Seeing vs focal lenght

0.00 Alvaro Küper ... · view views · Share link
Hi all, greetings from nothern Italy, first post overhere! there is a concept that based only on my experience, i would want to ask you all if im right or am i missing something? referred exclusively to AP: "getting a bigger focal lenght scope (+1000mm ) doesn't worth under my average home sky because of the seeing" i arrived to this conclusion after making AP first with a 200/1000 and struggled to get nice images, but barely. then got a ED80 and was piece of cake to get nice sharp images. i know we are talking here also of image scale. but, when imaging at 1000mm focal length i think going further would only make things worst. due to the average seeing i get here. im referring only to DSO. i would love to get a C8 or something around there, but i think my local sky would not "support" it. Meteoblue says that my local skies varies from 0,8 to 1,3 , very often under 1.0 how trusty is it? ah, im under bortle 6. thanks for your considerations, i hope my question(s) are clear enough CS! Like

0.00

...

Hi all, greetings from nothern Italy, first post overhere!

there is a concept that based only on my experience, i would want to ask you all if im right or am i missing something?

referred exclusively to AP:
"getting a bigger focal lenght scope (+1000mm ) doesn't worth under my average home sky because of the seeing"

i arrived to this conclusion after making AP first with a 200/1000 and struggled to get nice images, but barely. then got a ED80 and was piece of cake to get nice sharp images. i know we are talking here also of image scale. but, when imaging at 1000mm focal length i think going further would only make things worst. due to the average seeing i get here. im referring only to DSO.

i would love to get a C8 or something around there, but i think my local sky would not "support" it.
Meteoblue says that my local skies varies from 0,8 to 1,3 , very often under 1.0 how trusty is it? ah, im under bortle 6.

thanks for your considerations, i hope my question(s) are clear enough smile

CS!

7.16 Christian Großmann ... · view views · 1 like · Share link
It's not just a matter of the scope. It all depends on the combination of scope, camera and seeing. If you just look at the angle per pixel size, you are right. Then it makes no sense to buy a huge telescope and a huge camera to get the same field of view. But there is another fact that adds to the equation. Without getting into this technically, you can get better details in your image with a scope with bigger aperture than with a smaller one. This only depends on the aperture itself. I saw some images where somebody had taken the same subjects with its 4,5" refractor and the 8" EdgeHD. The difference was night and day to me and I decided to buy one. I did not regret it. But you have to expose much longer because of the focal ratio. The fact that you get better images with the ED80 than with your 200/1000 Newton might be another question. These scopes are worlds apart. The ED80 is much smaller and does not weight as much as the Newton. My EQ6 mounts really can handle the Newton, but even with my TS115/f/7 I get much sharper images because it can handle the weight much better. I assume, you use the same camera for both of them. Then the pixel scale is much different and you will get sharper images in terms of guiding errors anyway with the smaller scope. Little movements does not effect the image as much as with a longer focal length. It took me quite some time to handle those 1000mm focal lengths and more. At this stage, I would not recommend buying the EdgeHD, yet. It is even longer and you need some experience to handle it. Try to handle your Newt first. Otherwise, you won't have much fun with the Edge. CS Christian Edited ... Like

7.16

Christian Großmann

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It's not just a matter of the scope. It all depends on the combination of scope, camera and seeing. If you just look at the angle per pixel size, you are right. Then it makes no sense to buy a huge telescope and a huge camera to get the same field of view. But there is another fact that adds to the equation. Without getting into this technically, you can get better details in your image with a scope with bigger aperture than with a smaller one. This only depends on the aperture itself. I saw some images where somebody had taken the same subjects with its 4,5" refractor and the 8" EdgeHD. The difference was night and day to me and I decided to buy one. I did not regret it. But you have to expose much longer because of the focal ratio.

The fact that you get better images with the ED80 than with your 200/1000 Newton might be another question. These scopes are worlds apart. The ED80 is much smaller and does not weight as much as the Newton. My EQ6 mounts really can handle the Newton, but even with my TS115/f/7 I get much sharper images because it can handle the weight much better. I assume, you use the same camera for both of them. Then the pixel scale is much different and you will get sharper images in terms of guiding errors anyway with the smaller scope. Little movements does not effect the image as much as with a longer focal length.

It took me quite some time to handle those 1000mm focal lengths and more. At this stage, I would not recommend buying the EdgeHD, yet. It is even longer and you need some experience to handle it. Try to handle your Newt first. Otherwise, you won't have much fun with the Edge.

CS
Christian

Edited ...

0.00 Alvaro Küper Topic starter ... · view views · Share link
This only depends on the aperture itself. I saw some images where somebody had taken the same subjects with its 4,5" refractor and the 8" EdgeHD. The difference was night and day to me and I decided to buy one. I did not regret it. But you have to expose much longer because of the focal ratio. Hi Christian, thanks for your reply! if i understood correctly, does it mean that for same focal length, bigger aperture will get sharper images? it makes sense because bigger aperture = bigger resolution, im assuming perfect guiding and collimation, and the only variable the sky conditions. on all my 3 scopes with same camera, give me a good sampling. so im not considering pixel size at this point. my mount handles very well the the newt 200/1000, have sub 0.8 guiding if i balance properly and there is no gust winds. but star size looks definitely different. no that i cant get goods images with it, see: https://www.astrobin.com/u6luoz/ got recently the quattro 200/800 f4, stars look middle way between ed80 and 200/1000 so thats why i though going to a longer focal length (let say 2000mm) would be disappointing under my skies, so would have to go at least for a C11 to get a decent improvement! thkns ! Like

0.00

Alvaro Küper

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This only depends on the aperture itself. I saw some images where somebody had taken the same subjects with its 4,5" refractor and the 8" EdgeHD. The difference was night and day to me and I decided to buy one. I did not regret it. But you have to expose much longer because of the focal ratio.

Hi Christian, thanks for your reply!

if i understood correctly, does it mean that for same focal length, bigger aperture will get sharper images?
it makes sense because bigger aperture = bigger resolution, im assuming perfect guiding and collimation, and the only variable the sky conditions.

on all my 3 scopes with same camera, give me a good sampling. so im not considering pixel size at this point.

my mount handles very well the the newt 200/1000, have sub 0.8 guiding if i balance properly and there is no gust winds. but star size looks definitely different. no that i cant get goods images with it, see:

https://www.astrobin.com/u6luoz/

got recently the quattro 200/800 f4, stars look middle way between ed80 and 200/1000

so thats why i though going to a longer focal length (let say 2000mm) would be disappointing under my skies, so would have to go at least for a C11 to get a decent improvement!

thkns !

15.16 Wei-Hao Wang ... · view views · 1 like · Share link
Don't take the face value of meteoblue seeing. The relative values may have some meaning, but the absolute values are terribly under-estimated. Actually seeing is way more complicated. You may need to multiply it by 2 or even 3 to match the actual seeing. Like

7.63 Scott Badger ... · view views · 1 like · Share link
Wei-Hao Wang: Don't take the face value of meteoblue seeing. The relative values may have some meaning, but the absolute values are terribly under-estimated. Actually seeing is way more complicated. You may need to multiply it by 2 or even 3 to match the actual seeing. Agreed. My seeing is what I get for short exposure star fwhm's when collimated and focused. Winter is worse than summer, and at any time it can more than double (or halve) in a single night. Common thought is your image scale (206.265*pixel_size(um)/focal_length(mm)) should be about 1/3 of your seeing and you want your image scale set for best possible seeing conditions. E.g. if your best seeing is 1" (I wish!....), then an image scale of 0.33" is appropriate. According to these formulas, any smaller image scale (i.e. a longer focal length), won't improve your detail resolution, but I've also heard that increased aperture does, so going to a bigger scope, even if already oversampled, might still be an improvement? Cheers, Scott Like

7.63

Scott Badger

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Wei-Hao Wang:
Don't take the face value of meteoblue seeing. The relative values may have some meaning, but the absolute values are terribly under-estimated. Actually seeing is way more complicated. You may need to multiply it by 2 or even 3 to match the actual seeing.

Agreed. My seeing is what I get for short exposure star fwhm's when collimated and focused. Winter is worse than summer, and at any time it can more than double (or halve) in a single night. Common thought is your image scale (206.265*pixel_size(um)/focal_length(mm)) should be about 1/3 of your seeing and you want your image scale set for best possible seeing conditions. E.g. if your best seeing is 1" (I wish!....), then an image scale of 0.33" is appropriate. According to these formulas, any smaller image scale (i.e. a longer focal length), won't improve your detail resolution, but I've also heard that increased aperture does, so going to a bigger scope, even if already oversampled, might still be an improvement?

Cheers,
Scott

7.16 Christian Großmann ... · view views · 1 like · Share link
Alvaro Küper: on all my 3 scopes with same camera, give me a good sampling. so im not considering pixel size at this point. Godd sampling is just a term to see if your camera pixels get enough light to be efficient. Because the pixels in your camera stay the same size every time, the tiny part of the sky you can capture with every pixel is different with every focal length. So a smaller focal length seems to be sharper (and actually is) than with a longer focal length. On longer focal lengths, you have more pixels for the same star disc size. You will be a bit more undersampled with a shorter focal length and a bit more oversampled with a longer one. This is just part of the calculation. The star sizes of the Edge are for sure bigger than what you get with all your current scopes if you use the same camera and binning. That has nothing really to do with the seeing. If the seeing stays the same, you have to get different star sizes. But with your image of M51 your are on a good level. Don't judge the very tiny stars in many images around here. People do a lot of work to get them in post processing. They might not come out of their scopes this way. As said, there is nothing wrong with your image of M51. You just have some reflections around the bright stars that can come from the filters or the coma corrector. But the stars look very good. Like

7.16

Christian Großmann

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Alvaro Küper:
on all my 3 scopes with same camera, give me a good sampling. so im not considering pixel size at this point.

Godd sampling is just a term to see if your camera pixels get enough light to be efficient. Because the pixels in your camera stay the same size every time, the tiny part of the sky you can capture with every pixel is different with every focal length. So a smaller focal length seems to be sharper (and actually is) than with a longer focal length. On longer focal lengths, you have more pixels for the same star disc size. You will be a bit more undersampled with a shorter focal length and a bit more oversampled with a longer one. This is just part of the calculation.

The star sizes of the Edge are for sure bigger than what you get with all your current scopes if you use the same camera and binning. That has nothing really to do with the seeing. If the seeing stays the same, you have to get different star sizes. But with your image of M51 your are on a good level. Don't judge the very tiny stars in many images around here. People do a lot of work to get them in post processing. They might not come out of their scopes this way.

As said, there is nothing wrong with your image of M51. You just have some reflections around the bright stars that can come from the filters or the coma corrector. But the stars look very good.

0.00 Alvaro Küper Topic starter ... · view views · Share link
Scott Badger: i always go for oversampling if happens, no matter how some pals demonize it undersampling is a different story Like

0.00 Alvaro Küper Topic starter ... · view views · 1 like · Share link
Christian Großmann: As said, there is nothing wrong with your image of M51. You just have some reflections around the bright stars that can come from the filters or the coma corrector. But the stars look very good. Thanks Chirstian for your comments on it, the OTA is actually a rebranded GSO(?) as "Delta Optical" a Polish brand, there is still theyrs e-shop, i have no clue how it arrived to Italy, i buyed it used, and keep it because i suspect that mirror "born" with better quality than the average GSO mirror, there is no coma corrector used on that image, you can see on star bright goes slightly to the borders, no filter used. reflections may be just not perfect ligths calibration. making flats with an ASI294MC is a pretty random beast. true i used BlurX and NoiseX but i can tell star's shape are pretty much the same on the raw images CS! Like

0.00

Alvaro Küper

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Christian Großmann:
As said, there is nothing wrong with your image of M51. You just have some reflections around the bright stars that can come from the filters or the coma corrector. But the stars look very good.

Thanks Chirstian for your comments on it, the OTA is actually a rebranded GSO(?) as "Delta Optical" a Polish brand, there is still theyrs e-shop, i have no clue how it arrived to Italy, i buyed it used, and keep it because i suspect that mirror "born" with better quality than the average GSO mirror, there is no coma corrector used on that image, you can see on star bright goes slightly to the borders, no filter used. reflections may be just not perfect ligths calibration. making flats with an ASI294MC is a pretty random beast.

true i used BlurX and NoiseX but i can tell star's shape are pretty much the same on the raw images

CS!

2.39 Rob Lyons ... · view views · 3 likes · Share link
The point of diminishing returns for resolution is probably somewhere near 8-10" aperture in average seeing conditions. I would say 10". For this reason, I run a pair of Edge HD 8" scopes from my Bortle 9 location. I can tell you that you want to dial in your guiding over 1000mm to be somewhere around .5rms. I can see the difference in the quality of images I get from sensors with different sized pixels on my setup due to the effects of seeing on my guiding. At 0.55 arc seconds per pixel, things look just a touch soft, but at 0.67 arc seconds per pixel, they appear a little bit sharper. This is because I would have to be guiding with a better accuracy than 0.55rms to get sharp images with the smaller pixels. I am usually around .5-.7 on average. Once I wrapped my head around this I noticed improvement on my very first image. Long story short I think a C8 would be just fine as long as you are not too over or under sampled and your guiding is great. Like

2.39

Rob Lyons

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The point of diminishing returns for resolution is probably somewhere near 8-10" aperture in average seeing conditions. I would say 10". For this reason, I run a pair of Edge HD 8" scopes from my Bortle 9 location. I can tell you that you want to dial in your guiding over 1000mm to be somewhere around .5rms. I can see the difference in the quality of images I get from sensors with different sized pixels on my setup due to the effects of seeing on my guiding. At 0.55 arc seconds per pixel, things look just a touch soft, but at 0.67 arc seconds per pixel, they appear a little bit sharper. This is because I would have to be guiding with a better accuracy than 0.55rms to get sharp images with the smaller pixels. I am usually around .5-.7 on average. Once I wrapped my head around this I noticed improvement on my very first image. Long story short I think a C8 would be just fine as long as you are not too over or under sampled and your guiding is great.

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