comparing optics

[icytailmark]

hi guys. I have had a celesron c14 for a few years now and i have noticed the optics are no where near as good as skywatcher optics. I had a Skywatcher 12" Dob and i think the 12" Dob showed more fine details with planets.(Observing)

[Astro_Bot]

Well, that's not much to go on. Frankly, it sounds a bit like bait. I suppose it's been a while since there was an SCT vs Newtonian slanging match, or a "my scope's better than yours" punch up. :P

You had a 12" Dob and think it showed more details? Any side-by-side comparison? Or just going from memory? If not side-by-side, then there could have been vast differences in seeing and transparency. Did you keep detailed logs of conditions, time, target elevation, exactly what details you saw, etc.? Were you using the same eyepieces (doubtful, given large differences in scope focal length)? But, possibly, you may have a lemon. Quality of optics can be a factor, but without a controlled test, it's hard to draw firm conclusions.

When was the last time you collimated your SCT? It'll still need it, from time to time. The corrector may be slightly off centre - I (and others) have found that to be a potential issue - the cork shims can shrink/compress allowing corrector movement. Are you letting your scope cool down? A C14 will take a couple of hours, possibly longer.

No matter what scope you have, in poor seeing, planetary views are crappy.

An afterthought: Isn't your mount a G11? A C14 (and accessories) must be pretty close to the limit for that mount (not that it'll affect the planetary view - I'm just curious).

[icytailmark]

this is from my personal experience. The only thing it could be is the local weather has been worse since i got my c14??? But i highly doubt it. I am going to buy another skywatcher dob to test my theory. Also i give my c14 plenty of time to cool down before using it.

[Astro_Bot]

Local weather is not necessarily the limiting factor in seeing. But what I was trying to get across was the myriad of factors involved, not just one thing. Nevertheless, you may have a lemon.

Seriously, though, have you collimated recently? And how long is "plenty" when cooling? It matters - e.g. one hour isn't enough.

Another afterthought: If you're mostly a visual observer, and particularly if you're casual or time-poor, then a Dob may be a better choice of scope regardless.

[multiweb]

I think high price tag equipment is better built mechanically so you're assured to have consistent results. Lower range, well, sometime you can get an exceptional item but quality can be far in between.

[icytailmark]

i bought some bob knobs recently i have yet to put them on. How do you know its a good night to collimate? I have made small adjustments over the year but have not notice any performance improvements.

[Astro_Bot]

Well, nights of better seeing (as opposed to the Knights of Better Seeing - the name by which they were known before they were the Knights Who Say "Ni" :) ) are best for collimating. I guess the "twinkle" test is easiest to pick a good night - I'll try and find a link for some reading material. I use an artificial star for initial collimation (especially if I've cleaned the corrector, which I do when removed) then do fine adjustment with a bright star. Bob's Knobs definitely make it easier.

There are two schools of thought about whether to collimate with diagonal in place, given the diagonal can potentially be a source of problems. I've tried both and found no appreciable difference, but your mileage may vary.

As far as cooling/comparison goes: Paul Haese has a C14, which he adapted for Peltier cooling, and he should know more about the specifics of that scope than I do.

Edit. Here's an article about measuring seeing, though I admit I don't think it's as clear as it might have been:

Astronomical Seeing (http://www.handprint.com/ASTRO/seeing2.html)

There's a table half way down entitled, "ratings on the standard scale of seeing" that has the "twinkle" scale on the LHS.

[casstony]

Proper cooling is critical for good planetary performance in SCT's and the larger the SCT the harder it is to get cooled. Even in Sydney I expect you would need active cooling for a C14 since there are significant differences between night and daytime temperatures.

[Allan_L]

I'll take the bait, Mark.
I agree with you.
I had a CPC1100 and I found my Skywatcher 10"DOB provided a better viewing experience for me too.
(The optics on my CPC1100 were excellent, attributed by various people who know a lot more than I).

I too was surprised, better contrast, brighter image, more detail.
But I did read that his is the accepted norm. For example:

[Renato1]

The big central obstruction in the SCT relative to the smaller one in an equivalent reflector affects contrast and sharpness. Check it on a star at high power, more light goes from the central star disc out into the diffraction rings.

My MAKs and SCT are never as contrasty on the planets as their equivalent sized reflectors are. That's just the physics of the design, it sacrifices some optical properties for the convenience of a handy, very versatile and easy to handle and use telescope.

If you want sharp contrasty images from the C14, perhaps make an aperture mask (5 or 6" diameter) which just misses the central obstruction and goes out to the edge of the mirror.
Regards,
Renato

[casstony]

It's easier to get a newt to perform at it's best and SCT optics vary in their quality on a sample by sample basis, but if you have a good SCT which is properly collimated and cooled it's performance will come close to the same aperture newt.

[Steffen]

Um, no. In my experience Maks are more contrasty on planets than both Newtonians and SCTs of the same (or even larger) aperture. Some of them (like the Russian made Rumak types) are said to rival only slightly smaller APO refractors in contrast and fidelity.

Cheers
Steffen.

[Paul Haese]

Ok central obstruction does give more contrast to viewing the planets with a Newtonian.

However, things you need to ensure performance on an SCT is as follows.

1: the scope must be at ambient. SCT's often take 4-6 hours to cool down to ambient. There are several methods for cooling SCT's down.

2. Seeing is king. If you don't know what seeing is, then it is a sure sign that you are not collimating at the highest level possible for an SCT, nor are you able to see anything due to poor seeing. Only view the planets during periods of good-great seeing. The poorer contrast of an SCT needs good seeing, otherwise the view will appear to be worse than a Newtonian.

3. If you are not already doing it, collimate your scope regularly. SCT's have collimation shifts even without being moved off the mount. The primary can get slop in it and that can cause collimation errors. Only collimate in good seeing.

4. You can help contrast on an SCT by flocking it well. It will not give you as good a contrast as a Newtonian but it can improve the performance.

5. Use very high quality eyepieces. Good quality optics make a difference with viewing the planets. Barlows and Powermates also impact on detail being seen.

Besides all this, you cannot really compare both types of scopes. Each has its strengths and each has is weaknesses. Just a fraction of miscollimation on an SCT makes a huge difference.

Hope this all helps.

[Amaranthus]

A Duncan mask (looks similar to a radiation symbol) is VERY useful for collimating the SCT. I use this to get it near-perfect on my SCT, and then, when the seeing is superb, I 'top up' with Airy disk checks.

[raymo]

Steffen is spot on, my Mak has stunning pinpoint stars and great contrast. The image quality[not brightness], is a level up from my
8 and 10" Newts. It splits Antares much better than either of my Newts.
raymo

[Wavytone]

+1 agree with Steffen.

Comparing my 7" f/15 Mak with 8-12" newtonians:

1. Contrast in the Newtonians depends on the planet, and also the size and configuration of the diagonal spider. A traditional Newtonian with 4-vane spider will show horrid spikes that degrade the image of Mars quite badly, yet the view on Jupiter / Saturn isn't so bad.

Operating at the same magnification the 7" f15 Mak out-resolves all of the 8" newtonians I've tried it against. A 12" f5 is a tad better - just - but there's the shorter focal length to contend with, which means the eyepiece selection is fundamentally different.

A long Newtonian, say f7 or f8 with a single curved vane spider, or alternatively a maksutov Newtonian (no vanes at all) should be considerably better. By way of example the 16" at Mt Bowen (f7) provides superlative views of the planets and easily better than a C14.

The best - bar none - is a schiefspiegler (unobstructed off-axis reflector, for the uninitiated). But at f23 they're not much use for anything else.

2. The larger catadioptric scopes (like the 14" Celestron) have a very long focal length - so long that it only provides high magnification, or extreme. At the images scale of these scopes poor seeing will badly degrade image quality and it's true that you may well see more by stepping down to a smaller scope around 8".

[ausastronomer]

Congratulations, you have just managed to defy the laws of physics.

This is an interesting article and comparison conducted by Ed Ting a few years ago. Note the Mak in this review is an Intes MN61 (6" Mak newt) which will outperform a similar aperture MCT due to the fact it has a smaller central obstruction (18% Vs over 30%) and one less reflective surface (diagonal)

http://www.scopereviews.com/best.html

Cheers,
John B

[Steffen]

You can't pick and choose your laws of physics. There are other things that affect contrast besides central obstruction size. Only by looking at an optical design (and its implementation!) as a whole can you make predictions about its performance. Only by looking through an optical instrument can you verify its performance. Saying that contrast must be worse because the central obstruction is larger is simplistic and wrong.

Cheers
Steffen.

[ausastronomer]

If you think you can only get exceptional planetary images from a long focus Newtonian you are still in the stone age. With modern mirror making techniques and scope construction techniques it is easy to have Newtonians as fast as F4.5 delivering exceptional planetary views, the equal of long focus Newtonians. The critical things are that the ability and skills of opticians has improved enormously in their ability to make mirrors in the F4 to F5 range very well and the scope designs and low profile focusers have enabled the secondary obstruction to be kept under 20%. Both my F4.5 scopes have an 18% Central Obstruction.

Assuming you have found a good optician and have a central obstruction under 20% the only optical disadvantages of a fast Newtonian these days are:-

1) Collimation is more critical to produce good performance. It's no harder to collimate a fast scope but the faster the scope the better the collimation needs to be to produce high grade images.

2) Depth of focus is reduced making it more difficult to hold a good focus at higher powers under variable seeing conditions.

There are of course a large number of mechanical factors which make a fast scope more desirable than a slow scope

Cheers,
John B

[ausastronomer]

Don't ever set your MCT up beside my 10" Suchting powered SDM or my 14" Zambuto powered SDM because you will go home terribly disspappointed.

Cheers,
John B

[Steffen]

I've had the joy of looking through quite a few big Dobs lately, including some premium SDM ones. They blew me away on the faint stuff, but on Mars or Saturn my puny little Mak held its own quite impressively. And while the big Dobs impress I wouldn't want to own one (and not being able to afford it is only part of the reason).

The funny thing is, owners or proponents of big Dobs won't even bother to look through a 6" Mak at star parties etc. I think they're the ones missing out.

Cheers
Steffen.

[Satchmo]

Hi Steffen

Large dobs have to be carefully collimated and cooled and the seeing has to be good to work at their best for planetary observing - just you look through a scope that has a certain name tag on it does not mean it is working optimally !

On the Thursday night at SPSP we spent time later in the night on Saturn with Mark Axisa's newish 18" F4 SDM. After I gave the collimation a tweak The seeing was such that frequently the view went rock steady and the Cassini Division looked almost laser etched .

Including the crepe ring we saw the ring broken up into 9 odd rings and the pentagonal polar darkening was obvious. I've been looking at Saturn through various scopes for 40 years and this was by far and away the best view I've ever had - the combination of image scale ( X350 ) - a nice bright image and the incredible resolving power combined to give what everyone in the group agreed was the best ever view of Saturn .

While I'm sure your Maksutov would give diffraction limited sharp view on most nights due to the average size of seeing cells generally being 6" to 8" - I would not trade that restriction for the potential offered by a large quality scope. The views in this scope on the Thursday night were simply mind blowing .

[Steffen]

Very true, I'm amazed that some owners of high-end telescopes can't be bothered with accurate collimation.



Actually, nights that allow me to observe at the Dawes limit (about 0.77 arc seconds) are rare. I wish I had spent more time on Saturn that Thursday night, I was mostly chasing doubles, which at 360x was quite rewarding nonetheless.

Cheers
Steffen.