Aperture vs Seeing

[ollypenrice]

27 minutes ago, Northernlight said:

Olly out of interest, can you post the full versions of those images as I would like to compare them side by side without pixel peeping 

They are a bit big but let's see what we can get away with...

First the reflector image:

...and then the TEC 140

 

The reflector used an SXVH36 (Kodak) full frame camera, the TEC an Atik 460 (Sony) with much smaller chip and smaller pixels. As mentioned earlier, the refractor image had a lot less exposure and has gone deeper but this will be more down to camera than much else. Any theorist wanting to insist that more aperture has to be faster will probably have to rely on this explanation. For those who believe in F ratio per se there is not much in it, F6.8 for the reflector, F7 for the refractor (or F7. a bit because the TEC flattener slightly extends the focal length by an amount which TEC don't disclose. It will be about 30mm.)

Hmmm, when I look through a telescope stars are round... ?

lly

 

[newbie alert]

7 hours ago, Northernlight said:

Olly, put us out of our misery, which one is which 

The spike on the large star gives it away I think

[Northernlight]

yup but when olly posted the original images, it was cropped at center of galaxy and that very obvious star was not in the original image when i posted that comment......

 

Olly, for me the ODK image looks better, not necessarily becuase of the resolution, the ODK image just looks more natrual, but i think a lot of that might come down to processing as the Tec image looks a little overcooked on the processing side as if the curves have been pushed just a little too far as well as a tad too much sharpening on the core.

So with similar processing, i think the difference could be less pronounced between the 2 images and close the gap a bit.....

It's certainly food for thought.......Do i want a bit of extra resolution or do i want a simple life..........i think that's what it comes down too rather than comparing apertures.

 

 

[ollypenrice]

14 minutes ago, Northernlight said:

yup but when olly posted the original images, it was cropped at center of galaxy and that very obvious star was not in the original image when i posted that comment......

 

Olly, for me the ODK image looks better, not necessarily becuase of the resolution, the ODK image just looks more natrual, but i think a lot of that might come down to processing as the Tec image looks a little overcooked on the processing side as if the curves have been pushed just a little too far as well as a tad too much sharpening on the core.

So with similar processing, i think the difference could be less pronounced between the 2 images and close the gap a bit.....

It's certainly food for thought.......Do i want a bit of extra resolution or do i want a simple life..........i think that's what it comes down too rather than comparing apertures.

 

 

I've nothing to argue against in any of that. If the thread has helped you in your decision then the forum's working as it should!

Olly

[Northernlight]

Well not quite made my decision yet, but it's certainly helped, so many thanks for presenting your images. Unfortunately like most things in Astronomy the more you dig the more worms you unearth.

Whilst I know in my heart a 150mm would be the easier option, and would be very much plug and play in comparison to a newt, I'm now faced with the question : - do I want  to pay £2,000 or £4,000 for a scope.

[ollypenrice]

14 minutes ago, Northernlight said:

I'm now faced with the question : - do I want  to pay £2,000 or £4,000 for a scope.

Of course you do! But, naturally enough, you want to blame the rest of us. That's what we all do. I never wanted to buy a TEC140 but SGL made me do it, bless 'em! It's a self-help community... We all aim to please.

lly

 

[Northernlight]

So Olly can I also send my wife in your direction when she finds out that I spent a small fortune.......Olly made me do it.

My wife's pending rage aside, I genuinely can't decide between a 150mm apo and a 10" newt 

[Rodd]

The dust lanes look better in the refractor image--no question--they are almost completely missing in the 14" image--I submit that the 14" is better resolved--but the refractor has more detail (count the dust lanes)--The original crops I refer to.

Rodd

[DaveS]

I see the dust lanes just fine in the ODK image, though will concede the TEC has more contrast. I do not know if that is intrinsic (I suspect it is, Apos are known for their contrast) or due to processing.

[vlaiv]

2 hours ago, DaveS said:

I see the dust lanes just fine in the ODK image, though will concede the TEC has more contrast. I do not know if that is intrinsic (I suspect it is, Apos are known for their contrast) or due to processing.

It is due to processing alone. Optics characteristics don't play a part in this level of contrast in images.

There are two types of contrast for which refractors can have edge over mirrored system of same aperture and light gathering power. Neither of them has impact on DSO imaging such that would manifest in contrast in the image.

First one is fairly simple to understand and it can be noticed when observing night sky around bright stars - its light scatter type of contrast. Mirror systems need to be very smooth in order not to produce this effect - it's often said for high quality smooth mirrors (we are not talking about figure here, but rather micro smoothness) that they have "refractor like" image. Mirror reflect most of the light as they are designed to, but some small portion of the light gets reflected all over the place. This makes sky around very bright stars look slightly washed out comparing to refractor view - sky is darker in refractor because of this (on same magnification). This is low power effect.

Other effect where it is said that refractors have more contrast is very high magnification thing - it relates to MTF of aperture and here central obstruction plays a part - it's about resolving planetary detail for example. Refractor of same aperture will provide more contrasty image of tiniest features resolved on plants, because it's unobstructed scope. Again this holds for scopes of same aperture. 8" Newtonian will have higher contrast on same scale features over 5" refractor for example (and show smaller features than 5" is able to do - it has higher resolving power).

Contrast in image above is only due to processing and SNR. Resolution is to coarse for MTF type contrast to have impact, as resolution of both images is dominated by seeing / guiding rather than aperture alone (aperture adds small difference observed, but this is still no planetary level of detail in images). Other type of contrast act just like slightly higher LP levels - it impacts SNR, but with enough exposure you can easily overcome that.

[DaveS]

Thanks for clarifying that Vlaiv.

[Northernlight]

Sod it, I'm up for a challenge, 10" F5 newt with quartz mirror @ half the price and double the trouble 

[jimjam11]

This is very closely related to my thread from a few weeks back, trying to work out where my FWHM values were coming from especially because I am suspicious of the mirror in my 150p.

It definitely appears as though less aperture can keep up with more in all but the best conditions we get here?

Having said that, the required fettling of the newt gives you something to do on the 350+ nights of cloud each year...

 

[Northernlight]

That mirror looks like the coating has broken down, so optically speaking the surface will be very rough.

[jimjam11]

On 14/03/2019 at 11:46, vlaiv said:

Looking back at above formulae, I'm not sure they are correct!

I'm often get confused by this - "Airy disk size", sometimes people use diameter and sometimes radius. We need to check above approximation of Airy disk with Gaussian, because I think that I over estimated sigma for Gaussian by factor of 2 (I was looking at formula for radius instead for diameter).

According to wikipedia article on airy disk, and other sources, first minima in airy pattern appears at sin(angle) = 1.22 * lambda / aperture_diameter, that would be the radius, and diameter is therefore 2.44 * lambda / aperture_diameter. Gaussian approximation (on the same page) gives sigma to be 0.42 * lambda / aperture_diameter ( there is actually another numerical constant here 0.45, depending if one approximates by volume or by peak intensity, not sure which one should be used - probably one that gives least sum squares error).

Reference: https://en.wikipedia.org/wiki/Airy_disk

According to this page:

http://www.wilmslowastro.com/software/formulae.htm#Airy

Diameter of airy disk is given by: 2.44 * lambda * F/ratio, and angular size (I think it now stands for Airy diameter) is given by 2 * arctan( 1.22 * lambda / apereture_diameter),

This means that sigma in radians would be 2 * arctan( 0.42 * lambda / 2*aperture_diameter) - two times arc tan of half of "pattern", but we can use small angle approximation here so it would be 2 * 0.42 * lambda / 2 * aperture_diameter, and two's cancel out, we have 0.42 * lambda / aperture_diameter that we need to convert from radians into arc seconds.

(0.42 * lambda / aperture_diameter) * 180 / PI * 60 * 60  = 3600 * (0.42 * lambda / aperture_diameter) * 180 / PI

Where lambda and aperture_diameter are in same units - meters or mm or um, ... - it does not matter since unit cancels out, but we need to convert one to other because wavelength is specified in nm and aperture in mm.

(60 arc minutes in one degree, 60 arc seconds in one arc minute)

If above is correct, and again someone better check if it is as half the time I'm likely to get it wrong by factor of 2, then above one that I used is not correct as it has 7200 * ..., instead of 3600 * ...

Well, good news if this is now correct! Ratio of 5" to 10" scope resolving power won't change much, and one still needs very good conditions to see the difference, but it is actually feasible to use 1"/px resolutions under good circumstances.

For example, with this reduced Airy sigma, 10" can utilize 1"/px with 1" FWHM seeing and 0.5" RMS guiding.

 

I cannot quite match your results using the linked calculator for the airy disc:

Diameter=250mm

Airy disc (ad) = 1.03"

Guide rms (g)= 0.5"

Seeing FWHM (s)= 2"

ad^2=1.0609

g^2=0.25

(s/2.355)^2=0.72

Sqrt(ad2+g2+s2)=1.42

× 2.355 = 3.36"

If I repeat for a 127mm diameter I get

ad=2.18"

The resultant FWHM is therefore 5.63"

This seems very high?

[vlaiv]

14 minutes ago, jimjam11 said:

I cannot quite match your results using the linked calculator for the airy disc:

Diameter=250mm

Airy disc (ad) = 1.03"

Guide rms (g)= 0.5"

Seeing FWHM (s)= 2"

ad^2=1.0609

g^2=0.25

(s/2.355)^2=0.72

Sqrt(ad2+g2+s2)=1.42

× 2.355 = 3.36"

If I repeat for a 127mm diameter I get

ad=2.18"

The resultant FWHM is therefore 5.63"

This seems very high?

Ok, let's do the case for 250mm diameter, guide RMS of 0.5" and seeing of 2" FWHM.

RMS we leave as is (it is already sigma) - so s1 = 0.5"

Seeing we need to "convert" from FWHM value to sigma, and that is done by dividing with 2.355, so s2 will be 2"/2.355 = ~ 0.85" = s2

For sigma of gaussian approximation of airy disk we use: 3600 * (0.42 * lambda / aperture_diameter) * 180 / PI. We can use 550nm for example (being "center" of 400-700nm range).

3600 * 0.42 * 0.00055 mm * 180 / ( 250mm * PI) = ~0.19" = s3

s_total = sqrt( 0.5^2 + 0.85^2 + 0.19^2) = ~ 1.0043"

To get back to FWHM we need to multiply with 2.355, so resulting star FWHM in above case will be: 2.365"

We can do the same for 127mm

s3 in this case will be: 3600 * 0.42 * 0.00055 mm * 180 / ( 127mm * PI) = ~0.3752"

So s_total will be ~ 1.05512" , or in FWHM it will be (multiply with 2.355): ~ 2.485"

[vlaiv]

Only difference to your case is that you used diameter of airy disk, and you need to use sigma of gaussian approximation.

Diameter is calculated from lambda and diameter with factor of 2.44 and sigma is calculated the same but factor is 0.42, so your values of airy disk size should be multiplied with 0.42 / 2.44

Let's check that:

For 250mm Airy disk is 1.03 and 1.03 * 0.42 / 2.44 = 0.1773, while in my calculation it was 0.19" (this is due to rounding errors)

In 127mm case airy disk is 2.18" or when converted to sigma it is 0.375 and my value is 0.3752 - (again rounding errors)

This brings interesting point , you don't need to do complex formula above, just take airy disk diameter value and multiply by 0.42 and divide with 2.44 to get sigma of gaussian approximation.

[jimjam11]

Thanks, I knew I was missing something because the airy disc calc wasn't a sigma.

My results now agree with yours.

This now allows me to revisit my other post and assess my FWHM against what should be possible. I can now compare results to my recently acquired ed72 to try and make an informed decision about the state of my mirror.

[vlaiv]

2 hours ago, jimjam11 said:

Thanks, I knew I was missing something because the airy disc calc wasn't a sigma.

My results now agree with yours.

This now allows me to revisit my other post and assess my FWHM against what should be possible. I can now compare results to my recently acquired ed72 to try and make an informed decision about the state of my mirror.

These should be considered approximate for couple of reasons - first, we have no idea about actual seeing FWHM and it's not stable - it changes with time as things get better or worse. This also assumes that there is no correlation between guiding error and seeing. There are cases when this is simply not true - so called chasing the seeing. Guide RMS should also be measured with enough precision - that depends on mount and guide resolution used. For many setups, small guide scopes (like 250mm or so) will not provide accurate enough guide RMS. They are ok for guiding - stars will be round, but sometimes, especially if mount is good enough - higher guide resolution can provide tighter stars (smaller guide RMS).

To complicate things further, there is a fourth component to blur that we did not account for. It's so called pixel blur. This is consequence of pixels having surface as opposed to being point sampling devices. Larger the pixel compared to other factors - larger pixel blur associated with it. Above is true if we strictly consider / use point like pixels.

Even airy disk calculations are based on single wavelength. Airy disk size is about twice as larger for 700nm as it is for 400, so there is difference in there as well (like comparing Ha subs to OIII subs, although in reality Ha subs tend to be tighter than OIII because longer wavelengths are less susceptible to seeing effects, and seeing is most often dominant component in calculation).

But for general comparison and estimation, above calculation is good enough predictor of star FWHM in images.

[jimjam11]

17 hours ago, vlaiv said:

These should be considered approximate for couple of reasons - first, we have no idea about actual seeing FWHM and it's not stable - it changes with time as things get better or worse. This also assumes that there is no correlation between guiding error and seeing. There are cases when this is simply not true - so called chasing the seeing. Guide RMS should also be measured with enough precision - that depends on mount and guide resolution used. For many setups, small guide scopes (like 250mm or so) will not provide accurate enough guide RMS. They are ok for guiding - stars will be round, but sometimes, especially if mount is good enough - higher guide resolution can provide tighter stars (smaller guide RMS).

To complicate things further, there is a fourth component to blur that we did not account for. It's so called pixel blur. This is consequence of pixels having surface as opposed to being point sampling devices. Larger the pixel compared to other factors - larger pixel blur associated with it. Above is true if we strictly consider / use point like pixels.

Even airy disk calculations are based on single wavelength. Airy disk size is about twice as larger for 700nm as it is for 400, so there is difference in there as well (like comparing Ha subs to OIII subs, although in reality Ha subs tend to be tighter than OIII because longer wavelengths are less susceptible to seeing effects, and seeing is most often dominant component in calculation).

But for general comparison and estimation, above calculation is good enough predictor of star FWHM in images.

Yes, it seems like a good predictor of best case FWHM. I ran the calc into some tables based on varying aperture and guide rms:

 

 

[jimjam11]

Question @ollypenrice

What kind of FWHM do you see with your high end refractors; how do they compare to the tables?

Once you get beyond 130mm it looks like the gains are minimal unless you get exceptional seeing and are able to guide sufficiently well. Your data appears to show the same in a more graphical way! 

 

I looked at some data I got from an iTelescope FSQ106 and the best FWHM was 4"

 

[ollypenrice]

20 minutes ago, jimjam11 said:

Question @ollypenrice

What kind of FWHM do you see with your high end refractors; how do they compare to the tables?

Once you get beyond 130mm it looks like the gains are minimal unless you get exceptional seeing and are able to guide sufficiently well. Your data appears to show the same in a more graphical way! 

 

I looked at some data I got from an iTelescope FSQ106 and the best FWHM was 4"

 

I'll give you the luminance values for 3 second focus subs.

With the FSQs at 3.5"PP we sometimes get FWHM down to 3.25 arcsecs. The pixel value may get down to just less than 0.9. 

The range of values with the TEC at 0.9"PP is far wider, meaning the rig is more sensitive to seeing. If we are very lucky it might be down to 1.35 arcsecs but it can be 2.25 or even worse, again in short focus subs of 3 seconds. When it's like that we tend only to shoot colour.

Our guide RMS usually falls between 0.25 and 0.5.

I assume the high values from the Taks arise from undersampling on this widefield setup. If we drop TEC data into Tak to enhance key areas of a widefiled image it is very obvious that the TEC stars are both smaller and more numerous.

Olly

[Northernlight]

Does the focal ratio or optical design for a given aperture change  these  figures, e.g 150mm doublet @F8 vs a 150mm triplet at F7 

Cheers,  Rich.