Reflector central obstruction size.

[Alfian]

I've been mulling over the purchase of a  SW 150PL or maybe a GSO 150 f6.  The virtues of the 150PL being an f8 and having a smaller c/o are well documented and as a result of this I've endeavoured to get more acquainted with the whys and  wherefores of C/O size in relation to aperture focal length etc. The linked article from Sky and Telescope is interesting. On the bottom of the fourth page (page 123) there is a graph and the text above seems to indicate that a less than full edge illumination is, if not unnoticeable - quite acceptable and in the case of the graph an 8" F6 reflector has a C/0 of 1.57 inches which is 40mm. This is the same size as the secondary on my old Celestron 130/f5 with a 1.25" focuser.   The 150PL has a C/O of 36mm (as per Ade Ashford's review) and this is considered small but clearly from this article given the nature  of this 'scope and its likely use it could usefully be smaller. I have read other stuff that says that the importance of the C/O size in terms of actual observational terms, gets a bit overstated so I'm not getting particularly hung up on this  - just interested. The question in my mind is that - is the use of secondary mirrors which for visual use may be a touch oversized, deliberate to ensure full illumination and/or create some ability to do AP. I'm feeling my way  with this so I may not have got this at all right but would appreciate any comments for further illumination. (pun intended!)

https://s22380.pcdn.co/wp-content/uploads/GS-Adler-Secondary.pdf

[vlaiv]

Best to think of it in terms of scope primary use.

If main purpose of the scope is to do high power visual - like planets and double stars, then it's worth having smaller secondary. For any other use - like mixed use, or AP or whatever - don't worry too much about secondary size - or rather go with larger one - like 25% by diameter.

Difference to planetary performance will probably be smaller than average to very good optical figure of primary mirror. You can "investigate" actual performance difference by searching for MTF graphs of different options - like impact of CO and various other aberrations, like spherical correction, etc ...

There is a program called Aberrator 3.0 that you can use to generate PSF and MTF of various aberrations and COs

http://aberrator.astronomy.net/html/download.html

Have a look at this graph for example:

(found on this page: https://www.telescope-optics.net/obstruction.htm)

It represents MTF graph of perfect aperture, obstructed aperture, and lens / mirror that has 1/4 wave lower order spherical aberration. Graph should be read like this: on right side are low frequency components ("larger detail") - on right are high frequency components ("smaller detail"). Both larger and smaller detail is a bit misleading - you should think more in terms of contrast rather than actual features - contrast between large features vs contrast in small details.

Curve represents relative attenuation for given frequency. Lower it is - more contrast will be lost at particular scale. You will notice that at some point to the right all curves reach 0 - this is theoretical resolving power of the telescope. You can magnify image above this point - but you won't see any contrast - image will look featureless / soft due to lack of contrast in small scale detail (all will be the same color/intensity of the light).

It is clear from the graph that 32.5% linear obstruction on perfect aperture is a bit better in performance than 1/4 wave LSA on same aperture. With newtonians, you will probably have both - certain level of CO and also less than perfect figure. This graph also shows what to expect in comparison to perfect unobstructed objective lens - no features will be missing, but image will look more "washed" out as some medium frequency detail will have lower contrast in obstructed and aberrated scope.

I'm sure you will be able to find comparison graphs on internet that show difference between 20 and 25% CO for example, or compound CO + certain level of SA.

This page is also worth reading:

http://www.damianpeach.com/simulation.htm

 

[Alfian]

Thanks vlaiv, I'll try to get my head round that. Unfortunately I can't use the first link as I'm a non-MS os user but the second link is interesting.

[Alkaid]

I thought that the ‘frequency’ axis of the curve referred to the wavelengths of light.  Low frequency (left side) being reds, oranges, like the surface of Jupiter.  Right side of curve being higher frequencies of light like green, blue. 

[DaveS]

No, it refers to the frequency in cycles per mm, ie how fine the detail you're observing. Coarse detail on the left, fine detail on the right.

Vlaive will, of course, say I'm being simplistic, which I am.

[vlaiv]

51 minutes ago, DaveS said:

No, it refers to the frequency in cycles per mm, ie how fine the detail you're observing. Coarse detail on the left, fine detail on the right.

Vlaive will, of course, say I'm being simplistic, which I am.

That is actually spot on.

1 hour ago, Alkaid said:

I thought that the ‘frequency’ axis of the curve referred to the wavelengths of light.  Low frequency (left side) being reds, oranges, like the surface of Jupiter.  Right side of curve being higher frequencies of light like green, blue. 

It's not related to wavelength of light, but it is related to wavelength - that of signal. It's a bit strange concept, but if you think about image being 2d intensity function (horizontal as X axis and vertical for Y axis for example), here we are talking about Fourier transform of that signal, and Fourier transform is in frequency domain - here being number of cycles per unit length (can be mm in focal plane or angular measure - like arc seconds in general), or corresponding wavelength - again in units of length.

Above graph actually represents Fourier transform of corresponding Airy pattern and it describes how convolution of image with that Airy pattern reduces frequencies in actual image. Airy pattern is low pass filter and above diagram is frequency response of that filter.

[NGC 1502]

 

I used to obsess over central obstruction size, but looking back I’m sure I needn’t have.  Not saying it doesn’t matter at all of course.

Some would say that a slightly larger than strictly necessary secondary is fine. It means that the edges of the secondary are not in use and it’s the edges where optical flatness may sometimes be compromised, especially the end closest to the primary where the glass is thinner.

I think the main requirements for sharp views are :-

Good quality optics, accurate collimation, cooled to ambient, steadiness of the atmosphere, good eyesight, a practised observer, etc etc........

Lots of views on this of course ?

Ed.

[John]

I tend to think that a larger obstruction is OK as long as the scope is kept in accurate collimation. When the collimation is off I feel that image quality reduces more noticably in scopes with larger CO's.