SCT/Newt/central obstruction debate [split from 16" Dob]

[sejanus (Gavin)]

hey guys out of curiousity, does a dobsonian of equal size to a SCT absolutely kill it optically? is the difference in brightness really big?

i.e. is the SCT's only advantage compactness?

please note I don't want a 10" or 16" dob I am just curious


cheers

[janoskiss (Steve H)]

Originally Posted by sejanus

hey guys out of curiousity, does a dobsonian of equal size to a SCT absolutely kill it optically? is the difference in brightness really big?

No. They will be similar in performance.

i.e. is the SCT's only advantage compactness?

Yes, probably. And once you figure the mount and tripod into it the advantage might not be not that great (esp. if you opt for an EQ mount).

[norm]

Would a SCT be marginally dimmer compared to a dob due to the corrector plate ?

[mick pinner]

Steve, what do you mean similar in performance? 3050mm fl compared to 1500mm l don't think so.

[Starkler (Geoff)]

Originally Posted by sejanus

hey guys out of curiousity, does a dobsonian of equal size to a SCT absolutely kill it optically? is the difference in brightness really big?

That would depend on the quality of the optics as manufactured. Having said that, newtonian optics are easier to make to a high standard than sct optics which are more complex with more components and surfaces involved.

As for image brightness, I doubt you could pick a difference between a newt and an sct of equal aperture at the same magnification.

[janoskiss (Steve H)]

Originally Posted by mick pinner

Steve, what do you mean similar in performance? 3050mm fl compared to 1500mm l don't think so.

I mean for the same aperture the two types of scopes are capable of showing you similar things. One might be an f/6 or f/5, the other an f/10, but if they have the same aperture, you'll get close to the same brightness and amount of detail in DSOs, on planets and the Moon. Wrt FL the main difference there is that you might need a 20mm eyepiece with the SCT when with the Dob you need a 10-12mm for the same power. But what you see will be similar.

Originally Posted by norm

Would a SCT be marginally dimmer compared to a dob due to the corrector plate ?

Marginally, but I doubt you'd notice. And the contrast might be a bit less because the central obstruction of the SCT is larger. OTOH on the Newt you have the spider. I wouldn't worry too much about it either way. But if you do, look through some scopes before buying one.

One other thing that I'm only guessing is that Newts would reach thermal equilibrium quicker because the tube is open - esp if a fan is used behind the primary mirror.

[Satchmo]

Originally Posted by sejanus

hey guys out of curiousity, does a dobsonian of equal size to a SCT absolutely kill it optically?

I wouldn't say `kill' but SCT's usually suffer from rough optical surfaces and the 40% obstruction at the secondary baffle lowers image contrast significantly over the smaller Newt obstruction. A 33% obstruction for example lowers the contrast curves equivelent to an unobstructed system with about 1/2 wave wavefront error. Throw in some surface roughness , and real optical errors and even good SCT's are struggling to get a Strehl ration in the low 0.70's. A well made Newtonian with small obstruction and single smooth primary surface can easily keep a Strehl above 0.95 ( Strehl ratio of a perfect system is 1.0).

Mark

[wavelandscott (Scott)]

Originally Posted by Mark Suchting

I wouldn't say `kill' but SCT's usually suffer from rough optical surfaces and the 40% obstruction at the secondary baffle lowers image contrast significantly over the smaller Newt obstruction. A 33% obstruction for example lowers the contrast curves equivelent to an unobstructed system with about 1/2 wave wavefront error. Throw in some surface roughness , and real optical errors and even good SCT's are struggling to get a Strehl ration in the low 0.70's. A well made Newtonian with small obstruction and single smooth primary surface can easily keep a Strehl above 0.95 ( Strehl ratio of a perfect system is 1.0).

Mark

So out of curiosity, what kind of difference would this difference in Strehl ratio translate in terms of difference in observable magnitude? Another way of asking is, for a visual observer what difference might a "reasonably skilled" observer notice?

Thanks!

[janoskiss (Steve H)]

Originally Posted by CoombellKid

This is not correct, the central obstruction is bigger in a SCT. To the point
where most say 6" SCT is eqivilent to a 8" newt.

Yes, as I mentioned about that larger CO will marginally reduce contrast in an SCT. But don't you mean 6" Newt to 8" SCT?? But even so I don't think so. The effect of a change in CO from 25% to 30% CO is minimal compared with that of change of aperture from 8" to 6".

How did you arrive at this
conclussion?

I've looked through a few SCTs, Maks and Newts, and aperture seems to be by far the most important factor when it comes to seeing detail in either DSOs or planets. e.g., a C9.25 SCT is certainly more capable than an 8" Newt, despite the larger CO of the SCT. Aperture rules. Everything else is icing on the cake, or lack thereof.

[Satchmo]

Originally Posted by CoombellKid

I've looked through a few myself, and compared even a C9.25 with my own
8" newt. The C9.25 show no more than my 8" newt, infact I was un-impressed
as at the time my 8" was clearly showing detail you were seeing in the C9.25.

One has to be careful of such comparisons, the seeing may have been limited to 8" cells on that night. However its generally accepted that a simple rule of thumb, assuming nice smooth optical surfaces is that you subtract the diameter of the central obstruction from the aperture to give you equivelent performing scopes in terms of equal contrast transfer function ( or MTF )

In other words , a C9.25 , 8inch Newt and 6" Apo refractor may all be capable of giving similar views , but I would say the 8" Newt would have the best potential for smooth wavefront with inly two surfaces in the optical train compared to 4 in the SCT.

Mark

[janoskiss (Steve H)]

Originally Posted by CoombellKid

Never yet seen a newt with a 25% obstruction, and I believe Most sct's CO is closer to 40%.

Most Newts around have about 25% CO. The 8, 10 and 12" GSO Dobs (i.e. Newts) all have 25% CO. The 6" f/8 Dob looks like it has a bit less but I never measured it. 8" f/4 Newts have more, about 30% for mine, but that's a very fast scope. I did not realise most SCT's had such large COs.

[Satchmo]

Originally Posted by janoskiss

Most Newts around have about 25% CO. The 8, 10 and 12" GSO Dobs (i.e. Newts) all have 25% CO.

Maybe in those small sizes. But 16" F5 ATM scopes typically uses 66mm flat ( 16%), 18" F4.5 uses 79mm ( 17% ) and 20" F5 using 79mm ( 15% ).

SCT's have bigger obstruction than you would think when you measure the actual diameter of the secondary stray light baffle.

Mark

[Starkler (Geoff)]

Originally Posted by CoombellKid

How do I figure CO out Mark. Do I measure the flat optical surface of the
secondary? then do some equation to allow for it sitting at an angle?

Its simply the primary mirror size divided by the minor axis of the secondary.

eg. 10" primary with a 2" secondary has a CO of 20%.

But I would like to add, get a second smaller scope as well for when you can't be botherd setting up a large instrument.

Good advice. In fact I wouldnt recommend a 16" of this type as an only scope unless you live in a good dark sky location and never want to shift it.

[Starkler (Geoff)]

Originally Posted by CoombellKid

how did you come to 20%

Sorry I divided the wrong way. A two inch secondary is 1/5 the diameter of a 10 inch primary, hence 20%.

[janoskiss (Steve H)]

Originally Posted by CoombellKid

If you had not realise why or did not know, why say anything? I would
of thought being a moderator on here, doesn't give you the right to
just say anything. thats 2 times now in a couple of weeks

Goodness me, Rob! You're a worry...

The facts:

- Geoff obviously meant the reciprocal of what he said, i.e. obstruction diameter divided by primary mirror diameter, so 2"/10" = 50/250 = 0.2 = 20%.

- GSO Dobs have about 25% central obstruction, i.e., the diameter of the secondary obstruction is about 25% the diameter of the primary mirror.

- The difference between light gathering area of two equal-aperture scopes with 25 and 40% CO is about 10%, which would correspond to an effective aperture difference of only 5%. Not enough to make too much of a fuss about. The effect of the larger CO on the point transfer function of the instrument is a much more important factor as it impacts on contrast.

[janoskiss (Steve H)]

Originally Posted by CoombellKid

Do you always find yourself answering other people msg's I think
me and Geoff can chat easy enough between us. I was going to answer
your post, but since you keep patronizing, then whats the point.

Well, I had not realised Geoff already answered your qn, and I could not have made my other points without clarifying that one first. No patronising intended by it.

I stick by my earlier statement that a Dob and an SCT of equal aperture will be similar in optical performance. Not identical but similar. That is certainly true within the context of someone like sejanus looking to buy their first scope off the shelf, so that we're talking about the likes of LX90 vs GSO Dob, not purpose-built hand-tuned Newts. But we probably managed to scare poor sejanus off by now anyway.

[jase (Jason)]

Oh how I love debates on contrast and resolution based around central obstruction.

If you looking for some hard facts on obstruction, go no further than here:
http://legault.club.fr/obstruction.html

Infact, step back a notch and look at http://legault.club.fr. Check out the "About high resolution" section. Nice information of collimation, sampling etc.

Enjoy the educational experience.

[Karls48 (Karl)]

You should also consider effect of focuser being fully racked in to reach focus with some eyepieces and cameras when comparing SCT with Newtonian. Defocus star image and you will see dark rectangle intruding to light circle. Surely, it must have impact on the image contrast

[bird (Anthony Wesley)]

It's very hard to properly compare scopes of different designs, especially if you're doing it at a star party where there are lots of different scopes set up and in use.

A lot of factors can effect the performance that you might not be able to measure or have any control over, e.g. whether or not the scope is properly cooled or collimated - these things can be radically different in scopes of different designs but the same aperture even when they are sitting side by side.

From what I've seen and read over the years, it seems to me that the humble newtonian can be the best performer, but it's also a bit cantankerous and needs TLC to get the most out of it. I think that's why the SCT has become so popular - sure the performance is down a bit compared to the equivalent newt, but it's a lot easier to transport and use.

If you want the best performing scope, and don't mind getting your hands dirty with some tinkering and maintenance then the newt is very hard to beat.

cheers, Bird

[74tuc]

Hi All,

Straight comparison of the two telescopes will show that t Newtonian can be made to perform better than an SCT. If you assume that the Strehl ratio is, initially, the figure of merit. The strehl of the 'scope is the product of the strehl ratios of the individual components in the light path.

For the Sct the Strehl ratio is the product of the strehl ratios of:

1. Corrector plate; 2; Primary mirror; 3; Secondary mirror;4.Secondary obstruction; 5; SR due to diffraction of central obstruction.

For a Newtonian the SR is the product of :

1. SR due to obstruction; 2. SR due to diffraction around central obstruction; 3. SR of main mirror; 4; SR of secondary mirror.

The Newtonian, because of its smaller CO, near perfect secondary (SR = 0.98) and with a hand figured main mirror, can be made diffraction limited for a small field of view.

The SCT, on the other hand, will have to have "neigh on perfect" components to approach the same performance of a Newtonian of similar diameter.

If you multiply the overall SR by the area of the main mirror you might consider this to be a figure of merit - The bigger the better.

Please note SR is meaningful for a field that displays no distortion, spherical aberration or coma a narrow field for most SCT's and Newtonians

Rgds,

Jerry.