Schmidt Cassegrains and DSOs

[emadmoussa]

I'm a refractor fan and any opinion I might give her may carry within it a degree of subjectivity. But I want to make sure I am not just that - biased.

The sharp contrasty views through a refractor have always been the major appeal to me. I noticed that in a Schmidt Cassegrain - be it the larger C11 - I don't really get these rich sharp images a refractor provides me. My C11 is in perfect collimation, and despite it's massive light collecting capabilities it still doesn't give me this sort of satisfaction especially on deep sky objects. Don't get me wrong, I still think this scope is magnificent with planets, but not DSOs.

I'm aware there's a certain degree of light loss due to tube mirror obstruction, but so does a reflector which as far as I remember showed slightly richer, sharper views.

So, what's the story? Am I just imagining?

[Gib007]

I'm a refractor fan and any opinion I might give her may carry within it a degree of subjectivity. But I want to make sure I am not just that - biased.

The sharp contrasty views through a refractor have always been the major appeal to me. I noticed that in a Schmidt Cassegrain - be it the larger C11 - I don't really get these rich sharp images a refractor provides me. My C11 is in perfect collimation, and despite it's massive light collecting capabilities it still doesn't give me this sort of satisfaction especially on deep sky objects. Don't get me wrong, I still think this scope is magnificent with planets, but not DSOs.

I'm aware there's a certain degree of light loss due to tube mirror obstruction, but so does a reflector which as far as I remember showed slightly richer, sharper views.

So, what's the story? Am I just imagining?

Focal ratio is what defines how fast you saturate images. SCT telescopes tend to have high focal ratios like f/10 and these are dead-slow for imaging. You can of course do it, but it's really, really slow. The way to go here is to use a focal reducer. I think Celestron makes a 0.65x focal reducer for their SCT range and that would bring down the focal ratio from f/10 to f/6.5. Do take into account that this also widens your FOV so you won't be "zooming in" as much on DSOs. This may be good or bad depending on your needs. In any case, f/6.5 is when you can start thinking about imaging in ample time.

[emadmoussa]

Focal ratio is what defines how fast you saturate images. SCT telescopes tend to have high focal ratios like f/10 and these are dead-slow for imaging. You can of course do it, but it's really, really slow. The way to go here is to use a focal reducer. I think Celestron makes a 0.65x focal reducer for their SCT range and that would bring down the focal ratio from f/10 to f/6.5. Do take into account that this also widens your FOV so you won't be "zooming in" as much on DSOs. This may be good or bad depending on your needs. In any case, f/6.5 is when you can start thinking about imaging in ample time.

I think I didn't make myself clear. I was talking purely about visual observing - imaging is set aside for the time being.

[AlexB67]

From what I read ( not from experience ) apart from the most astute observers the loss in contrast in a reflector is hardly visible at 20% obstruction size ( not by area, radius ), and becomes more noticeable above that figure. In terms of obstruction, it is actually an interesting fact that the airy disk radius shrinks with increasing obstruction size , thus increasing "resolution" but that would be not a good way to look at it in terms of the overall result, more importantly is the effect on the rings in terms of intensity and how it affects the final result.

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

and

http://www.hoflink.com/~mkozma/obstruction.html

some interesting simulated images and reading about the topic.

That being said, I've never heard anyone complain much about lack of good planetary views in the SCTs having an obstruction of 35% or so. Horses for course I suppose in each design. SCTs, nice slow compact scopes, low aberrations, a bit of contrast loss, ooh, and I forget, very expensive if you want some aperture on top

oops, sorry I forgot the topic actually stated DSOs and the articles are more about planetary viewing, but anyway, still interesting reading.

[Gib007]

My apologies! I know close to nothing about visual use, besides sticking an eyepiece in and looking through it so my contribution to this thread is null, hehe.

[emadmoussa]

Yes, agreed. Planetary observing in the C11 is stunning, but my main interest ia DSOs which renders me a bit disappointed with SCTs not being able to achieve similar resolution as refracting telescopes.

Unless you have a minimum of 8" high end refractor to view the planets, SCTs always win. Deep sky objects However are beighter with higher res in good refractors...let alone the wider field of view.

This is my experience, some might disagree.

[emadmoussa]

My apologies! I know close to nothing about visual use, besides sticking an eyepiece in and looking through it so my contribution to this thread is null, hehe.

No worries, sometimes I babble and babble without making my point clear... you can ask the wife

[nytecam]

Yes, agreed. Planetary observing in the C11 is stunning, but my main interest ia DSOs which renders me a bit disappointed with SCTs not being able to achieve similar resolution as refracting telescopes. Unless you have a minimum of 8" high end refractor to view the planets, SCTs always win. Deep sky objects However are beighter with higher res in good refractors...let alone the wider field of view. This is my experience, some might disagree.

You seem to be comparing bright planets and faint fuzzies without reference to your own eyes. DSO, by their nature, are never as crisp as the moon etc unless with lower powers via a short fl refractor that you prefer. Personally refactors make great finders and SCTs great for DSO imaging

[Luke]

I think a refractor will have an edge in sharpness because of no obstruction but it was close on DSO's between my C11 and a good 12 inch Skywatcher dob. The dob was slightly better but I put that down to the touch more aperture. Was your SCT definitely cooled down?

[kirkster501]

I was not impressed with DSO on my C11 (actually a CPC1100) either. So I sold it. The 12 Dob and the ED80 refractor are cracking visual instruments. The C11 is superb too for lunar and planetary but simply too long a FL for DSO I found.

[YKSE]

I have no experience about bigger refractors or C11, but from the scopes I've had, when view DSOs, I would rather use 130P than the 80ED, simply because the aperture wins on these faint fussies. The only disadvantage of C8 (I asumed it's true about C11) is the narrow field of view, which makes some large DSO can't be framed in wide-angle EP, but there are only very limited number of these DSOs, other than that, C8 wins hands-down but every possible account.

[Moonshane]

Yes, agreed. Planetary observing in the C11 is stunning, but my main interest ia DSOs which renders me a bit disappointed with SCTs not being able to achieve similar resolution as refracting telescopes.

Unless you have a minimum of 8" high end refractor to view the planets, SCTs always win. Deep sky objects However are beighter with higher res in good refractors...let alone the wider field of view.

This is my experience, some might disagree.

are you comparing similar magnifications? i.e. a 2.8m focal length will naturally produce higher magnifications with similar eyepieces. e.g. using a 32mm plossl would equate to 87x in the C11 and around 30x in a 1000mm focal length frac. this in turn produces a narrower field and apparently brighter image in the frac.

personally, I'd do nothing until you have had a look through several scope types and can then make an informed choice. you say you are not a reflector guy but surely that's what a SCT is? I have never looked through a SCT but am very happy with views through my newts and have found the images in my newts as good or better than any fracs I have used - including 6" f5 newts.

[Moonshane]

ps, if your main interest is fainter DSO's then in my opinion, dark skies then aperture matter most.

[emadmoussa]

I was not impressed with DSO on my C11 (actually a CPC1100) either. So I sold it. The 12 Dob and the ED80 refractor are cracking visual instruments. The C11 is superb too for lunar and planetary but simply too long a FL for DSO I found.

I think we have similar opinion on certain issues. I was actually considering a 10 inch dob and a 110mm ED frac with the price of both the C11 and 80ED. I am not sure though about the dob, I never got on with reflectos, but probably for a dob it does take getting used to?

[ollypenrice]

I've no axe to grind because I have a 10 inch Meade SCT (well collimated) and a TEC140 apo, or rather the business does! I am a bit like Emad because I would never dream of putting the SCT on the mount for a visual session just for me. Give me the TEC any time. It isn't as bright but... it makes my heart sing. You are just out there in space. The SCT gives a bright, informative and detailed image which looks like an image. With the TEC I'm up there.

Olly

[emadmoussa]

ps, if your main interest is fainter DSO's then in my opinion, dark skies then aperture matter most.

I not comfortable with the eyepiece position mainly...

[Mr Spock]

My C9.25 will produce lower magnifications than I need for DSOs. I can compare it with a Newt - I used to have a 250mm f4.7. A 13mm eyepiece in the 250mm gave very similar views to s 22mm in the C9.25. Optically I find very little difference between these two for DSOs or for planets.

I think any perceived lack of image quality in a SCT is either psychological or due to inexperience.

[emadmoussa]

My C9.25 will produce lower magnifications than I need for DSOs. I can compare it with a Newt - I used to have a 250mm f4.7. A 13mm eyepiece in the 250mm gave very similar views to s 22mm in the C9.25. Optically I find very little difference between these two for DSOs or for planets.

I think any perceived lack of image quality in a SCT is either psychological or due to inexperience.

Perhaps psychological but I happened to conpare SCT next to a frac...the frac had a better resolution. Then again I had and have 6 fracs in total and 1 SCT...

[earth titan]

IMHO, my C9.25 outperforms any frac on DSOs for the price point. You show me a frac for a £1000 that gives the same level of detail in DSOs and I'll eat my hat!

Hardly a fair test though, as the SCT has a much bigger mirror than a premier frac and if you compared a premier newt for the same price, I bet that would be even better still.

I think the mistake people make with a SCT is to use the same eyepiece they would with a faster scope and as a result gain magnification at the expense of detail.

These days I do all my DSO stuff with my C9.25 as I haven't got room for another big scope, and it is the best all rounder I've found, but I do use a 2" 40mm eyepiece most of the time, seldom dropping down below a 1.25" 25mm.

I agree a frac gives lovely views of larger objects such as the Pleides, where as I can't fit this into the FOV on the C9.25, but I've never found the smaller DSOs with my APO which cost the same.

The benefit of a slower scope, also seems to be a flatter field.

Each to their own though! Would be dull if we all had the same scopes.

[Moonshane]

It surely cannot have had better resolution as aperture governs resolution. it's semantics really but I presume you man better contrast?

[RikM]

I have only looked through smaller SCT's C5/C6 at deep sky objects. I friend has a C6 and have compared to my 150P Newtonian and a 5" achromat. At equivalent magnification, both the reflectors were essentially the same and slightly out performed the refractor on everything except producing pinpoint star shapes in open clusters. Could this be what you mean by better resolution? Certainly the Orion nebula was more extensive in the larger telescopes.

[HD-AP]

I'm a refractor fan and any opinion I might give her may carry within it a degree of subjectivity. But I want to make sure I am not just that - biased.

The sharp contrasty views through a refractor have always been the major appeal to me. I noticed that in a Schmidt Cassegrain - be it the larger C11 - I don't really get these rich sharp images a refractor provides me. My C11 is in perfect collimation, and despite it's massive light collecting capabilities it still doesn't give me this sort of satisfaction especially on deep sky objects. Don't get me wrong, I still think this scope is magnificent with planets, but not DSOs.

I'm aware there's a certain degree of light loss due to tube mirror obstruction, but so does a reflector which as far as I remember showed slightly richer, sharper views.

So, what's the story? Am I just imagining?

It's probably down to a combination of large central obstruction ( compared to a newtonian a refractor has none ) and poorer optical quality ( compared to a refractor and a good newtonian)

apeture dosen't compesate for the deficiencies of large central obstruction and poorer optical quality.

[michael.h.f.wilkinson]

Sorry to say it but a load of people are still banging on about slow scopes not being suitable for DSO observing. That is one whole load of foetid dingo's kidneys. An F/10 scope with a 20mm EP gives the same visual appearance as an F/5 with a 10mm (assuming similar optical quality). The amount of light per unit of solid angle reaching the eye is exactly the same. For visual, aperture is king. The only drawback of a slow scope is that the maximum FOV is a bit more limited (1.34 deg in my 8" F/10) for visual work.

I have both an 80mm F/6 triplet APO (from APM), and an 8" SCT. Guess what? The SCT is the better DSO hunter by far on about 95% of the 793 DSOs (including 395 galaxies) I have bagged to date. This is simply a matter of aperture being king in visual. My C8 bagged me a mag 13.2 galaxy this summer; try that with an 80mm. Only for a handful of wide-field objects (M31, M33, the Veil, the North America Nebula and Pelican, the M45, the Magellanic Clouds, and perhaps half a dozen more) does the 80mm come into its own with its 5.3 deg FOV with the 31T5. The view of pinpoint stars is stunning, but remember that this only in part due to the lack of central obstruction. The remainder is due to the simple fact that you do not use as much magnification in the smaller scopes (65.5 times is the minimum mag I use in my C8, vs 15.5x for the 80mm, the C8 regularly gets good views at 203x, which I never use with the 80mm). This means I will see much more of the contribution of the atmosphere in my C8. With a C11, these problems are worse.

So, for me, which is the better DSO hunter, for the vast majority of faint fuzzies, given the choice of an 11" SCT or a 6" APM-class APO? The 11" SCT, any time. Will a 12" Dob outperform it visually on DSOs (and at a fraction of the cost)? Sure it will, assuming the optical figures are similar.

The APM does show more contrast at a given exit pupil size. At the same magnification the situation can be different, as the resolving power, and light gathering power of the C8 are so much larger.

[michael.h.f.wilkinson]

It's probably down to a combination of large central obstruction ( compared to a newtonian a refractor has none ) and poorer optical quality ( compared to a refractor and a good newtonian)

apeture dosen't compesate for the deficiencies of large central obstruction and poorer optical quality.

Optical quality is most critical in planetary work, as you are looking at very fine detail. A scope which excels at planetary and lunar can hardly be poor quality. One more point to realize is that a scope below about 6" in diameter (and few of us have APO scopes beyond that) "looks" through a single seeing cells (roughly), which means seeing tends to make stars wobble about a bit, but not to blur so much. In bigger scopes, light from the same point source passing through different parts of the aperture travel through different seeing cells at each point along the path, which results in more blurring and less wobble.

[earth titan]

As another 'slower' visual scope fan, can I point out its easier to figure slower optics than fast ones. Therefore more bang for your buck and more tolerant of lower quality eyepieces.