thing is - you can't illuminate ASI6200 with 8" F/8 RC - at least not natively without flattener. And even with flattener - it is really question of how good corner stars will be at such large sensor.

 

Thanks vlaiv, that’s helpful. Broadly speaking then, is it accurate to say that a larger aperture:

* Helps a lot to increase resolution for planetary imaging;
* Helps to increase resolution (i.e. sharpness and ability to separate close objects) for DSOs, but seeing and mount performance are also big factors;
* Makes a faster system (i.e. lower focal ratio) possible;
* Is important to get good quality all the way to the edges of a large sensor.

[vlaiv]

55 minutes ago, Lee_P said:

Thanks vlaiv, that’s helpful. Broadly speaking then, is it accurate to say that a larger aperture:

* Helps a lot to increase resolution for planetary imaging;
* Helps to increase resolution (i.e. sharpness and ability to separate close objects) for DSOs, but seeing and mount performance are also big factors;
* Makes a faster system (i.e. lower focal ratio) possible;
* Is important to get good quality all the way to the edges of a large sensor.

Yes, except maybe avoid use of lower focal ratio in context of speed.

It is regularly used that way - but that only holds true if you use fixed resolution - like comparing impact with same camera without introduction of binning.

As soon as you introduce resolution in the story - things change.

Think of it this way: larger aperture often means larger focal length. Larger focal length means higher resolution with same pixel size. Binning reduces resolution and brings it back to starting one - in the end of this process we have larger aperture at same resolution - which is faster system.

It is aperture at resolution that is important bit.

[newbie alert]

15 hours ago, vlaiv said:

Big scopes tend to illuminate big sensors - and that is advantage

Not really ..a 8inch sct is a big scope aperture wise

A sharpstar 61 edph is a small scope aperture wise

So which has the bigger imaging circle to illuminate a big sensor

[vlaiv]

1 hour ago, newbie alert said:

Not really ..a 8inch sct is a big scope aperture wise

A sharpstar 61 edph is a small scope aperture wise

So which has the bigger imaging circle to illuminate a big sensor

As you get above say 10" or so - most telescope come with 3" or larger focusers - that is because of field illumination.

SCT is really not very good DSO imaging scope. If you want to compare - compare it with EdgeHD, or maybe RC type scopes.