Oh boy, another question...hope I'm not bombarding the forums too much....
I'm wondering if different f-ratios have an effect on field of view or contrast or something similar? The example I'm thinking of is a 12" f/6 vs a 12" f/4.5
I'm aware of how coma increases as the f-ratio gets faster. I'm mainly interested to know if the different f-ratios are for convenience, keeping the overall scope length down, or if there are other reasons as well, beyond avoiding optical aberrations.
Smaller f ratio means more coma, more critical collimation, a larger diagonal mirror, leading to some light loss and less contrast. It is also harder to make a short focus mirror, so for a commercial enterprise, a "good enough" f4.5 may not be as good as a "good enough" f6. A short focus scope is more portable and doesn't demand as much from your mount if you are imaging. Coma can be corrected with a coma corrector.
It also depends on what you look at most. For planets go as long as possible with the smallest possible diagonal to maximise contrast, for close equally bright double stars, the larger diagonal of a short focus job actually helps--light from the Airy disc is pushed into the first diffraction ring, so the Airy disc is smaller.
Field of view for visual is not affected. It depends on magnifying power and eyepiece--100 power with a 50 degree eyepiece will have the same FOV in both scopes. For imaging, the smaller f ratio will put more of the sky on your chip.
Everything is a trade-off, nothing is perfect.
Geoff
I think my interest is definitely moving towards objects such as clusters and nebulae. Over Christmas I had my first real glimpses of the LMC, and it's really caught my imagination. My first view of 47 Tuc blew me away, and was totally unexpected....Planets are great, but I want to build a scope to help me go deeper. I don't want to do any imaging at all with this, so all my consideration will be towards best visual setup...
I have a 12 inch F5 and observe often with a friend who has an F6. The main difference is I sometimes need a a couple of steps to reach the eyepiece in the F6 (I'm 5 ft 10.). My feet are always on the ground at F5. If the mirror is well made it will perform well no matter the F ratio but a well made short focus mirror is likely to be expensive. I don't use a coma corrector at F5 but some people do.
Smaller f ratio means more coma, more critical collimation, a larger diagonal mirror, leading to some light loss and less contrast. It is also harder to make a short focus mirror, so for a commercial enterprise, a "good enough" f4.5 may not be as good as a "good enough" f6. A short focus scope is more portable and doesn't demand as much from your mount if you are imaging. Coma can be corrected with a coma corrector.
It also depends on what you look at most. For planets go as long as possible with the smallest possible diagonal to maximise contrast, for close equally bright double stars, the larger diagonal of a short focus job actually helps--light from the Airy disc is pushed into the first diffraction ring, so the Airy disc is smaller.
Field of view for visual is not affected. It depends on magnifying power and eyepiece--100 power with a 50 degree eyepiece will have the same FOV in both scopes. For imaging, the smaller f ratio will put more of the sky on your chip.
Everything is a trade-off, nothing is perfect.
Geoff
The airy disk doesn't actually decrease in size with increasing central obstruction, but rather less percentage of light is concentrated inside the airy disk and more in the diffraction rings which translates into a loss of contrast.
The size of the disk only decreases as you increase the aperture and vice-versa.
The airy disk doesn't actually decrease in size with increasing central obstruction, but rather less percentage of light is concentrated inside the airy disk and more in the diffraction rings which translates into a loss of contrast.
The size of the disk only decreases as you increase the aperture and vice-versa.
Yes, that's correct--its just that you can't see the fainter bits, so it looks smaller, so it makes splitting equal brightness double stars easier. In fact, some DS observers actually put a mask over the spider to block out the central area in order to enhance this effect
Note added: I dug out this pic from Suiter's "Star Testing Astronomical Telescopes" which gives a nice illustration of the effect of increasing the central obstruction. It is easy to see why we get a loss of contrast. Also easy to see why it will be easier to split a close equal double. As I said above, almost all aspects of telescopes involve some type of trade-off
Yes, that's correct--its just that you can't see the fainter bits, so it looks smaller, so it makes splitting equal brightness double stars easier.
Quote:
Originally Posted by pgc hunter
The airy disk doesn't actually decrease in size with increasing central obstruction, but rather less percentage of light is concentrated inside the airy disk and more in the diffraction rings which translates into a loss of contrast.
The size of the disk only decreases as you increase the aperture and vice-versa.
Actually, no that's not correct sorry. While it is true that the airy disk size depends on the aperture, it (as well as the size of the diffraction rings) are also affected by any obstructions in the entrance pupil. Larger central obstructions will not only result in more energy being distributed in the diffraction rings, but the angular size of the airy disk and the rings will decrease as well. Chapter 9 of Suiter's book descibes this ("The spot size is smaller in obstructed instruments, ... ").
I also happen to have written a Windows program that will provide a graphical display of the point spread function (ie. diffraction pattern) of a star based on user defined parameters (such as obstructions, wavelength, scale, etc) as well as measurements of airy disk size and diffraction ring diameters taken from the calculated data. The basic
algorithm is based on an article published in Sky and Telescope from 1987 and involves simulating thousands of random light rays to calculate the resultant phase difference at each pixel and convert to an intensity to form an image. I have attached a screenshot showing the results of 2 runs, one for an unobstructed aperture of 200mm, and the other
having a very large obstruction of 180mm. From the unobstructed results shown, you can see that the measurement of the diameter of the resultant Airy disk is 1.33 arc-seconds (in perfect agreement with theory), whilst the second (obstructed) run shows an Airy disk of only 0.89 arc-seconds. You can also see that the diameters of the diffraction rings are also smaller for the large obstruction (and that the relative intensity of the rings has increased as well).
The size of the disk only decreases as you increase the aperture and vice-versa.
This to me seems counter-intuitive, as I'm under the impression larger aperture = larger object size.
Quote:
Originally Posted by David Fitz-Henr
Larger central obstructions will not only result in more energy being distributed in the diffraction rings, but the angular size of the airy disk and the rings will decrease as well.
In terms of observing, does this mean a larger central obstruction would make it easier to resolve stars in globular clusters? What would the effect be on observing faint galaxies, such as those in the Grus quartet?
In terms of observing, does this mean a larger central obstruction would make it easier to resolve stars in globular clusters? What would the effect be on observing faint galaxies, such as those in the Grus quartet?
In the specific case where the separation of 2 stars are at or near the Rayleigh criterion (ie. radius of the Airy disk) then yes, a telescope with a central obstruction will actually resolve these stars better than an unobstructed one. However, at lower spatial frequencies (ie. greater angular separation for the double star) the unobstructed telescope will perform better. Yes, it is counter-intuitive but a by-product of the wave nature of light. In practice however, you would need excellent seeing to take advantage of this in order to split a double star that has an appropriate separation for your aperture.
For extended objects such as faint galaxies there will be a loss of contrast due to the fact that the image is comprised of many overlapping point sources, so the airy disks / rings get "smeared" all over each other. Since an unobstructed aperture has less energy distributed to the rings, this smearing is kept to a minimum and results in better contrast.
In relation to F ratio, I guess it depends on where you are sourcing your mirror. If you are planning on buying an off the shelf GSO mirror then the F ratio will be F5 for a 10 or 12 inch and F4.5 for a 16 inch. If you are getting a mirror custom made by someone like Mark Suchting, then you as the customer can specify any reasonable F ratio with an expectation of a high quality diffraction limited mirror as an outcome.
In a practical sense for visual observing, assuming high quality optics and well built scope structure, other than requiring a coma corrector somewhere below F6 and experiencing tighter focusing margins with reducing F ratio I suspect you will find the visual image quality will be about the same regardless of F ratio.
I suggest you set out some basic criteria such as maximum eyepiece height, maximum weight, any size constraints ( both packed up and assembled), minimum F ratio ( I suggest not below F4) and see where that leads you.
For the above discussion, eyepiece height is usually about the same as the primary mirror focal length.
I find it a fascinating voyage every time I build a scope, with compromise required at every turn
It's already been fascinating, and I'm learning a lot...there are so many people on Ice In Space who are very generous with their knowledge and experience.
At this stage I don't think a custom mirror will be possible, so I'm thinking about alternatives....I've seen a couple of options at telescopes-astronomy.com.au that look interesting...
12" pyrex or BK7 at f/5
16" glass at f/5
I'm 180 cm tall so the 16" f/5 focal length of 200cm might put the eyepiece a bit high. 16" at f4.5 (f/l 180cm) might just be possible though...
These mirrors are stated to have strehl rating of better than 0.88 and wave rating of 1/8. I'm wondering how these figures stack up - trying to find out what the minimum numbers are for "very good but not premium" optics.
As I work towards a more definite idea of what I'm looking for I'm going to keep my eyes open for something second-hand as well, as this might be the most cost-effective way to go.
While the technical aspects of optics and light are fascinating, aperture and portability are by far the primary concerns when choosing a scope for general observing (IMHO).
If collimation is touchy you can buy a better collimator and you can buy a paracorr if needed, but there's not much you can do about images that are too dim or a scope that's too cumbersome. The central obstruction gets more attention than it deserves too (for a general purpose scope).
So I think you should get the biggest aperture, in a structure that doesn't flex and moves smoothly, and a scope that's not too big to deter you from setting it up.
