Seeing, Resolving Power and the rest.......

[scotty38]

Hi all I created this post as I don't mind admitting it still confuses me despite the best attempts of @vlaiv to educate me otherwise. I hope he doesn't mind but I stole part of one of his posts from another thread as this is what I want to speak about.

You can all read what he has written below but here is my scenario. Let's say I wanted to buy a longer scope for imaging galaxies and let's say I decided upon a 10" RC, the carbon fibre truss version that FLO sells for roughly £3k. Let's also say that I recognise I need some serious mount/guiding to be in place so I also buy a 10 Micron GM2000. In actual fact the specific scope/mount don't matter you get the point because my question is that given I'm in the UK and at 10m above sea level, looking at Vlaiv's words is there really any point in me having a setup that will image at this level?

 

On 09/11/2022 at 09:54, vlaiv said:

Resolving power of the telescope grows with aperture size. Large aperture resolves more than small

For given pixel size longer focal length will give higher sampling than shorter focal length (more pixels per arc second).

It turns out that these two grow the same for fixed F/ratio - thus if 6" scope has optimum sampling rate with F/15 for given pixel size - so with 8" scope as difference in resolving capability between 6" and 8" is the same as difference in sampling rate between 2250mm of FL and 3000mm of FL

Sampling is being measured in arc seconds - but there is simple relationship between pixel size in microns and arc seconds for given focal length (this shows that sampling rate in arc seconds is not fixed and grows with aperture since F/ratio is fixed - larger scopes resolve more in terms of arc seconds or have higher angular resolution).

There is relationship between mount RMS and FWHM that is given by FWHM = 2.355 * RMS

Stock Heq5 has guide RMS of around 1"

Modded/tuned Heq5 has guide RMS 0.7-0.8 and even down to 0.5" depending on all the mods applied (same is for EQ6 really).

Higher tier mounts can go as low as RMS 0.2"-0.3" (Mesu200 for example).

Average seeing is 2" FWHM. Good seeing is 1.5" FWHM. Excellent seeing is 1" FWHM and below (below 1" happens only on mountain tops / deserts - special sites).

You will often find that seeing is below 2" - on a cloudy day / night - but when it is night time and clear - it is going to be around 2" (stable atmosphere helps with cloud formation).

In the end - here is sort of rule of the thumb related to aperture size:

with <100mm scopes - limit yourself to widefield imaging as you'll be limited to say about 2"/px

100mm - 150mm - upper sampling limit would be 1.8"/px

150mm - 200mm - 1.5"/px

above 200mm - 1.3-1.4"/px

One can attempt 1"/px - only on the best of nights using very good mount (RMS 0.5 or less).

In all reality - odds are - you won't get image that has detail below 1"/px in 99.99% conditions / equipment.

 

 

 

 

[vlaiv]

35 minutes ago, scotty38 said:

is there really any point in me having a setup that will image at this level?

For me there is, if you can afford it and "field"/operate it, but question is - what do you hope to achieve?

Sure, someone with excellent seeing and maybe not so good equipment (like 8" scope) will be able to match your image as far as resolution goes, but resolution is only part of the story.

There is also speed part. Larger scopes have potential to be faster (if matched with appropriate camera and handled accordingly).

Here is an example.

You take 10" RC and pair it with say ASI6200, while someone else takes APO from LZOS 130 (so 5.1") and pairs it with ASI533 or ASI183

That will give you same FOV, so images will be comparable - but you will image yours with 10" of aperture for photon collecting while someone else will do that with only 5" -  so 1/4 of aperture by surface - or you will be about x4 faster if you also match resolution / sampling rate (which I would advise you to do).

Even if person with 5" APO uses larger sensor - they will need to crop on most galaxies to get nicely framed result. Most of the galaxies are small enough for smaller FOV offered by longer FL.

While larger telescope and better mount can provide you with higher resolution - even if your seeing prevents that from happening - there is always speed factor to take into account.

[scotty38]

Ok thanks so cost aside if I can operate it correctly if nothing else I have the speed to take advantage of. Has to be said that’s a high price to pay though as I’m not picking up on much else really yet plenty of folk have them and surely I can’t be the only one not on the top of a mountain 😀

thanks for the feedback Vlaiv, much appreciated!

[wuthton]

3 hours ago, vlaiv said:

 

You take 10" RC and pair it with say ASI6200, while someone else takes APO from LZOS 130 (so 5.1") and pairs it with ASI533 or ASI183

 

Colour me confused, my apologies if I've misread your post but isn't the LZOS 130/ASI183 (F6, 0.63"/px) significantly faster than the 10" RC/ASI16200 (F8, 0.38"/px), regardless of the aperture?

[scotty38]

24 minutes ago, wuthton said:

Colour me confused, my apologies if I've misread your post but isn't the LZOS 130/ASI183 (F6, 0.63"/px) significantly faster than the 10" RC/ASI16200 (F8, 0.38"/px), regardless of the aperture?

Hope I get this bit right at least but in telescopes focal ratio, f8, f6 etc only really impacts the field of view all other things being equal, it's the aperture that has the big impact on speed. I'm sure someone else will give you the warts and all but I think that's the gist of it.

Edited November 14, 2022 by scotty38

[AstroMuni]

2 hours ago, scotty38 said:

so cost aside if I can operate it correctly if nothing else I have the speed to take advantage of.

+1 .... for folk like me who can only image during weekends and NOT having an obsy and dont wish to break the bank, thats an important consideration (esp when you live in UK and are at the mercy of the clouds). So the choices for me are large aperture, fast Newts. 🙂

Edited November 14, 2022 by AstroMuni

[scotty38]

3 minutes ago, AstroMuni said:

+1 .... for folk like me who can only image during weekends and NOT having an obsy and dont wish to break the bank, thats an important consideration (esp when you live in UK and are at the mercy of the clouds). So the choices for me are large aperture, fast Newts. 🙂

Yes, if you're saying what I think you're saying then there are cheaper ways to achieve speed........ Let's park ultimate quality as my example probably wasn't at that point anyway.....

Edited November 14, 2022 by scotty38

[AstroMuni]

1 minute ago, scotty38 said:

Yes, if you're saying what I think you're saying then there are cheaper ways to achieve speed........

The way I see it is - for a given FOV the best bang for buck is a Newt with the largest aperture that your mount can handle AND if you can get a faster F-ratio then you will need to spend less time imaging! As an example for my HEQ5 Pro you can get a SW Quattro 150 f/4 with a coma corrector that makes it even faster at f2.5 for around £499. If I wanted to get something like this in a refractor with a 150mm aperture, I would need to spend a lot-lot more and the HEQ5 would probably not be capable of taking its weight 🙂

[scotty38]

2 minutes ago, AstroMuni said:

The way I see it is - for a given FOV the best bang for buck is a Newt with the largest aperture that your mount can handle AND if you can get a faster F-ratio then you will need to spend less time imaging! As an example for my HEQ5 Pro you can get a SW Quattro 150 f/4 with a coma corrector that makes it even faster at f2.5 for around £499. If I wanted to get something like this in a refractor with a 150mm aperture, I would need to spend a lot-lot more and the HEQ5 would probably not be capable of taking its weight 🙂

Agreed it's much more cost effective than a refractor no doubt but it's not a galaxy scope unlike the 10" RC at 2000mm, yes it's f8 but it's still 254mm so still plenty of aperture so it's not "fast" in those f ratio terms. This takes us back to my original question (I think) though actually as I got sidetracked with the speed question 🙂 🙂

[wuthton]

15 minutes ago, scotty38 said:

Hope I get this bit right at least but in telescopes focal ratio, f8, f6 etc only really impacts the field of view, it's the aperture that has the big impact on speed. I'm sure someone else will give you the warts and all but I think that's the gist of it.

Focal length has the big effect on field of view, more aperture (at the same focal length) gives you more speed. Larger pixels on your camera also give you more speed. 

This is certainly an interesting example as an F6 at a lower sampling rate should be faster than an F8 at a higher sampling rate but this is the same field of view, and the ASI16200 has larger pixels which leads me to think that I might be missing something.

[AstroMuni]

25 minutes ago, scotty38 said:

Agreed it's much more cost effective than a refractor no doubt but it's not a galaxy scope unlike the 10" RC at 2000mm, yes it's f8 but it's still 254mm so still plenty of aperture so it's not "fast" in those f ratio terms. This takes us back to my original question (I think) though actually as I got sidetracked with the speed question 🙂 🙂

When you add many variables then it makes it harder, but my assumption is that your mount and camera are fixed. So in my case with a pixel size of 3.76 on ASI533 and using the 150/600 quattro I would get 1.29"/px without the CC (@f4) and 1.5"/px with the CC (@f2.5). To get to < 1"/px I would have to go for larger FL scopes.

EDIT: with the CC its f3.45

Edited November 14, 2022 by AstroMuni

[scotty38]

13 minutes ago, wuthton said:

Focal length has the big effect on field of view, more aperture (at the same focal length) gives you more speed. Larger pixels on your camera also give you more speed. 

 

I did add "all things being equal" as you mentioned 130mm f6 versus 254mm f8. If we take 130mm as being fixed then f6, f8 etc etc won't make it faster, it'll only change the fov because the focal length has changed of course. That's what I was trying to say, maybe not very well I accept.

[AstroMuni]

8 minutes ago, scotty38 said:

If we take 130mm as being fixed then f6, f8 etc etc won't make it faster, it'll only change the fov because the focal length has changed of course.

Didnt understand that bit 😞 If you compare a 130/f5 and a 130/f4 then the latter would need less exposure time to get same level of detail, is my understanding. Have I misunderstood what you are saying?

[Fegato]

You have to take account of focal length and aperture really (and then the camera). Not really much help saying something is "faster" if the FOV is completely different. But if you have two similar focal lengths, and one has larger aperture - it will be quicker for a similar FOV (assuming same camera).

 

[CraigT82]

1 hour ago, wuthton said:

Colour me confused, my apologies if I've misread your post but isn't the LZOS 130/ASI183 (F6, 0.63"/px) significantly faster than the 10" RC/ASI16200 (F8, 0.38"/px), regardless of the aperture?

Actually, no. If we take 'faster' as meaning taking less time to acheive a specific SNR in an image.

Very useful calculator here by @dan_adi.  Plugging in the scopes outlined above and subbing in the 533 in place of the 6200 as it's not in the app database (same pixel size) you can see the 10" RC setup is faster than the 5" frac. Have to make some assumptions about reflectivity/light throughput.

 

http://clearskies.go.ro:8501/

[vlaiv]

1 hour ago, wuthton said:

Colour me confused, my apologies if I've misread your post but isn't the LZOS 130/ASI183 (F6, 0.63"/px) significantly faster than the 10" RC/ASI16200 (F8, 0.38"/px), regardless of the aperture?

No

Ok, on one side - you might be right, and it could be faster as is - but we would need to calculate that.

Speed is defined as "aperture at resolution" - or to be more precise, aperture surface times pixel surface (in arc seconds)

So LZOS would natively be 130^2 * 0.63^2 = ~6707 Units and 10" RC would be 254^2 * 0.38^2 = 9316 Units

10" RC would be about x1.5 faster natively (if everything else is equal - QE of sensor, transmission of optics and so on).

However, part of original post is quoting what can be achieved in terms of resolution - and I advise for both scopes not to aim over their maximum.

I would bin 130 at least x2 to get 1.26"/px and I would bin 10" RC at least x3 to get 1.14"/px

When we do that, applying again above formula we get:

130^2 * 1.26^2 = ~26830

254^2 * 1.14^2 = ~83845

That is 83845 / 26830 = ~3.125 times faster.

(if you wonder why we multiply squares above - from each point in the sky - all photons that fall on aperture are focused in single point - aperture surface is proportional to square of diameter, and all those photons in all points that fall  on one pixel are added up to form pixel signal - there is "surface in the sky" that is covered by pixel number of such points - which is again proportional to pixel side squared).

That is why I said that 10" scope can be roughly x4 faster. In fact, if you exactly match sampling rate between the two (by binning or using appropriate sized pixels) - 10" scope will be exactly x4 faster than 5" scope (10 * sampling rate / 5 * sampling rate)^2 = (10/5)^2 = 2^2 = 4.

Make sense?

[vlaiv]

1 hour ago, AstroMuni said:

The way I see it is - for a given FOV the best bang for buck is a Newt with the largest aperture that your mount can handle AND if you can get a faster F-ratio then you will need to spend less time imaging! As an example for my HEQ5 Pro you can get a SW Quattro 150 f/4 with a coma corrector that makes it even faster at f2.5 for around £499. If I wanted to get something like this in a refractor with a 150mm aperture, I would need to spend a lot-lot more and the HEQ5 would probably not be capable of taking its weight 🙂

Well, how about 8" RC on Heq5?

You see, if one wants to image galaxies - one does not need much in terms of FOV. Most galaxies will be nicely framed by FOV that is half a degree or so. There are only hand full of galaxies that are larger than that.

Now, we have half a degree - say in width. That is 30 arc minutes or 30 x 60 = 1800 arc seconds.

If we assume that we are going to obey physical limits of what the telescope can resolve in long exposure - then there is no point in going above 1.5"/px.

That is only 1200px in width.

Most newer DSO cameras will waste FOV on such a fast scope.

On the other hand, If we bin camera to get larger pixels - then it really makes sense to get camera with say 3600px or 4800 px in width. Then we need scope with enough focal length to bring those binned pixel to wanted resolution.

Fast scopes come with their fair bit of headache inducing problems.

For one - they are not diffraction limited, which means that their spot diagram will further limit sharpness of the image / achievable resolution.

Speed is not always in F/number, and once you account for everything - newtonians might not be the cheapest scopes .  Both 6" RC and 6" CC are less than £500 - and don't need corrector over 4/3 sensor. They are light and compact and will ride well on HEQ5

 

[vlaiv]

1 hour ago, Fegato said:

You have to take account of focal length and aperture really (and then the camera). Not really much help saying something is "faster" if the FOV is completely different. But if you have two similar focal lengths, and one has larger aperture - it will be quicker for a similar FOV (assuming same camera).

 

You don't have to have similar focal length.

Similar focal length is important if you want to keep the pixel size the same - but you don't have to do that.

Imagine you have F/5 scope of say 4" and F/10 scope of 4".

99% of people will say that F/5 scope is faster than F/10 scope - but that is not true. In fact there is no true statement.

F/5 can be faster than F/10

F/5 can be equally fast than as F/10

F/5 can be slower than F/10

F/ratio is not indicator of speed. It is only useful for comparison if we keep pixel size fixed across setups (use the same camera).

If we use the same camera as is on F/5 and F/10 - F/5 will be faster

If we use the same camera on F/5 and F/10 - but on F/10 we bin pixels x2 - they will be the same speed (twice larger pixel cancels out twice longer focal length - as sky surface is the same and aperture surface is the same because both are 4" scopes).

If we use camera with larger pixels on F/10 and bin x2 and we use camera with smaller pixels on F/5 natively - F/10 will be faster.

 

Edited November 14, 2022 by vlaiv
I type too fast for my brain to follow ... :D

[Lee_P]

Re: 8" RC, one for sale here 😁

 

[900SL]

Bookmarked. 

[AstroMuni]

15 hours ago, vlaiv said:

Both 6" RC and 6" CC are less than £500 - and don't need corrector over 4/3 sensor. They are light and compact and will ride well on HEQ5

Thats a good suggestion and one to keep in mind for next buy. 🙂 

Since I have the ASI224 & ASI533 I can get both zoomed in and wider field views on a 130 or 150mm scope. Both cameras have similar pixel size too.

[scotty38]

@vlaiv Let's say I did buy a Stellalyra 10" RC and used it with my QHY 294m, that would give me a resolution of 0.48"/pixel.

If I intended binning later x3 or even x4 to give me either 1.43" or 1.91" do I still need to guide at 0.48" in the first instance?

Apologies if this is a dim question but in my head I think I can make arguments both ways......

[vlaiv]

6 hours ago, scotty38 said:

@vlaiv Let's say I did buy a Stellalyra 10" RC and used it with my QHY 294m, that would give me a resolution of 0.48"/pixel.

If I intended binning later x3 or even x4 to give me either 1.43" or 1.91" do I still need to guide at 0.48" in the first instance?

Apologies if this is a dim question but in my head I think I can make arguments both ways......

I'm not sure that I understand what you are asking?

Guiding is independent of resolution at which you are imaging. You can guide the mount and not have scope and camera on it at all.

What you want to do is get your guiding and mount performance in general to the best level you can comfortably achieve.

Guide RMS is contributing factor in overall FWHM that you'll be able to achieve. Other two being seeing and aperture size (for diffraction limited scope or spot RMS in general).

Those things combined will determine how sharp your image is and whether you are over sampling or not given your target sampling rate. It is the only factor that you have some influence over after you've chosen our scope.

You can't influence seeing. Once you have decided on your scope, only way to change its aperture size is to replace it (same thing goes for mount of course). Only thing that you can somewhat tweak is guiding parameters and mount in general (like some mods to make it more stable, more precise).

We often say things like "You need to guide at half of our sampling rate", but these are very rough guidelines. What we should really be saying - make your mount performance the best you can (and that is really inline with other recommendation - and that is that mount is the most important part of imaging kit) as it will in part be determining factor whether you should bin x3 or x4 later (if your primary goal is high resolution).

[scotty38]

1 hour ago, vlaiv said:

I'm not sure that I understand what you are asking?

Guiding is independent of resolution at which you are imaging. You can guide the mount and not have scope and camera on it at all.

What you want to do is get your guiding and mount performance in general to the best level you can comfortably achieve.

Guide RMS is contributing factor in overall FWHM that you'll be able to achieve. Other two being seeing and aperture size (for diffraction limited scope or spot RMS in general).

Those things combined will determine how sharp your image is and whether you are over sampling or not given your target sampling rate. It is the only factor that you have some influence over after you've chosen our scope.

You can't influence seeing. Once you have decided on your scope, only way to change its aperture size is to replace it (same thing goes for mount of course). Only thing that you can somewhat tweak is guiding parameters and mount in general (like some mods to make it more stable, more precise).

We often say things like "You need to guide at half of our sampling rate", but these are very rough guidelines. What we should really be saying - make your mount performance the best you can (and that is really inline with other recommendation - and that is that mount is the most important part of imaging kit) as it will in part be determining factor whether you should bin x3 or x4 later (if your primary goal is high resolution).

Thanks and yes maybe not the best question but you did sum it up with "We often say things like "You need to guide at half of our sampling rate"" as I think that's the basis of what I was asking.

Given you essentially said guide as best as you can are you saying that due to seeing etc etc that even with a setup that "could" deliver 0.48" even if I can still only guide at 0.8" with binning in software later I could still make good use of the kit? Maybe not as good as it could be but not a complete and utter waste of time?????

I'm purposely trying to give a bad example to try and make my point by the way 🙂

[ollypenrice]

On 14/11/2022 at 15:44, wuthton said:

Colour me confused, my apologies if I've misread your post but isn't the LZOS 130/ASI183 (F6, 0.63"/px) significantly faster than the 10" RC/ASI16200 (F8, 0.38"/px), regardless of the aperture?

As vlaiv has ably demonstrated, the answer is 'no.'  However, I'll offer an equivalent but alternative explanation, more verbal than mathematical.  You are, essentially, asking, 'Doesn't F ratio over-ride aperture?'  If you asked this of a daytime photographer they would probably say,  'How can it? F ratio is aperture.' In the shorthand of daytime photography, that's how F ratio or F stop is used. This isn't formally correct but it works because, in their context, the focal length is fixed. If they switch a lens from F5 to F10 they open the iris and increase the aperture.  In their case, the only variable is aperture since focal length is fixed, so lowering the F ratio physically increases the aperture. That's why they use F ratio and aperture interchangeably.   In astrophotography we have two variables, aperture and focal length, so F ratio is not pinned to just one of these values as it is with a camera lens.

We, therefore, have to wary of the term F ratio since, out of context, it has little value.

Olly