Practical difference in resolution between 6" and 8" Newtonian at the *same* focal length (750mm).

[MichaelBibby]

An F5 6" Newtonian has a focal length of 750mm, and a F5 8" Newt with a 0.75x reducer (Starizona Nexus) also has a focal length of 750mm (i.e., at F3.75). My question is quite simply: will the increase in resolution going from the 6" at 750mm to 8" at 750mm be noticeable?

Obviously there are some variables to consider, like the pixel size of the camera and the seeing conditions. In my case I am using the 585 sensor with a pixel size of 2.9um which will be oversampled at 750mm. Will the difference in resolution only be noticeable in the very best seeing conditions, or would it be noticeable in average seeing conditions?

Another use case would be going from 6" F5 Newt with 0.75x reducer (about 585mm) vs a 8" F4 with 0.75x reducer (about 600mm). Obviously we would be comparing an F5 scope vs an F3 scope at the same focal length, but again the question arises: will the greater theoretical resolving power of the 8" be noticeable in real world conditions?

Any guidance here would be helpful (and what would be super helpful is a direct comparison of two different images taken under comparable conditions).

 

Edited October 22, 2023 by MichaelBibby

[MichaelBibby]

To be clear, the question is: what kind of gains, in terms of resolution, can you reasonably expect from increasing aperture at these focal lengths?

[Mr Spock]

None. The resolution is a function of focal length and pixel size in this case as your focal length v sensor size is 0.8" per pixel. The only thing aperture affects here is the amount of photons reaching the sensor, so the 8" lets in more light than the 6" so the exposure time will be less.

[Clarkey]

Just to add to @Mr Spock comment above, technically with the larger scope better resolution is possible. but both of these are less than your pixel scale so you will not see any difference. If you go to do any planetary imaging using the longer FL you would see a difference.

You can work out the numbers here: astronomy.tools

 

[DaveS]

Plus, you'll be hard put to achieve any real resolution below 1"/px unless you have a night of exceptional seeing and a premium mount that can guide below 0.5".

[MichaelBibby]

Thanks everyone.

I understand that in a vacuum angular resolution is a function of aperture and wavelength, and although I understand the supervening atmosphere imposes limitations beyond those of the wave nature of light itself, am still getting my head around the relevant maths. 

I was looking at buying an 8" F4 but it looks like a cheap but good as new 8" F5 has landed in my lap, which I will be using with a Nexus CC that will bring it down to F3.75 at 750mm, so I should be able to make some direct comparisons, under varying conditions, with my 6" F5 which is also 750mm.

I'll post some comparison images in the coming months so that hopefully others can learn along with me (targets like m42 and the Horsehead nebula are rising on the East coast of Australia), but obviously the biggest confounding variables I'll have to find ways to mitigate or at least record is seeing conditions and integration time with different focal ratios.

Edited October 31, 2023 by MichaelBibby

[johnturley]

On 22/10/2023 at 05:17, MichaelBibby said:

 

Obviously there are some variables to consider, like the pixel size of the camera and the seeing conditions. In my case I am using the 585 sensor with a pixel size of 2.9um which will be oversampled at 750mm. Will the difference in resolution only be noticeable in the very best seeing conditions, or would it be noticeable in average seeing conditions?



 

Based on the optimum focal ratio being 3x the pixel size, the optimal focal ratio would be around f9, so at 750mm focal length you will be UNDER rather than oversampled with both the 6in and 8in telescopes, more so with the 8in if they have the same focal length.

You haven't stated whether you are talking about planetary or deep sky imaging, for deep sky imaging what is more important is being able to fit the desired object in the available field of view. For planetary imaging the theoretical optimum with both scopes would be achieved with about a 2x Barlow, but with a native focal length of just 750mm, you might find the resultant images a bit on the small size, and find a 3-4x Barlow gives more pleasing results.

 I think some people get too hung up about what should be the optimal focal ratio, and it might be worth doing a bit of experimenting to see what gives the best results. Some planetary imagers get excellent results with focal ratios as long as f20 or even f25.

John  

Edited October 31, 2023 by johnturley

[MichaelBibby]

6 minutes ago, johnturley said:

Based on the optimum focal ratio being 3x the pixel size, the optimal focal ratio would be around f9, so at 750mm focal length you will be UNDER rather than oversampled with both the 6in and 8in telescopes, more so with the 8in if they have the same focal length.

You haven't stated whether you are talking about planetary or deep sky imaging, for deep sky imaging what is more important is being able to fit the desired object in the available field of view. For planetary imaging the theoretical optimum with both scopes would be achieved with about a 2x Barlow, but with a native focal length of just 750mm, you might find the resultant images a bit on the small size, and find a 3-4x Barlow gives more pleasing results.

 I think some people get too hung up about what should be the optimal focal ratio, and it might be worth doing a bit of experimenting to see what gives the best results. Some planetary imagers get excellent results with focal ratios as long as f20 or even f25.

John  

Sorry John, deep sky objects, not planetary.

[ONIKKINEN]

What coma corrector were you planning on using for the 1.0x comparison scope? For a newtonian this is the most important optical component and will make or break the image.

Have not seen too many images with the nexus reducer, and even fewer mentions of star sizes it produces. Would be very interested in seeing some data taken with that thing as i could see myself getting one some day.

If you compare the nexus scope to an uncorrected scope, the nexus will win easily in terms if actual resolved detail. If you compared to a quality 1.0x coma corrector, then i would guess the nexus is second even in the bigger scope as reducers are often borrowing detail for speed. Just a hunch of course, as have not seen much of it.

[ollypenrice]

The theory will be overwhelmed by the practice in comparisons like this. An 8 inch scope has higher optical resolution than a 6 inch if both are diffraction limited. To give the same FL the larger scope has to be faster, making it more susceptible to optical imperfections in the grinding of the mirror, the collimation and the mechanical alignments. Which will be the real world winner? It depends on the specific instruments in use.

In DS imaging, sharpening in post processing has an effect on final resolution and more signal can take more sharpening than less signal. This favours the faster scope.

If you are over sampled the optical resolution comparison goes out of the window because you'll be seeing limited, but the larger, faster scope gets more light. See previous point.

I firmly believe that these discussions are academic and that the success or failure of an image derives from a dozen other things before the niceties we are discussing here have any contribution to make. I also know from experience that not all reducers play nicely with all optics and, given the very questionable wording of Starizona's Hyperstar advertising, I would read anything they write with a high degree of critical distance. For example, 

The HyperStar 8 lens converts a standard Celestron 8" SCT from f/10 to f/1.9, making the system 25 times faster. That means 25-times shorter exposures!   For this to be true, the Hyperstar lens would have to increase the area of the clear aperture by 25 times and then we could consider it to be the same 'system.'

Olly

[gorann]

For a while I have been running a rig with two RASA8 and a piggybacking Samyang 135. All are at F/2 and the cool thing with the Samyang (FL 135) is the extreme wide field. But when if comes to both resolution and catching the faint stuff, the Samyang is nowhere near the RASA. The simple reason is aperture: 67.5 mm (Samyang) versus 200 mm (RASA). So there is no doubt in my mind that aperture rules when it comes to resolution and depth, as long as both instruments do not get limited by seeing or guiding. This may be an extreme comparison and the difference between a 6" and 8" SCT may not be so clear, but I assume it will be there.

Edited October 31, 2023 by gorann

[MichaelBibby]

As I understand it, there is also the effect of improved 'contrast' with the wider aperture, which in and of itself can have the effect of making an image look 'sharper'?

[ollypenrice]

11 hours ago, gorann said:

For a while I have been running a rig with two RASA8 and a piggybacking Samyang 135. All are at F/2 and the cool thing with the Samyang (FL 135) is the extreme wide field. But when if comes to both resolution and catching the faint stuff, the Samyang is nowhere near the RASA. The simple reason is aperture: 67.5 mm (Samyang) versus 200 mm (RASA). So there is no doubt in my mind that aperture rules when it comes to resolution and depth, as long as both instruments do not get limited by seeing or guiding. This may be an extreme comparison and the difference between a 6" and 8" SCT may not be so clear, but I assume it will be there.

I certainly agree that the Samyang is mightily out-resolved by the RASA but here, we are comparing massively different apertures.

As regards depth, I think it's 'aperture per pixel' which matters. I'll go back and compare our own RASA and Samyang data on the same targets. I've blended the two without paying particular attention to this.

Olly

Edited November 1, 2023 by ollypenrice
False click

[MichaelBibby]

While playing with Microscofts generative 'A.I.' model called 'Bard' I thought I would test it with this question and see what it came up with:

Me: "if I were to compare images taken through each of those telescopes using a camera with a pixel size of 2.9um, and was guiding at a total error of 0.5 arc seconds, would I notice the difference in resolution under 'average' to 'good' seeing conditions?"

Bard: "Yes, you would likely notice the difference in resolution between images taken through a 150mm Newtonian telescope and a 200mm Newtonian telescope, using a camera with a pixel size of 2.9um and guiding at a total error of 0.5 arc seconds, under average to good seeing conditions.

The 150mm telescope has a theoretical resolution of 0.8 arcseconds, while the 200mm telescope has a theoretical resolution of 0.6 arcseconds. This means that the 200mm telescope can resolve finer details.

The pixel size of the camera is 2.9um, which corresponds to a sampling rate of 0.12 arcseconds per pixel. This means that the camera is capable of sampling the images from both telescopes at a high enough rate to capture the difference in resolution.

The guiding error of 0.5 arc seconds is relatively small, so it will not significantly degrade the resolution of the images.

Under average to good seeing conditions, the atmosphere will typically introduce a blurring of about 1 arcsecond. This means that the effective resolution of both telescopes will be limited to about 1 arcsecond. However, the 200mm telescope will still be able to resolve finer details than the 150mm telescope, even under average to good seeing conditions.

In conclusion, you would likely notice the difference in resolution between images taken through a 150mm Newtonian telescope and a 200mm Newtonian telescope, using a camera with a pixel size of 2.9um and guiding at a total error of 0.5 arc seconds, under average to good seeing conditions.

Here is a table that summarizes the expected resolution of the two telescopes under different seeing conditions (Please note that these are just estimates. The actual resolution of the images will depend on a number of factors, including the quality of the optics, the observing conditions, and the processing of the images.):

 

Seeing	Expected resolution of 150mm telescope	Expected resolution of 200mm telescope
1 arcsecond	0.9 arcseconds	1.1 arcseconds
0.5 arcseconds	0.8 arcseconds	0.9 arcseconds

Edited November 4, 2023 by MichaelBibby

[ollypenrice]

8 minutes ago, MichaelBibby said:

While playing with Microscofts generative 'A.I.' model called 'Bard' I thought I would test it with this question and see what it came up with:

Me: "if I were to compare images taken through each of those telescopes using a camera with a pixel size of 2.9um, and was guiding at a total error of 0.5 arc seconds, would I notice the difference in resolution under 'average' to 'good' seeing conditions?"

Bard: "Yes, you would likely notice the difference in resolution between images taken through a 150mm Newtonian telescope and a 200mm Newtonian telescope, using a camera with a pixel size of 2.9um and guiding at a total error of 0.5 arc seconds, under average to good seeing conditions.

The 150mm telescope has a theoretical resolution of 0.8 arcseconds, while the 200mm telescope has a theoretical resolution of 0.6 arcseconds. This means that the 200mm telescope can resolve finer details.

The pixel size of the camera is 2.9um, which corresponds to a sampling rate of 0.12 arcseconds per pixel. This means that the camera is capable of sampling the images from both telescopes at a high enough rate to capture the difference in resolution.

The guiding error of 0.5 arc seconds is relatively small, so it will not significantly degrade the resolution of the images.

Under average to good seeing conditions, the atmosphere will typically introduce a blurring of about 1 arcsecond. This means that the effective resolution of both telescopes will be limited to about 1 arcsecond. However, the 200mm telescope will still be able to resolve finer details than the 150mm telescope, even under average to good seeing conditions.

In conclusion, you would likely notice the difference in resolution between images taken through a 150mm Newtonian telescope and a 200mm Newtonian telescope, using a camera with a pixel size of 2.9um and guiding at a total error of 0.5 arc seconds, under average to good seeing conditions.

Here is a table that summarizes the expected resolution of the two telescopes under different seeing conditions (Please note that these are just estimates. The actual resolution of the images will depend on a number of factors, including the quality of the optics, the observing conditions, and the processing of the images.):

 

Seeing	Expected resolution of 150mm telescope	Expected resolution of 200mm telescope
1 arcsecond	0.9 arcseconds	1.1 arcseconds
0.5 arcseconds	0.8 arcseconds	0.9 arcseconds

Where is the artificial intelligence in all this? It is just a regurgitation of numbers already out in the public domain, with one vitally important number replaced by the adjectives 'average' and 'good.' Since these are the numbers which define everything that the system will produce, what are they and where are they?

I would rate this answer average to poor.

lly

[MichaelBibby]

28 minutes ago, ollypenrice said:

Where is the artificial intelligence in all this? It is just a regurgitation of numbers already out in the public domain, with one vitally important number replaced by the adjectives 'average' and 'good.' Since these are the numbers which define everything that the system will produce, what are they and where are they?

I would rate this answer average to poor.

lly

I asked it:

Me: "give me the scientific definition of 'average' and 'good' seeing conditions."

Bard:

"Average seeing conditions are characterized by atmospheric turbulence that causes the image of a star to blur to a diameter of about 1 arcsecond. This is equivalent to the resolution of the human eye. Under average seeing conditions, it is possible to resolve double stars that are separated by about 1 arcsecond, and to see the details of larger astronomical objects, such as planets and galaxies.

Good seeing conditions are characterized by atmospheric turbulence that causes the image of a star to blur to a diameter of about 0.5 arcseconds or less. This is better than the resolution of the human eye. Under good seeing conditions, it is possible to resolve double stars that are separated by about 0.5 arcseconds, and to see finer details of astronomical objects.

The following table shows a more detailed classification of seeing conditions:

Seeing condition	Image blur (diameter)
Poor	2 arcseconds or more
Fair	1-2 arcseconds
Average	1 arcsecond
Good	0.5-1 arcsecond
Excellent	0.25-0.5 arcseconds
Exceptional	0.25 arcseconds or less

drive_spreadsheetExport to Sheets

Seeing conditions can vary depending on the location, the time of day, and the weather. The best seeing conditions are typically found at high altitudes, in dry climates, and during clear nights.

Astronomers use a variety of techniques to improve the resolution of their images under average and good seeing conditions. These techniques include:

• Adaptive optics: This technology uses a deformable mirror to correct for the effects of atmospheric turbulence.
• Lucky imaging: This technique combines many short exposures of an object to select the best images that are not affected by atmospheric turbulence.
• Stacking: This technique combines multiple images of an object to reduce noise and improve the signal-to-noise ratio.

By using these techniques, astronomers can obtain high-resolution images of astronomical objects even under average and good seeing conditions."

 

tuneshare

 

more_vert

 

 

 

 

Edited November 4, 2023 by MichaelBibby