SCT central obstruction

[TSRobot]

Hi. I've been looking at the quote figures for the size/area of the central obstruction for SCTs. 2 percentages are given. 1 by area and 1 by diameter. The percentages are very different. What do they really mean?

[vlaiv]

They should be same thing.

Central obstruction in SCT is circular - same shape as aperture. So you can define it's size in various ways. One is to quote diameter or radius of it. Other is to give percentage of diameter / radius of aperture. This is handy way of quoting it, because it lets you quickly calculate radius / diameter in mm from percentage. For example 8" scope (200mm) with 25% CO will have 50mm diameter of central obstruction. Size of central obstruction is related to design of scope, so it makes sense to state it regardless of aperture (same design can have 6", 8", etc). Other important implication is that since CO is usually given as percentage of aperture radius / diameter, some common figures can be noted and impact of CO on resolution can be defined in terms of these common figures. Like less than 25% CO has virtually invisible impact on resolution of the scope - so 6" 20% CO scope will in terms of resolution perform pretty much the same as 6" unobstructed scope (both having perfect curve). Scopes with >40% CO will have significant impact on resolution and contrast.

On the other hand stating CO as percentage of the area is the same thing - simple math and relation that surface of circle is radius squared times PI should give you easy way to convert between CO percentage of diameter / radius or surface. This way of quoting it has more to do with light gathering than resolution - how much of light collecting area is blocked by CO, so scope with certain CO behaves as smaller unobstructed scope in terms of light gathering (again CO percentage let's you see how smaller unobstructed scope is).

[Louis D]

vlaiv is correct.  To summarize, the smaller number is CO by area which affects light gathering and is the less important number.  The larger number is by diameter and is the more important one as it affects contrast which affects perceived resolution.

A typical SCT will have a 35% to 40% CO by diameter compared to a 20% to 25% CO for Newtonians.  By area, it will be much less for both.  Even getting above 30% CO, contrast will start to noticeably go down.  By 40%, it's quite obvious in A/B testing with a low CO Newtonian on low contrast objects like Jupiter.

Even at 20% CO, a Newtonian looks noticeably worse than an equivalent APO refractor when it comes to contrast.  Of course, that APO will cost much, much more than the Newtonian on a per aperture basis.

SCTs cost more than equivalent Newts as well because you're paying for the convenience of a folded, more compact light path which necessitates the larger CO.  TANSTAAFL

[michael.h.f.wilkinson]

My old 6" F/8 Newtonian had a very small secondary: 20% by diameter, 4% by area (because 0.20^2 = 0.04), and had great contrast, my C8 has 34% CO by diameter, or about 11.6% by area, and contrast is lower. I do actually see more detail with it on Jupiter, simply because the aperture is bigger, but the detail is less "in your face", and you have to tease it out more. The big advantage over the Newtonian (apart from better DSO performance due to aperture) is that I can take it along on holidays without having to put the kids or the missus on the roof rack

 

[TSRobot]

Very good explanations chaps. Thanks very much. 

[vlaiv]

1 hour ago, Louis D said:

Even at 20% CO, a Newtonian looks noticeably worse than an equivalent APO refractor when it comes to contrast.  Of course, that APO will cost much, much more than the Newtonian on a per aperture basis.

Is it really that much worse? According to physics and math, there should be some difference, but I'm not sure how much of it would be noticeable at the eyepiece.

Here is graph:

(reference: http://www.telescope-optics.net/telescope_central_obstruction.htm)

and if you look at 0.2 - 0.4 normalized frequency range, line for 32% CO does have like 1/3 less intensity of unobstructed, but at the same range 16% CO has at most 5% loss. It does not show line for 20%, but I think it would be less than 10% reduced compared to unobstructed.

I have not had a chance to actually compare the same apertures at 20% CO (20% CO Newtonian is hard to find in less than 6", and on the other hand 6" APO is not something that I readily have access to ). Maybe one could fashion simple spider and CO to put on 4" class APO/ED scope and compare - like aperture mask but CO instead (much higher probability anyone wishing to try could have access to).

One more thing to note, that mathematics of CO does not take into account - non linear response of human eye (both on perceived brightness depending on scope size / magnification, and perceived brightness depending on color / wavelength of target). So some targets might suffer more and some less from CO.

 

 

[Louis D]

2 hours ago, vlaiv said:

Is it really that much worse?

I find it easier to pick out the GRS in my 72mm ED scope at the same exit pupil than with my 8" Dob.  It's smaller, but it's more obvious.  I can also pick out the E and F components of the Trapezium much easier with the frac than the Dob.

Having any CO, no matter how small, limits how pinpoint each point of focus can be.  Stars are more bloated as well in the Dob than the frac.  It's not the mirror, it's a premium, hand figured mirror in the Dob with an 18% CO by diameter secondary.  Part of it is surface roughness of the enhanced and dielectric coatings.  Silver would be better, but no one does them for mirrors.

[vlaiv]

2 hours ago, Louis D said:

I find it easier to pick out the GRS in my 72mm ED scope at the same exit pupil than with my 8" Dob.  It's smaller, but it's more obvious.  I can also pick out the E and F components of the Trapezium much easier with the frac than the Dob.

Having any CO, no matter how small, limits how pinpoint each point of focus can be.  Stars are more bloated as well in the Dob than the frac.  It's not the mirror, it's a premium, hand figured mirror in the Dob with an 18% CO by diameter secondary.  Part of it is surface roughness of the enhanced and dielectric coatings.  Silver would be better, but no one does them for mirrors.

That is quite interesting, I compared my 80mm apo to my 8" dob and it is no contest at all (things are both easier to spot and more detailed in dob). Granted, might be my diagonal - it is TS 2" 99% dielectric, so not premium by any means. Must try straight thru on APO when I get a chance and compare the two.

Optically APO should be way ahead, having ~ 0.94 Strehl in green, while Dob tested to ~0.8 (system strehl so both primary and diagonal combined).

[dweller25]

When I had my 10” dob I would usually observe Jupiter at around x180 and used a neutral density filter to dim the amount of light getting through to the eye. This gave me really great views that my 5” APO could not match.

[michael.h.f.wilkinson]

12 hours ago, Louis D said:

I find it easier to pick out the GRS in my 72mm ED scope at the same exit pupil than with my 8" Dob.  It's smaller, but it's more obvious.  I can also pick out the E and F components of the Trapezium much easier with the frac than the Dob.

Having any CO, no matter how small, limits how pinpoint each point of focus can be.  Stars are more bloated as well in the Dob than the frac.  It's not the mirror, it's a premium, hand figured mirror in the Dob with an 18% CO by diameter secondary.  Part of it is surface roughness of the enhanced and dielectric coatings.  Silver would be better, but no one does them for mirrors.

The central peak actually becomes narrower (but lower!) as central obstruction increases, as shown in the graph below. This matches the graph posted by vlaiv, where the higher frequencies are enhanced, at the cost of the mid frequencies. The mid frequencies contribute most to the contrast and apparent sharpness of the image. At the same magnification the central peak of the psf of my 8" scope is 2.5x smaller than that of my APM 80mm F/6 triplet (I don't have Strehl figures for either scope, the APM 80mm is very, very sharp indeed, and my C8 is simply a good quality one). The graph below shows that even the first diffraction ring lies well within the central peak of the PSF of the 80mm. I have found it much easier to spot E and F with the C8 than with the 80mm APO. At the same exit pupil, the colour intensity of the red spot is more striking than in the C8 (and that is a contrast issue), but for detail the C8 has the little frac beaten by a mile

image from http://www.telescope-optics.net/obstruction.htm

[vlaiv]

Pardon my ignorance, but what does using same exit pupil when comparing two different apertures (3" and 8") ensures (in terms of equalizing, except of course exit pupil size)?

On the matter of "redness" of GRS when using APO vs Reflector, I wonder how much is there due to reflectivity curve of particular coating on reflector?

According to this and similar graphs:

Enhanced aluminum has a dip in red compared to other wavelengths (dependent on AOI, so even greater dip at angles compared to normal, I believe - so faster scopes might be even more affected by this - maybe that is why 6" F/8 is quoted as excellent planetary scope).

[michael.h.f.wilkinson]

12 minutes ago, vlaiv said:

Pardon my ignorance, but what does using same exit pupil when comparing two different apertures (3" and 8") ensures (in terms of equalizing, except of course exit pupil size)?

On the matter of "redness" of GRS when using APO vs Reflector, I wonder how much is there due to reflectivity curve of particular coating on reflector?

According to this and similar graphs:

Enhanced aluminum has a dip in red compared to other wavelengths (dependent on AOI, so even greater dip at angles compared to normal, I believe - so faster scopes might be even more affected by this - maybe that is why 6" F/8 is quoted as excellent planetary scope).

Using the same exit pupil ensures the Airy disk is shown at the same scale, that is all. The effect of heavier wings of the PSF in the case of CO affects contrast in much the same way as spherical aberration, which affect the wings more than the sharpness of the centre peak of the PSF. Many "portrait" lenses (like my Zeiss Planar 85mm F/1.4) show quite a bit of SA at full aperture, which lowers contrast more than it affects resolution. As you stop down, SA vanishes very quickly, and the image becomes razor sharp and contrasty. At full aperture, it still gives the impression of sharpness but the soft nature of the image masks small imperfections in the skin nicely, hence the use in portrait photography.

[Peter Drew]

What do CA and CO have in common?  They are both overrated!   

[michael.h.f.wilkinson]

My best Jupiter image with the 80mm

vs one of my best Jupiter captures in the C8

No contest, I would say

[paulastro]

I know all the folks contributing to this thread here will know it, but it's worth a reminder that seeing plays an important part.   A bigger telescope will always perform comparatively worse than a smaller one visually on the Moon/planets in poor seeing.  Of course with an image, it's cheating (in the nicest possible way ) in that by selecting frames and stacking them you can get much better details with an image than you can visually observing on  any night almost irrespective of the seeing.  It's not a coincidence that many top imagers use scopes with large central obstructions for imaging the planets.

As a visual observer myself, I know how good Newtonians and even SCs can perform on the planets in ideal conditions, it's just that they only generally come along every few years, if you have clear sky to take advantage of it.  In my view it makes sense to have a nice moderate aperture refractor and a bigger obstructed scope for when seeing allows you to get the best out of it.

[michael.h.f.wilkinson]

Seeing is important, but I have found that even under poorer conditions, it pays to lug out the bigger scope, and stay at the EP long enough to catch those fleeting moments of stable air. Patience and comfort when observing are often overlooked. There is also a difference in the way seeing affects smaller and larger scopes. On the whole in smaller scopes, the image tends to wobble more than blur, whereas in very big scopes, blur is the main effect. The transition is caused by the number of seeing "cells" that fit into a single aperture. In big scopes, different parts of the aperture are looking through different cells, which causes the final image to be a superposition of multiple slightly shifted images. As the shifts are random, on average the position is the same. If the scope's aperture is smaller than a typical seeing cell, most of the light is refracted by the air in roughly the same way so wobble is more noticeable than the blurring effect, which is certainly still there. Of course, because the central peak of the PSF is wider in a small scope, seeing has to be worse to affect it noticeably.

In practice, all my lunar and planetary observations are done with the C8, and the 80mm triplet is used mainly for wide-field observing, and solar observing and imaging.

[paulastro]

1 hour ago, michael.h.f.wilkinson said:

Seeing is important, but I have found that even under poorer conditions, it pays to lug out the bigger scope, and stay at the EP long enough to catch those fleeting moments of stable air. Patience and comfort when observing are often overlooked.

Yes Michael, of course observing technique is paramount to get the best out of any telescope - I took that for granted and didn't mention it in my post.  There are occasions of course when the aperture of some scopes just don't play ball.  It's then a matter of judging how long it's worth persevering with the larger scope before resorting to stay with a smaller aperture instrument.  Also, many people observe in locations when they are just going to have to make the best of only brief clear conditions.  In cases where the clear periods are very limited I may just use my smaller telescope, depending on the nature of the observing I'm doing to make the best of the time available.

[michael.h.f.wilkinson]

2 minutes ago, paulastro said:

Yes Michael, of course observing technique is paramount to get the best out of any telescope - I took that for granted and didn't mention it in my post.  There are occasions of course when the aperture of some scopes just don't play ball.  It's then a matter of judging how long it's worth persevering with the larger scope before resorting to stay with a smaller aperture instrument.  Also, many people observe in locations when they are just going to have to make the best of only brief clear conditions.  In cases where the clear periods are very limited I may just use my smaller telescope, depending on the nature of the observing I'm doing to make the best of the time available.

Fair enough, but I haven't yet encountered conditions in which my smaller scope spotted more detail than the big one on planets. Of course, if there is a threat of clouds or rain, the big scope is problematic in that it needs to cool down, which is REALLY necessary to get the most out of it, whereas the small frac really just works out of the box. In those conditions, I would certainly get out the frac, even for moon and planets. The smaller scope can also be rushed inside much faster than the C8 (let alone a big Dob), which can be a real bonus in our fickle climate.

[paulastro]

Yes, an 80mm doesn't do enough for me now I'm more aged ().  My refractor is a 102mm and will perform better than my 180mm SW Mak (on long-term loan from a very good friend of mine) on some occasions, but not that often as you indicate.  I generally keep the mak in my observatory but bring it in if it look like there is going to be a prolonged period of poor weather.  My observatory is rather dilapidated  and rather damp to leave it out permanently.  The refractor is used more often as it's very easy to take out at short notice.

I liked your comparison images by the way, I wish I could see 'views' like that through either of my scopes .