Light gathering of Cassegrain vs Newtonian

[osbourne one-nil]

Sometimes I ask stupid questions and pretty instantly wish I hadn't...this is probably going to be one of those times. 

I know the relationship between aperture and light gathering, the square of the aperture etc etc. But a Newtonian mirror doesn't have a stonking great hole in the middle like a Cassegrain-type instrument does, so surely for any given aperture, any Newtonian will collect more light than a Cassegrain as it has a bigger surface area...or does the secondary have the same impact as the "hole"?

[ronin]

A CST may have a hole but both have a secondary that should be central and so straight above where the hole of the CST is.

Really if Newtonian, CST, Refractor it is not really worth trying to take holes and secondary's into account. A refractor after all has absorbtion through the glass and no-one appears to consider that a mirror is not 100% refectivity, usually down at the 92-95% mark.

[Naemeth]

I'm sure there will be a difference, albeit small - but perhaps not noticeable if you compare a Cassegrain to a Newtonian side by side. The number of elements in an eyepiece also makes a difference, but often the quality of the eyepiece will matter more than how many elements it has.

Are you thinking of a C14 vs 14" Dob?

HTH

[osbourne one-nil]

Ha - is it that obvious?!

[Naemeth]

Ha - is it that obvious?!

It's something I've thought of myself .

[Stu]

The secondary size can have an impact on contrast by causing scatter etc. SCT central obstructions tend to be larger than Newts. The obstruction on a C8 is for instance 34% by diameter, which works out at around 11% by area, so it is a real effect. My f6 newt has a 16.7% obstruction which is 2.8% by area, significantly smaller.

There are lots of other effects obviously, refractor glass and diagonals all lose light to an extent, as do newt mirrors and SCT front plates and mirrors.

I believe, but am quite happy to be corrected, that the scattering/contrast impact is more significant than aperture loss.

Stu

[brantuk]

The central obstruction of both scopes does affect light gathering - but for the same aperture and assuming the obstructions are a similar diameter - it's going to be negligible. The main difference is going to come from the width of the fov, which will be down to the difference in focal length, focal ratio, and how fast each scope is.

The newtonian will have a shorter focal length and faster f-ratio typically around f-5 and will give a much larger fov. The longer focal length cassegraine will give sharper views but narrower field of view. If you compare both scopes on a planet you'll get higher contrast and detail from a Mak or Sct. The newtonian will still give a good view but with slightly less contrast.

Turn them both on a faint dso and you'll get a nice wide field with the object very visible (maybe using averted vision). But the cassegraine will have a narrower field and at f-10 may struggle to even see it (depending on which object of course). If it does see it - the view is clearer in the newtonian.

That's my experience at least - others may differ and there's a lot of variables: eyepiece quality, health/age of eyes, seeing etc. But all things being the same that's how it appears to me. Hth

[osbourne one-nil]

I'm definitely much more deep-sky orientated, so I've always tended towards Newtonians, and while a SCT might be sharper on planets, a good Newt is hardly a slouch I suppose?

[ollypenrice]

The newtonian will have a shorter focal length and faster f-ratio typically around f-5 and will give a much larger fov. The longer focal length cassegraine will give sharper views but narrower field of view. If you compare both scopes on a planet you'll get higher contrast and detail from a Mak or Sct. The newtonian will still give a good view but with slightly less contrast.

 

Since the SCT has a larger secondary obstruction than the Newt I think the Newt would have slightly more contrast, not slightly less, no?

Olly

[brantuk]

Actually Olly - I think you're correct - I did make a rather wobbly assumption that the size difference of the central obstruction in like for like apertures is similar.

Also I'm thinking about the views I get in the "closest aperture match" from my collection - the nearest I can achieve is a 200P newt v's a 9.25 Sct. The Sct is a spot brighter and more contrasty - but not by much despite it's inch and a quarter extra aperture. Of course, the secondary is much smaller in the newt.

So yes - I'm comparing apples and pears on a doubtful supposition and rightly stand corrected thank you.

I'd never argue with you on your birthday anyway - even in the unlikely event you were wrong lol

Many happy returns

[CodnorPaul]

My own experience in viewing through an 8" SW dob and my 8" C8 is that Brantuk is right - the contrast in my SCT gives a much sharper view on planets and bright objects, the 8" dob tends to do slightly better on DSO's but the difference is not vast, the benefit being it has a larger FOV.  The starbright XLT coatings on modern SCTs seem to make any deficiencies negligible.  Normally when I cant find an object in my SCT, I doubt I would find it in a newt given my own experience.

Obviously I cannot comment on how well collimated the newts I have looked through are - I know my SCT is well collimated so it performs very well, and has great optics which helps.  I also know I have never looked through a really premium newt which would be an interesting comparison.

Any upgrade I make to my kit will be a larger SCT rather than a larger newt - until I get real aperture fever and go 16" upwards...

[glappkaeft]

Like others said the central obstruction is the important factor. A rule of thumb is that the contrast and resolution of an obstructed telescope is reduced to that of an unobstructed telescope by the size of the obstruction. I.e. a 20 cm scope with a 6 cm secondary will match a 14 cm scope all other things being equal. Light grasp obviously depends on the % of the aperture area that is obstructed.

The hole in the main mirror actually has no effect at all since it is under the shadow from the secondary.