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Old 03-10-2018, 09:05 PM
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mental4astro (Alexander)
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Observing the Moon & Planets - the good juice, and cheats...

New comers to astronomy – WARNING!

Maxing out the magnification of your telescope is not the way to use it. And, most likely than not, you will also be exceeding the optical limits of your scope by maxing it out with the eyepieces and barlow lenses supplied with cheap telescopes.

Here’s the really bad news: your 60mm refractor IS NOT capable of 525X magnification! despite the box your scope came in having plastered on it 525 power telescope! Yes, there is a little telescope that is currently marketed by a high profile telescope company with this misleading claim on the box. Truth is, the most this little scope can ACTUALLY punch out is just 120X before the image craps itself!

Sorry for the rather dire start to this piece. It’s just that there are many misconceptions that new comers to astronomy have, and the worst one of the lot is that one of the outrageous claims of 402X, 450X and even 525X magnification from really small telescopes, accompanied by photos of Jupiter and Saturn taken by the Voyager spacecraft, deliberately misleading the ignorant beginner that this is how they will see the planets with THAT telescope.

In this article, I will describe just a few basic aspects of what a telescope is ACTUALLY capable of providing, how to maximize its efficiency regarding its optical limits, the Great Equalizer and severe task-master that is the atmosphere, and then a few tips on how to rip the most detail out of the Moon & planets. Believe it or not, there are even some cheats that can be done that cost nothing in order to improve the quality of your viewing when the atmosphere is not playing nice.

One thing I will try my utmost to do is keep the astro-techno-jargon to a minimum, and what I do use explain it as I go as some of this astro-techno-jargon is very useful shorthand and important to know. Not understand some of these aspects, and you won't know why your telescope "is not working", when in reality there may be several things working against you, some of which you have absolutely no control over.

The whole article will evolve over a few posts as I develop them, and in a way form separate "chapters", much like my earlier article on nebulae. And of course, any questions as we go are MOST WELCOME. I mean this most sincerely as I can overlook some items, and your questions will also help me develop this article. Don't be shy as the only dumb question is the one that is not asked, and we all start from knowing nothing.

Alex.
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Last edited by mental4astro; 03-10-2018 at 09:17 PM.
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Old 03-10-2018, 09:16 PM
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The Golden Rule.

There is actually an optical limit to the maximum practical magnification ANY telescope can provide before the image begins to degrade. And this is entirely related to the aperture of the telescope. This maximum limit is known as The Golden Rule

The Golden Rule is: 50X per inch of aperture

So, that 60mm (2.3”) telescope that Tasco boasts as punching out 525x, is actually limited to 120X. A 200mm (8”) telescope maxes out at 400X. And so on.

Exceed this optical limit, and the image goes to crap as the optics because the resolution limit of a telescope is a function of the aperture size. Of course, with the right combination of eyepiece and barlow you can push any telescope to outrageous “magnification”, but the image will not be useable AT ALL. It will be dim and a fuzzy mess. THIS is what Tasco is not telling you, and you are suckered into buying that scope because of that bullcrap implication that’s accompanied by those photos taken by spaceprobes…
I have a lovely 200mm (8”) telescope that I use just about exclusively to sketch the Moon and planets with. If I listened to what Tasco claims I should be able to do with its 60mm telescope, I should be able to rip 1750X out of the 8" scope! My 8” scope is 11X larger in area than that 60mm, I limit my max magnification to 400X, yet that Tasco can do 525X!?!? Someone is telling a porky here…

This is all I will say about maxing out your telescope. Eyepiece selection is another topic altogether, and not relevant to this article. Of course eyepiece quality is important, so all I will say about eyepieces is that BIG$$$ is not everything with eyepieces either. There are some very good eyepieces that are actually very modest in price. Like I said, this is another topic altogether. For now, learn to use the eyepieces that came with your telescope, obey the practical optical limits of your scope, and you will come to see that those very same eyepieces can actually do a commendable job!

Alex.

Last edited by mental4astro; 04-10-2018 at 04:11 PM.
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Old 03-10-2018, 09:30 PM
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The Atmosphere – the Great Equalizer & task-master

While you can get higher magnification with a larger telescope, how high you can actually take it ALL DEPENDS on the atmosphere on any given day! This is called “SEEING”.

Seeing means how thermally stable the atmosphere is. If the atmosphere is thermally active, the image through the telescope will shimmer and look like it’s boiling. It will look very much like it does looking across a hot road and you see the image in the distance just above the road surface shimmer and roll about – essentially it is a mirage. So in “poor seeing”, the image will begin to shimmer and boil as low as 100X (sometimes even lower). In “average seeing”, the max useable magnification will be around 250X. In “very good seeing” it is possible to go to 400X. BUT this is also limited to your aperture!

Here’s the kicker…
Sometimes, a smaller aperture is better to use than a large aperture!

The larger the aperture, while being able to provide higher magnification and having greater resolution, it is also much more sensitive to the prevailing seeing conditions of the night. So while I may prefer to use an 8” scope with the Moon and planets, sometimes a modest 80mm telescope is the better option. While I will only be able to max out the 80mm scope with 160X, my 8” scope at 160X in poor seeing might produce a totally boiled out image, but that 80mm scope at 160X will be just fine!

CHEAT!
Not everyone is in a position to have two or more telescopes. If you have just the one telescope, and seeing conditions are not great on a given night, there is one thing you can do to help sway things a bit more in your favour - crop the aperture of your scope.

Say your scope is 8" in aperture. As a smaller scope is less susceptible to the ravages of poor seeing, you can make a mask that has an opening of say 6" and put this over the front of your scope.

TRADE-OFF
Of course, with any cheat there is cost involved. By making the aperture smaller, you make the image a bit dimmer, and you reduce the resolving power of the neat aperture of the scope. But sometimes this cheat can mean the difference between a productive night and a night of frustration.

Alex.
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Old 04-10-2018, 10:23 AM
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Observing tips with the Moon and planets.

The Moon
The Moon is our closest neighbour in the sky. It is often the very first thing most people ever look at when they first look through a telescope. But this initial fascination all too often and all too quickly turns into loathing and disdain as its brilliance overwhelms the dimmer deep sky objects. Yet the Moon can provide a lifetime of observing joy, and every phase providing a unique treasure trove of details and features, can teach us a lot about our own Earth and even teach us about our own human history with the names its features carry.

Viewing the Moon is actually very easy. There are very few technical tips that make viewing easier, and sometimes less painful due to its brilliance. What is KEY to viewing the Moon however is understanding what can be seen on the Moon. Understand this, and the Moon will never be a dull and repetitive place.

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Telescope tips:

The EASIEST tip to follow about viewing the Moon is:
• You don’t need to turn all the lights off!
There is NOTHING subtle about the Moon. Your eyes WILL NOT be dark adapted. So there is not only no need to view the Moon in the dark, it can actually be dangerous to do so as your stunned eyes will not be able to see hazards when you walk around in the dark. Keep all the lights on that you want!

• The lower the magnification, the more painful the image to your eyes. To help control this as sometimes low power viewing is most pleasing, you can mask down the scope’s aperture or use a filter in the eyepiece.
There are many different types of filters available to deal with the Moon’s brilliance. Some are a green tinted filter (horrid thing), others are called Neutral Density filters which are essentially grey tinted filters that come in different grades of transmission. The difficulty with these is which filter do you buy??? Different phases have different brilliance impact, so to recommend any one Neutral Density filter I never do. The filter I do recommend is a Variable Polarized filter set – these two polarized filters are used together to regulate the amount of light that is transmitted, and so you can control the Moon’s brilliance to whatever amount you want regardless of the phase.

• High magnification doesn’t require a filter.
With my own lunar sketching, I use a large 8” telescope, which in turn collects A LOT of light, but I don’t use a filter. As the features being viewed will be along the Terminator, the brilliance of the Moon is more tempered, so a filter is not necessary. On those areas where the shadows are nonexistent on the Moon, yes its brilliance can be painful, but these are not areas where most details are visible.

If you are examining those areas where and when there are no shadows, then a filter is necessary. Examining these shadow-less areas is a niche aspect of lunar observing. Some features, most often volcanic in origin, can be impossible to view if there are strong shadows about, and the best time to view these unique features is when there are no shadows visible and you rely upon subtle variations in shading to identify these areas. A neutral density or variable polarizing filter set can be very handy here.

• Adapt the magnification you are using to best suit the prevailing seeing conditions!
When seeing is poor, the lunar surface can appear to shimmer, ripple like a flag in the breeze, and even boil. When this is happening, the best thing to do is turn down the magnification. As the night wears on, seeing conditions may improve (often happens), or even go pear shaped (sucks when this happens!), so you need to be aware that being adaptable to the conditions will mean a productive time or just maddening frustration.

What to see on the Moon?
Oh, yeah! Now this is what REALLY excites me about the Moon! Because there is no atmosphere, no water, no weather on the Moon, its surface is an open storybook of its entire violent and terrifying history. Like I mentioned earlier, there is NOTHING subtle about the Moon. And the processes that formed and keep forming its surface are nothing gentile and cute, but conjured up from the very depths of Hell itself!

And as I did with the Nebulae article, I will let heavy lifting of describing what can be seen by another brilliant page: Observing the Moon along the terminator.

Many of us are aware that the dark Seas and Oceans on the Moon’s surface is lava. But how many of us then consider that if there is lava, then there has to be volcanoes too! And if there were volcanoes, then it also stands to reason that there will also be the many different features associated with volcanic activity: lava rivers, collapsed lava tube, lava fields and folds, and flooding of craters. And a volcanically active body will also give rise to other planet forming forces such as tectonic forces creating different faults (sheer and rift), mountains, fractures. Of course, today none of these forces are active on our Moon. But as there is no erosion taking place on the Moon, its entire history is on display, and with a little understanding you will be able to identify a 4 billion year story all within one small area of the Moon, just by identifying the features visible, the forces that formed them, and hence the timing of the Moon’s age as to when these different features formed.

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Which is the best phase to view the Moon? EVERY PHASE is the best time to view the Moon!!!!

A very common misconception about the Moon is that “there is nothing to see during the full Moon”. And those people who profess this myth have never taken a close look at where the REAL TREASURE of the Full Moon lies – along its limb!

Here we don’t see craters or mountains from above. Instead all the lunar features are seen in profile, just like looking across a landscape here on our home planet. And the features, shadows and landscape all give us a totally different perspective of the lunar surface.

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~x.X.x~

I hope this chapter has given you some helpful tips on how to deal with the Moon, the many different features visible, some appreciation for the history it shows us every month and even some insight to the forces that formed the Moon are still active here on our Earth today!

Alex.

Last edited by mental4astro; 04-10-2018 at 04:09 PM.
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Old 04-10-2018, 11:09 PM
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100% Alex many people boast that they can get 200x and more with 60mm Unitron ?
I have an old Tasco 10TE-5 and have manages the magical 50x per inch (150x) for this scope on numerous occasions , thats why I still have it.,that scope however is only good for splitting stars , planets etc.
The highers magnification with my limited scopes was 250x thats with very clear sky and high altitude of 1650m Above sea level
The Moon is different and can have much higher magnification doe to its brightness
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Old 06-10-2018, 07:17 PM
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Thanks Richard for the confirmation!

It's also always nice to get some feedback on what one writes.

CHEAT!

There is one thing that both lunar and planetary observing require, and that is patience.

There is no shortcut to this. And the reason is the environment that our eyes are looking into – a high contrast situation of a very bright Moon or planet against a jet black background, and surface details that are actually of low contrast set atop of the bright Moon or planet.

If you rush, being impatient to see details, you will miss those fine details that our telescopes are actually very capable of providing.

Patience creates experience, and that experience will have you slow down so that you can get your eye “in” on the difficult conditions our eyes are being required to look into.

With the Moon, there are so many fine details that will only reveal themselves with patience. Lacework-like fine riles, clusters of tiny craterlets that pepper the floor of a flooded crater, identifying domes/volcanoes from the surrounding features, the fine and subtle shading & lines along the terraced walls of massive craters that reveal that these are the result of landslides – these are all the sorts of features that require a patient eye.

CHALLENGE!
There are three very small craters that I will use as one of two challenges I will make in this article. These three craters are Armstrong, Collins & Aldrin. They form a straight line in their orientation in Mare Tranquilitatus. Armstrong is the largest of the trio at 5km in diameter, Aldrin the smallest at about 2.5km in diameter. These sizes might sound BIG, but from here on Earth, they are getting to be the smallest craters we can see. If your scope is less than 5” in aperture, you will not see these as they are too small for scopes smaller than 5” to resolve.

Patience is the key to spotting this trio when the lunar phase is favourable (between 5 days old and 7 days old being optimal. Good seeing conditions are also necessary – Remember, these craters ARE VERY BLOOMING SMALL! You will need good seeing conditions.

Rush, and you will not see them. Give your eye the chance to adapt to the difficult brilliance of Mare Tranquilitatus. These three craters have no prominent landmarks close by to assist with spotting them. The available landmarks are far away and the entire exercise will test your patience. But they are visible.

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Oh, and you will not be able to see the Apollo 11 landing site. Not even the Hubble Space Telescope is capable of seeing the site - and they tried! The Apollo 11 landing site, and that of the other 5 landing locations were only visible just a few years ago by the Lunar Reconnaissance Orbiter.

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NASA photograph.

Alex.
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Old 06-10-2018, 08:09 PM
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Observing the Planets

TIP:
The planets will ALWAYS appear small in a telescope. You will never see any of the planets appear like the size of a dinner plate. The pretty photos that typically come on the box of a telescope are very misleading.

However, despite their small size, a surprising amount of detail can be seen. With your main tool being patience…

I am REALLY hammering this patience thing in these last two posts for a reason. Astronomy is NOT a “smash’n’grab” thing. It requires being calm, concentration, and a keen eye, and it doesn’t matter if it’s with the Moon, Saturn, the Orion Nebula, or the impossibly faint globular cluster Mayall 11 that belongs to the Andromeda Galaxy!

Be patient, and amazing wealth of exquisite details will become visible to you, such as the polar caps on Mars (Yes! Snow on another planet!!!), or like one of my greatest ever thrills, seeing clouds on Mars!!!

FILTERS help with the planets.

While not essential, colour filters can help a great deal with seeing details on the planets. What features and which filters is the subject of this part.

The way I will mention what features are visible will be contained in the following link to astronomical filters. Half way down the following link, you will find a list of the planets, what features are visible, and the best filters to see those features with:
https://www.myastroshop.com.au/guides/filters.asp

CAUTION!

If you do think about getting yourself some colour filters, GET NO MORE THAN THREE OR FOUR.

STUDY that filter guide very carefully, and then compile a list of no more than 4 filters. The reality is YOU WILL NOT USE THESE FILTERS VERY MUCH. So having just a clutch of them is all you will really need.

DO NOT BUY READY MADE FILTER SETS!!!! These are made up of colours that are way too deep and intense, and these filter sets are designed to appeal to novice eyes because of their strong colours. Truth is that these strong colours are not the best options for the planets. Instead, the better/more practical filters are more subtle in hue intensity.

Me, I only have four colour filters, and I use three of these the most, and not very often at that. Only sometimes do I ever bother using filters with the planets. A couple of these really I don’t use all that much, and sometimes I think about getting one or two others to replace them, but I’ve never acted on this as I doubt I would use these filters very much at all too. <sigh>…

Uranus and Neptune.

I will only expressly discuss these two planets because they are particularly challenging in that they really have no discernible disk to make out at all. Instead, the best way to identify these two is by their colour. Uranus has a very distinct bluish-green colour to it, and Neptune is an intense blue that is much too rich to be a star. Because there is no discernible disk with these two planets, don’t worry about using filters with them.

Uranus is actually visible to the naked eye! What you require is a very dark location, good transparency, and an accurate, up to date chart such as one of the astro apps. I’ve been able to see Uranus from up in the Blue Mountains just west of Sydney on many occasions, and I was also able to see fainter stars around Uranus to so I was not confusing things. And of course a quick look through a scope confirmed this too.

Uranus is also VERY EASY to spot with binoculars from under urban skies! At magnitude 6, it is not a challenge for a 7X50 pair of binos. Again I’ve managed this from my home in Sydney.


TIP
You will see more detail on the Moon and planets with a larger telescope due to a larger scope having a greater resolving capacity. But even so, a surprising amount of detail can be seen with smaller apertures. A wee 2” refractor will show the two main equatorial belt of Jupiter and reveal the four Galilean Moons. A good 80mm scope will reveal the Cassini Division in Saturn’s rings. A 14” scope will allow you to spy out the main five moons of Uranus and Neptune’s largest moon Triton.

CHALLENGE
This challenge will be a real test of the quality of your scope. You will also require a scope larger than 6” in aperture for this, and good seeing conditions too.
Saturn’s rings are made up of thousands of individual rings and even gaps. The Cassini Division is the largest of these (roughly 4000km wide). There is another ring division that is visible, the Encke Division. It is much smaller in size (roughly 400km), and to be able to resolve it in any scope over 6” in size is a testament to having a blooming brilliant set of optics in your scope! The Encke Division is a real test of optical quality.

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If you would like me to prepare other challenges, such as challenges for smaller scopes, deep sky objects, Moon, planets, whatever, let me know. I’d be only too happy to devise a few!

Alex.
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Old 07-10-2018, 08:53 AM
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Another awesome thread Alex,

I am learning so much from this & your nebula thread.

Looking forward to taking up your challenges.

Cheers

Carlton
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Old 07-10-2018, 10:47 AM
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Another ripper piece Alex.
Thankyou
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Old 07-10-2018, 02:26 PM
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Thanks Carlton and Brendon

RESOLUTION - What da???

I've been involved in astro for more than 35 years. Yet I've only recently come to understand one of its most oft mentioned but most poorly understood aspects - Resolution - and how there are two very different aspects to this with our scopes.

WARNING: Physics talk below!

When we read the specs table of our scope, frequently we will read a spec called "resolving power". In short it is the "smallest" size detail our scopes are supposed to be able to make out.

HOWEVER! This value is somewhat misleading! Our scope are actually capable of resolving much smaller details, close to 10X smaller than the quoted value!

There's two parts to this resolution thing. Let's start with the quoted figure and what it means. I promise to keep it simple.

Here's the thing with stars and our scopes. We may think that the stars we see through our scope are just pinpoints of light. They are actually a tiny, tiny disk (airy disk) with a series of faint rings (diffraction rings) that get fainter out from centre.

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You can actually see this Airy Disk and at least the first diffraction ring when you use high power on a very bright star and under good seeing conditions. So none of this is techno-mambo-jumbo! It's real and easy to see for yourself.*

So, being a tiny disk, the quoted "resolution" comes to be the smallest distance the a given aperture can provide a distinct separation - NOT a total separation.

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If you would like to follow this up with the maths that's involved with this, you can follow this excellent link that deals entirely with telescope optics:

Telescope Resolution

NOW, this resolution value is ONLY for two Airy disks.

When it comes to expended objects such as the Moon and planets, things are very different!

Because you are not dealing with point sources of light, the entire situation is different. The apparent diffraction patterns are totally disrupted. This in turn means that the actual resolving capacity of the optics is now greater than that provided by two airy disks.

The evidence for this is seen in those amazing photos of Saturn's Encke Division. It's angular size is 0.05 arcsec. This is more than 1/10th of the quoted resolving power of an 8" scope! Yet there are photographs of Saturn showing the Encke Division taken by 6" telescopes. I have also seen the Encke Division in a 7" and 8" scopes. I have also been able to resolve markings on the Moon that are smaller than 250m, and using simple trigonometry maths, these features come in at 0.15 arcsec.

So, this is why I am putting up these two particular challenges. These will show you finer detail than the quoted "resolving power" of your telescope.

~x.X.x~

Knowing this now, how does this affect you and your telescope?

Easy.

The "quoted" resolution of your scope you now need to think of only in terms of two stars. This resolution value will only affect the smallest separation that can be resolved say for double stars. When it comes to the Moon and planets, the resolving capability of your scope will be better than 1/10th of this

If you do not know the resolving power of your scope, the following telescope resolution calculator will give you this. All you need to do is input your aperture:

Telescope resolution calculator

This link has two values, the Dawes Limit and the Rayleigh Limit. The Dawes Limit is the smaller of the two, and you will be able to resolve finer details than 1/10th of this with the Moon and planets

Alex.

* One thing needs to be kept in the back of your mind when it comes to the Airy Disk. This disk is not the actual disk of any single star. All stars are actually impossible to resolve into there actual disk. Instead this a phenomenon to do with point sources of light meaning that ALL point sources of light will show an Airy Disk. So it does not matter if you look at Alpha Centauri (4.3 light years away) or the quasar 3C 273 in Virgo (which is 2.4 Giga light years from us, and visible in amateur scopes as it shines at magnitude 13!), both will show an Airy Disk.

Last edited by mental4astro; 10-10-2018 at 07:15 AM. Reason: Typo
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Old 07-10-2018, 02:53 PM
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Alex,

Would it be possible to put together a beginner's guide to compiling observation reports.

I read the ones posted but, have no idea about how to accurately compile one.

Thankyou in advance

Carlton
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Old 07-10-2018, 04:00 PM
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Carlton,

Writing formal reports is not something I normally do. However, I've asked a mate who does have a lot of experience in this if he could offer some insight into preparing astro reports. Les has also written many articles for various astro magazines. Les is The Man for this!

Alex.
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Old 07-10-2018, 04:47 PM
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Thanks Alex,

I'm just curious at this point & want to try & understand the reports that are in the visual observations sub-thread.

Then maybe have a go... see if it might become my thing..

Cheers

Carlton
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Old 09-10-2018, 11:02 AM
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Originally Posted by Outcast View Post
Alex,

Would it be possible to put together a beginner's guide to compiling observation reports.

I read the ones posted but, have no idea about how to accurately compile one.

Thankyou in advance

Carlton

http://www.iceinspace.com.au/forum/s...ad.php?t=12753
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Old 09-10-2018, 11:17 AM
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Brilliant, Mirko!
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Old 09-10-2018, 05:28 PM
morls (Stephen)
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Brilliant thread Alex, thank you!

I'm wondering if you could do a follow-up on your last post dealing with resolution, and explain how to do basic star testing of optics? I'm doing this a lot with my new scope, but I'm not sure what all the different rings represent, and whether they can be used to determine 'x-number of waves de-focus...

It could well be explained elsewhere, but you have a great way of presenting the information clearly...

Cheers

Stephen
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  #17  
Old 09-10-2018, 06:51 PM
Wavytone (Nick)
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Hi Stephen, Part I - your scope.

You will need a pretty steady mount for this, and one that tracks. On an unguided mount this is very difficult.

1. Choose a bright star near the zenith mag. 1-3 are ideal - altitude at least 70 degrees above the horizon.

2. Align scope on the star.

3. Make sure the optics are collimated to the absolute best you can, this is a subject in itself.

4. Precisely centre the star in the scope, and increase magnification to at least 2X per mm of aperture (changing eyepieces as needed) and centre it if need be.

5. Accurately focus. You should be able to clearly see the Airy disk and a couple of rings around the star at best focus.

Defocus one way (let’s assume inwards) by a small amount until the central Airy disk is gone and you have 2-3 concentric tiny rings. In the centre of these you will still see a tiny dot of light - the Poisson spot - which is caused by diffraction around the central obstruction in a Newtonian or catadioptric scope.

6. Compare with the images at

https://www.telescope-optics.net/aberrations_point.htm

And

https://www.telescope-optics.net/dif...berrations.htm

7. Refocus and then defocus the other way, and repeat.

Note that the Poisson spot is useful it shows where the exact optical axis is.

Lastly at step 5 there are situations where you may nit be able to clearly see the Airy disk and one or two rings:
- poor seeing,
- thermal issues within the telescope causing air currents,
- poor optics.

I have seen scopes that optically will not show the Airt disk - at high power they give a blurry mess. Sometimes the optics are pinched or strained because someone has over tightened something, or a mirror is badly supported. But some are just plain bad.

Last edited by Wavytone; 09-10-2018 at 08:05 PM.
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Old 09-10-2018, 07:53 PM
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mental4astro (Alexander)
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Thanks Nick. You were able to better explain this than myself,

Nick explained a formal method of star testing. There's a couple of empirical ways of testing.

The basis of this is one thing that Nick told me: "When all the photons go where they should go, then seeing conditions have less of a deleterious effect".

So when comparing two scopes, and seeing is less than perfect, these conditions are great for comparing the relative optical quality of the two scopes. The instrument with the better optics will be less affected by the poor seeing than the poorer set.

Stands to reason as with the poorer optics, photons are less well controlled, so the image will be more easily disrupted.

Another informal way of star testing can be done with splitting double stars. If you look at the second picture in post No. 10, you will see that splitting double stars does not require a full separation.

One last empirical test is done by pushing the optics to its limits with attempting to resolve the finest details possible. This is the reason behind the two challenges. The Encke Division has an angular size that is much smaller than the "theoretical" limit of many scopes 7" and larger. There are many 8" scopes that cannot resolve the Encke Division. The lunar crater resolution is more tricky as it requires the individual to compare the scope's image with images from a very detailed atlas, such as Virtual Moon Atlas. An atlas such as by Antonin Rukl is not detailed enough for this purpose.
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Old 09-10-2018, 07:58 PM
Wavytone (Nick)
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Hi Stephen,

Part II - the Rayleigh & Dawes criteria vs what you can actually see, and optical quality.

The Rayleigh & Dawes criteria concern the closest pair of stars - of equal brightness - that can be discerned visually as not a single star, assuming ostensibly perfect optics. It turns out - with the help of optical theory - that this depends solely on the aperture of the telescope, and the wavelength of the light. See https://www.telescope-optics.net/tel...resolution.htm In essence, the two central Airy disks overlap and resemble a dog-biscuit, just enough that an observer can say with confidence it is not a circular disk (ie single star). Note there is NO black gap visible between the stars.

Conversely, armed with a list of close bright double stars of similar magnitudes - and there are several - you can test the quality of the optics of your scope in the harsh real world if you are blessed with a night of excellent seeing. I guarantee you will find it very very difficult to split double stars at double the Rayleigh limit for your scope, never mind close to the Rayleigh limit. This also says something about the quality of the refractors available in the 19th century - some of them clearly did have exquisite optics despite the simple eyepieces of that era.

Note however that these resolution limits only apply in the context of two close bright stars of equal magnitude. In other contexts the limits to what you can see will differ - particularly double stars of unequal magnitude - a dimmer star is much harder to see and the minimum separation that can be clearly resolved will be greater than the Dawes limit.

Similarly the moment the subjects cease to be point objects (stars) the Dawes/Rayleigh criteria do not apply - eg observing Jupiter and its moons, or Saturn/Mars/Venus/Neptune/Uranus which can all be resolved in amateur scopes.

With excellent optics it is also possible to see features smaller than the Dawes limit, such as a dark linear feature against a bright background, for example a slender shadow on the moon cast by a crater rim or rille, or Cassini's division and Encke's division in the rings of Saturn.

Optical theory suggests - using somewhat simplistic maths - that a scope producing a wavefront error not exceeding quarter of the wavelength of light will resolve stars at the Rayleigh limit. For a reflector this means the surfaces of mirrors must conform to the required shape with 1/eighth of the wavelength as a defect in a mirror produces a wavefront defect twice as high.

A few telescope manufacturers will guarantee their products meet a specific criterion in terms of wavefront error - many do not.

Last edited by Wavytone; 09-10-2018 at 09:08 PM.
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  #20  
Old 10-10-2018, 08:27 AM
morls (Stephen)
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Thanks Nick and Alex,
Thanks for the info. There's a lot here to get my head around and I'm sure I'll have more questions, so rather than clutter this great article of Alex's I think I'll start a separate thread for learning about star testing and collimating my scope.
http://www.iceinspace.com.au/forum/s...40#post1399240


Looking forward to getting into the observation challenges put out in this thread...
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