Are central obstructions so bad?

[Tomjo59]

Having only just joined the Lounge, it seems that many people here have been through the same process as myself in trying to decide on which type of 'scope to buy for 'serious' observing.

My specific question is on the disadvantages of scope designs with a central obstruction, especially SCTs, where the obstruction is up to a third of the primary's diameter. Is the whole issue rather overblown?

If you listen to the hard-core refractor devotees, any obstruction significantly degrades contrast and resolution, and yet consider the quality of imaging work carried out by the likes of Damien Peach et al.

How many people on here have done an honest evaluation of 'scopes of the two types side-by side, under the same conditions?

It may be that if you tend to specialise in, say, planetary or double-star observation, then the difference is significant, but I have a suspicion that 'large central obstruction = poorer contast etc' is just received wisdom, rather than based on a more 'scientific' evaluation. Or does it depend on how experienced an observer you are, so that the majority of hobby astronomers wouldn't really notice much (if any) difference?

I shall stand back and wait for a possibly heated debate!!

[John]

I'm a great fan of refractors but I also think that central obstructions are not a huge issue if the obstructed scope is in good collimation. But good collimation is important in unobstructed scopes as well.

As someone who has owned many scopes I have done "back to back comparisons" and the results have been interesting and varied.

I think the whole concept of a "competition" between scope types is completely false - there are a range of designs, each with strengths and weaknesses. That's why folks often own more than one.

I've had my best views of Saturn with an 8" SCT but I've also really enjoyed the tight airey disks when observing binary stars with good refractors and the light grasp of a simple dobsonian.

I'm hedging my bets anyway - I presently own 2 refractors, a maksutov-newtonian and a traditional newtonian and all have shown me great views at one time or another

NB: I ought to point out that I'm purely a visual astronomer. I'm aware that imagers have another set of needs which will drive their response to this.

[barkis]

There is nothing better than a long focus reflector.

If it is designed as a lunar/planetary instrument, then the central obstruction need not be too destructive, as it would be quite small, especially coupled with a suitably low profile focuser.

It would excel as a double star instrument too. No chromatic aberration. The colours are real. Of course it would not be very suitable as a deep sky telescope. It is one of a selection of instruments one might wish to own.

Ron.

[MorningMajor]

I think the debate will always be tainted by a slightly blinkered view, with the various sides always maintaining "their" type of scope is better, Now, I'll be honest and say I haven't nearly has enough experience with different scopes, so couldn't enter the debate with any kind of informed view, but I love my scope. It's very much a beginners instrument, but it's teaching me a lot - it's not perfect by any stretch, and I know I could go and buy a better one - and probably will. if so, I'll plan to get something totally different and if I can afford it keep my current one as well.

I'd love to be in a position like John, and have an array of scopes to choose from because, one thing I've learnt, is that there is NO perfect scope - they all have strengths and weaknesses - so, even if the debate kicks off, no one will win it!!

The other man's grass?? - I don't think so

[barkis]

I don't think this thread should go down the road of which is the best scope, and I hope no one thinks my post intended that.

Of course when it comes to telescopes, it will always be horses for courses. No one type of instrument covers all the bases.

Ron.

[rfdesigner]

Been there..

got a windowed 12" with a small secondary... corrector on my "planned in" list

Not exactly off the shelf..

If you'll forgive me I'll ramble a bit as I've trodden this central obstruction path.

I wanted a scope capable of performing as well as possible over extened periods of time.. absolute field of view was a secondary concern to me. i.e. best strehl ratio in extended practice. This it does fairly well.

I find the lack of diffraction spikes a windowed instrument affords helps with planetary detail and with bright extened objects. The window also helps to keep tube currents controlled but it wasn't a simple bolt on. Adding a window to a poor newt will not help, the underlying instrument must be of good quality to benefit.. ditto SCT/RC etc. If you have a large enough instrument then obstruction really doesn't matter, my old 4.5" didn't hit diffraction limit that often, and the 12" will only do it on very rare occations.. so rare that I've only been looking a couple of times when it's happened.

Frankly you want quality and inches. If imaging you need to decide how much resolution you want. I wanted atmospheric limited or better, you may be happy with 3 or 4" fwhm and very wide field of view, you need to decide what you are trying to see. No point in a 24" f10 instrument for imaging M31 in one shot.

Also concider the HST, with a 2.4m primary and a 0.3m secondary, you're scope won't be in space, so don't worry if the secondary is twice that size (~25%)

Derek

[FraserClarke]

Also concider the HST, with a 2.4m primary and a 0.3m secondary, you're scope won't be in space, so don't worry if the secondary is twice that size (~25%)

That's mainly driven by the desired field of view of the instrument -- HST is a very narrow field instrument, so can have a relatively small secondary. If you want a wide field of view, you need a larger secondary. For example, LSST (proposed 8.4 meter wide field survey telescope) has effecitvely a 5m central obstruction == 60% obstruction!!

Fundamentally though, in a 'perfect scope', a central obstruction will always give lower contrast than no central obstruction (due to the way light diffracts from the different shapes). However, in practice, the performance is probably dominated by so many other factors that you'll be far better off with a good scope with a central obstruction than an OK scope without...

[Peter Drew]

Seeing conditions are the greatest influence on decent telescopes performance. .

[brianb]

Secondary obstruction has less effect than almost anyone imagines ... the worst effect is a slight reduction in contrast, which is critical for planetary work, yet all the world class planetary imagers are using scopes with central obstructions, many of then SCTs with 33% or more CO.

Optical quality has far more effect on the diffraction pattern than central obstruction. You pay for good optics! Refractors do work a bit better than reflectors of equal aperture when out of thermal equilibrium, but that's because of the number of times the light goes up & down the tube, not the central obstruction.

[Ad Astra]

You will never get a good answer to this question. Rather, let's reframe it a bit. "Is the Central Obstruction a problem for the way you observe most often?"

I have a really nice, long focus refractor and I love it to bits. Wouldn't change it for anything, but I love lunar and planetary observing. DSO's are nice, and I pick 'em off sometimes for fun when the Moon isn't out. For me, a central obstruction means less contrast and sharpness on the images I love best; so yeah, it's a big deal to me.

That said, if I have to travel with a scope, traveling with a SCT is easier and more trouble free than virtually anything else I can think of. They are very flexible and do almost everything well. And now with HyperStar / Fastar technology, they can shoot photographically at f/10, f/6.3 (focal reducer) and f/1.8 with HyperStar. Wow! No refractor ever thought of doing that! If flexibility and easy transport are high on your list, an SCT needs to be on your short list of scopes. Which one is another can of worms - there are lots of excellent choices out today.

So, I guess it comes down to tending the mote in your own eye, then. Will an obstruction mean that much to you? Just remember, everyone votes with cash!

Still in all, I'm still keeping my refractor... Apomax FTW!

Dan

[rfdesigner]

That's mainly driven by the desired field of view of the instrument -- HST is a very narrow field instrument, so can have a relatively small secondary

I thought the argument was the other way around: resolution first, which yields narrow field of view, partly due to obstruction, but mainly if you have ~0.02 arc sec pixels and fairly small CCDs by todays state of the art then FOV must be narrow. i.e. resolution was the driver.

Derek

[Jason D]

Anyone with a reflector can attach a larger obstruction mask than the secondary mirror to the spider vanes. Now using the exact same scope under the same conditions you can compare the views with the original secondary obstruction size versus the larger mask size.

It is a simple experiment that anyone can perform. It should provide firsthand experience on the difference.

Jason

[NGC404]

To get round the central obstruction on my 10" Dob , I use a 4" aperture mask.

The results you get on planets and multiple stars is increadible

Best piece of upgrade kit I ever bought

Price. Free of charge

A piece of cardboard with an off centre 4" aperture cut out

[FraserClarke]

I thought the argument was the other way around: resolution first, which yields narrow field of view, partly due to obstruction, but mainly if you have ~0.02 arc sec pixels and fairly small CCDs by todays state of the art then FOV must be narrow. i.e. resolution was the driver.

Yes, you could look at it that way. Resolution and field of view are not mutually exclusive though -- you just need a lot of pixels as you say! HST was never going to be a wide field instrument, because you can't afford to put that many pixels up there. So the optical design tends towards a narrow-field (smaller secondary) configuration. Also I guess, smaller secondary == lighter, which is a factor when you have to fling it 300km up

Guess we're going a bit off topic now :-\

[John]

To get round the central obstruction on my 10" Dob , I use a 4" aperture mask.

The results you get on planets and multiple stars is increadible

Best piece of upgrade kit I ever bought

Price. Free of charge

A piece of cardboard with an off centre 4" aperture cut out

Assuming your 10" dob is F/5, this gives you a 4" F/11.7 unobstructed scope - it should peform similarly to your TAL100 refractor although the refractor will transmit more light.

Personally I can't see the point of owning a 10" scope and then to stop it down to a 4" one, especially if I already owned a fine 4" F/10

[NGC404]

Assuming your 10" dob is F/5, this gives you a 4" F/11.7 unobstructed scope - it should peform similarly to your TAL100 refractor although the refractor will transmit more light.

Personally I can't see the point of owning a 10" scope and then to stop it down to a 4" one, especially if I already owned a fine 4" F/10

Keeps things simple if you are veiwing DSO,s and double stars in the same session, and dont want to keep swaping scopes.

Just takes 3 seconds to change from 10" to 4" without getting off the chair

Maybe not how everyone see,s it, but thats just (lazy) me

[John]

Keeps things simple if you are veiwing DSO,s and double stars in the same session, and dont want to keep swaping scopes.

Just takes 3 seconds to change from 10" to 4" without getting off the chair

Maybe not how everyone see,s it, but thats just (lazy) me

Good point - I'm all for simplicity

[ollypenrice]

I love refractors but would not make much of the contrast loss theory, within reason. It starts to be an issue with very fast astrographic designs (huge secondaries) which do have big contrast losses in visual use but that apart it is pretty forgettable. A third by diameter is not a third by area, remember!

Olly

[Ad Astra]

Assuming your 10" dob is F/5, this gives you a 4" F/11.7 unobstructed scope - it should peform similarly to your TAL100 refractor although the refractor will transmit more light.

Personally I can't see the point of owning a 10" scope and then to stop it down to a 4" one, especially if I already owned a fine 4" F/10

Stopping down the 10" will improve contrast on bright targets like Luna, Jupiter and Saturn - makes it easier to pull faint moons of Saturn in, too. Stopping down will also improve (decrease) the amount of 'swimming' the image does at higher magnifications when air conditions are turbulent. The improved contrast (no central obstruction) can also help in splitting doubles.

As to doing this if you already have a good 100mm f/11 - I dunno. Maybe a waste of time, but most of us aren't blessed with two fine pieces of kit like that!

Dan

[smudgeball]

I don't think this thread should go down the road of which is the best scope, and I hope no one thinks my post intended that.

Absolutely..........

But if it does my "central obstruction" is in my sig just below:D

[brianb]

Stopping down the 10" will improve contrast on bright targets like Luna, Jupiter and Saturn - makes it easier to pull faint moons of Saturn in, too.

Sorry but I never heard of faint objects being made more visible by reducing the aperture. If you're getting a "contrast improvement" maybe it's because the full aperture view is too bright - a filter would have a similar effect without wrecking the resolution. Alternatively it's because your reflector is not properly collimated or nowhere near in thermal equilibrium with the environment - no scope will perform to its potential under these conditions, but longer focal rations do tend to give a tider (but not better resolved) image in poor conditions.

Stopping down will also improve (decrease) the amount of 'swimming' the image does at higher magnifications when air conditions are turbulent.

That's true, but there's less information in the stopped down image - much less, unless the optics are of very poor quality (unusual these days).

[Ad Astra]

Sorry but I never heard of faint objects being made more visible by reducing the aperture. If you're getting a "contrast improvement" maybe it's because the full aperture view is too bright - a filter would have a similar effect without wrecking the resolution. Alternatively it's because your reflector is not properly collimated or nowhere near in thermal equilibrium with the environment - no scope will perform to its potential under these conditions, but longer focal rations do tend to give a tider (but not better resolved) image in poor conditions.

There is a marked difference between features that are difficult because they are low contrast and faint, extended objects like galaxies or dim nebulae. For these last, stopping down gets you nothing at all. For the former, you get a bonanza of detail. Things that benefit would include: gaps and ring structure on Saturn, splitting close double stars, cloud bands and surface features on Jovian planets, surface contrast features on Mars, lava flows (different color shades) on luna, etc. For these, taking out the central obstruction is the key, not reducing aperture. This is why an aperture mask improves contrast. You can also improve the view in turbulent air, but that has to do with the average size of the turbulence cell in the atmosphere. If you aperture is less than the turbulence cell size, they have far less effect on your view at the eyepiece. In this case, it IS aperture reduction that affects the improvement, not contrast enhancement. IRL, you will generally see both effects happening at once, and the result can be very pleasing. Think of it at 'tuning' your scope to give you peak efficience on your target of choice. There is a reflector design that is totally off-axis, and has no contrast problems with a central obstruction, they are called Schiefspiegler scopes, I believe (pardon my spelling from memory here).

Poor quality refractors sometimes benefit from a reduced aperture mask (as Galileo found) because it eliminates the outer regions of the lens which can be less accurately ground and polished than the center portion. Most commercial products are of sufficient quality today, that this is neither necessary, nor beneficial.

I hope that helps,

Dan

[John]

.....As to doing this if you already have a good 100mm f/11 - I dunno. Maybe a waste of time, but most of us aren't blessed with two fine pieces of kit like that!....

But the poster concerned is one such person

[Ad Astra]

But the poster concerned is one such person

The only thing to say there is that an aperture mask is an excellent DIY project that costs practically nothing.

Let's all be scientific about this - do the experiment and let Nature decide. The experimenter then accurately reports their data - and we go from there.

For what it's worth, the end of the optical train is always an eyeball, and a brain. Both instruments are so unique and mysterious, that we call the results subjective. Since the experiment costs little but time, and in no way harms or modifies either scope in a permanent way, why not give it a go and see what you think of the results? I've had great results with them on various reflectors over the years, and I still find that the results vary quite a bit both from instrument to instrument, and the seeing conditions had a big impact on success as well - the masks fared better compared to the unmasked view when the aperture was larger than 200 mm and the seeing was turbulent.

I for one would love to see how your results compare with mine.

Fiat Experimentum!

Dan

[brianb]

the masks fared better compared to the unmasked view when the aperture was larger than 200 mm and the seeing was turbulent.

Matching my practical experience. It's entirely a matter of bad seeing ruining definition, nothing to do with central obstruction - a smaller aperture gives a tidier but less resolved image & the worse the seeing is, the smaller the aperture needs to be before the effects of seeing become really troublesome. With strong solar heating, there's very rarely any advantage in going above about 6" for observing the sun - and, again, a refractor with one light trip down the tube will suffer less from tube currents than a reflector. The off-axis reflector designs may have no central obstruction but they still have tube currents & are much harder to make than conventional types - and collimation is a nightmare: one place where the central obstruction is very helpful!