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  #21  
Old 10-10-2018, 08:34 AM
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mental4astro (Alexander)
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Stephen, to finish answering your question about a numerical value that quantifies the ultimate quality of your optics, this can be approximated by visual techniques, but it is only possible following a lot of experience and knowing how to conduct bench tests, the various tests required and how to interpret the information these tests provide.

An example of this is the testing carried out by a Russian optical lab. They have the necessary equipment to thoroughly investigate a set of optics that access the different values and provide the ultimate quality value that is provided in a series of values, such as Strehl, RMS, P-V, and a few others. These all tell slightly different things about the optics, though not contradictory. Of these different values, the Strehl rating is the one that's most significant as it is the overall quality rating, as a ratio where 1.0 is "perfection", exceptional quality would be something like 0.974, mediocre quality is 0.767.

I'll chase up a link to this Russian lab.

Alex.
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  #22  
Old 10-10-2018, 10:51 AM
Wavytone (Nick)
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http://fidgor.ru/Observers/test.html

Back up your PC and make sure it has malware defences BEFORE you try that link as it is known to dish up some real nasty stuff.
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  #23  
Old 10-10-2018, 11:18 AM
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Quote:
Originally Posted by mental4astro View Post
your 60mm refractor
Alex.
What about the really small table top reflectors, like the 76mm reflector (Orion FunScope 76mm, Celestron FirstScope, Sky-Watcher Heritage 3 inch, Saxon Mini Dobsonian, ect) ?
Such supermarket/department store priced 76mm reflectors have even been sold by Aldi.
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  #24  
Old 10-10-2018, 12:01 PM
Saturnine (Jeff)
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Wavy, why provide a link that has known malware issues !!!

This thread is something that I'm sure that would be useful to a lot of amateurs, old and new, about how telescopes perform and what we are actually seeing through them and how to interpret what we are seeing. When it comes to fine lunar features it is obvious that they exceed the supposed resolution of the telescope in use, in good seeing of course.
Have had a few memorable nights where I've pushed the old Jaegers 4" f9 to 75X per/ inch on the planets and moon and double stars. My NG 5" triplet does perform better than the old achro by are fair margin even taking the extra aperture into account.

Alex and yourself have provided a great resource with your knowledge and insights, thank you.
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  #25  
Old 10-10-2018, 01:39 PM
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Originally Posted by JeniSkunk View Post
What about the really small table top reflectors, like the 76mm reflector (Orion FunScope 76mm, Celestron FirstScope, Sky-Watcher Heritage 3 inch, Saxon Mini Dobsonian, ect) ?
Such supermarket/department store priced 76mm reflectors have even been sold by Aldi.
Jenifur,

Now we head into the Dark Side of astro - the cheap.

What Nick and I have been discussing so far has been in relation to GOOD astro gear. To be able to push a scope to its theoretical limits, it needs to be a quality instrument. Sadly, the majority of what newcomers to astro buy is not good.

Before I go into tearing shreds off any item, I need to make something very clear, and something that is VERY CLOSE TO BOTH NICK AND MINE HEARTS:

"The best telescope is the one that gets used. If all you can afford is an inexpensive item, and you use it all the time, then it is just as good as the finest, most exquisite instruments humans have ever devised! And Damned be anyone who dismisses this."

So, knowing that cheap supermarket scopes are poor, we can discuss the Good, the Bad and the Ugly.

To make a scope cheap, then cheap and crude manufacturing techniques need to be used. And in turn, cheap scopes appeal to those people who unfortunately on a whole don't understand what they are buying. There's no right or wrong here, just a merge of two different forces - ignorance and market.

The requirements needed of an astronomical telescope are very stringent, and to produce such a high standard of instrument needs high quality materials and highly controlled manufacturing processes. And this costs money. Don't make the mistake of blaming China for cheap and nasty stuff. The Chinese are also very capable of making outrageously fine equipment, and today can rival and even surpass many traditional manufacturing powerhouses! Don't forget, some high profile Japanese and American brands have their entire product line now manufactured in China, and no one blinks an eye.

Those supermarket telescopes are not made using the optimal processes for a good astro telescope. They can't. These cheap scopes are entirely made to a price, and quality is only skin deep. Those 76mm reflectors are made with a poor quality spherical mirror, not a parabolic one. The eyepieces use plastic lenses, or uncoated glass lenses. This means that they are very poor at high magnification. These cheap scopes are best for low power wide field viewing, and they can deliver a very nice low power image. But they do not deliver a good high power image at all. They can't, and many people end up very disappointed with astronomy within 5 minutes and take it no further. And it's not just the figure of the primary mirror, but the cell it sits in, the focuser, the secondary mirror and its holder and the stalk that holds it. These can be altered to an extent to improve the little scopes performance, but these mods should not be something that the end consumer should be responsible for. These scopes are made to a price after all.

I have owned 3 of the Celestron FirstScope table top reflectors. All have been terrible at high power. I suspected this when I purchased my first, but I bought it as a had a particular use in mind with it, as big aperture finder scope, which meant I was exploiting its greatest strength - low power. And these scopes proved to be great for this. I had one mounted on my 17.5" dob!

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Would I recommend these cheap little scopes to anyone? It depends. These would not be my first or second suggestion, but for better or worse they do fill a niche. As a first telescope, they may work out to be a wonderful introduction to astronomy for a little kids - my own children when little enjoyed this little FirstScope. For older, more discerning kids and for adults, not a chance.

Last edited by mental4astro; 10-10-2018 at 01:58 PM.
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  #26  
Old 10-10-2018, 01:58 PM
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Quote:
Originally Posted by mental4astro View Post
Jenifur,

Now we head into the Dark Side of astro - the cheap.
Thank you very much for the reply, Alex.


In my case, getting the 130P reflector, price was important, but the physical size of the telescope mattered more due to where it would be set up.
Block of flats with small patio, and not much front to back depth. You can get an idea of how shallow a depth, from my avatar pic.
There's no way I could use a refractor here.
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  #27  
Old 10-10-2018, 02:06 PM
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Jen (I hope you don't mind me calling you Jen), I have only heard good things about that scope of yours. It is a clever design! Despite its price, you thought about it more than a lot of people to make sure you got something that suited your situation. That's blooming brilliant!

Don't get me wrong, some inexpensive scopes are real gems. Your's is one of those gems. You didn't buy that scope at Aldi. There's a few things you can do improve your experience with this little scope too. I'd be happy to PM you about some of these as this goes beyond the scope of this thread. Drop me a PM if you like

Alex.
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  #28  
Old 10-10-2018, 02:43 PM
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For buying a scope, the display of scopes in the window of my local Australian Geographic, caught my attention back in March. I had cash to spare and I could have all too easily bought a scope right there and then. Thankfully, I didn't.
At home, I started looking up info on beginner scopes, thinking about where I'd be using the scope, it's portability, storage size, the stuff that really matters when you're wanting to use a scope, and you live in a block of flats which only has stairs. I was thorough in doing the homework before heading to my nearest specialist astronomy shop here in Brisbane, Astro Anarchy, to 'kick tyres'. I wasn't going to spend a cent till I could actually see the scope, to be certain if it'd fit. I was lucky when I bought mine, as at the time it was on sale for about 40% off its usual RRP, so not expensive to start with and then sale price to make things even better.
In terms of what I've bought since getting the scope, that I use pretty well every viewing session, it's listed in my sig.
And that brings things right back to topic, on a point you raised. Eyepieces.
The stock ones with my scope would basically be a good way to dissuade anyone from astronomy. The 25mm has a nice wide field of view, but not enough magnification. The 10mm has better magnification, and bad chromatic aberration.
That meant getting better eyepieces was a must.
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  #29  
Old 11-10-2018, 06:30 AM
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Should this very useful Beginners thread be stickied?
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  #30  
Old 11-10-2018, 07:07 AM
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Jen, changing the stickies situation is one of the many things on the Administrator's list of "things to do"...

Ok, I've been asked to supply a few more challenges, so here goes

CHALLENGE NO. 3

Martian Clouds!
With the Martian global dust storm finally showing signs of breaking, it gives us an opportunity to chase down not just the polar caps (YES! Snow on another planet! And you can see it from here on Earth) which is easy, but the real teaser that is Martian clouds!

Steady seeing is a must. And another thing that helps is an #80A colour filter, or a #8 filter as it helps to tone down the glare of the red surface and allows the white clouds to show up more easily. But be patient! These clouds are not as dense as our Terrestrial ones. They are also best seen close to the limb of the disk (outer edge) as this is where they will be most dense in line-of-sight.

NOTE ABOUT COLOUR FILTERS:

Using colour filters with the planets will not necessarily make those features you seek stand out more obviously or how you expect them! Many features are very difficult to spot, and may appear in a way opposite to how you might expect. So be patient, understand what you are trying to see and give yourself time to interpret the image you are seeing.

Alex.
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  #31  
Old 11-10-2018, 07:23 AM
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Jen, changing the stickies situation is one of the many things on the Administrator's list of "things to do"...
Thanks Alex, I didn't know of the state of play for the to-do list.

Quote:
Ok, I've been asked to supply a few more challenges, so here goes
Right now, Brisbane is stuck with ugly stormy weather. Getting a long enough break to see anything is a challenge.

Quote:
CHALLENGE NO. 3

Martian Clouds!
With the Martian global dust storm finally showing signs of breaking, it gives us an opportunity to chase down not just the polar caps (YES! Snow on another planet! And you can see it from here on Earth) which is easy, but the real teaser that is Martian clouds!

Steady seeing is a must. And another thing that helps is an #80A colour filter, or a #8 filter as it helps to tone down the glare of the red surface and allows the white clouds to show up more easily. But be patient! These clouds are not as dense as our Terrestrial ones. They are also best seen close to the limb of the disk (outer edge) as this is where they will be most dense in line-of-sight.
This challenge is something I've wanted to be able to observe since getting my scope.
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  #32  
Old 12-10-2018, 07:27 PM
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CHALLENGE NO. 4

A cluster of volcanoes!

From just finding out that there are volcanoes on the Moon, this challenge is to spot one of the easiest clusters of lunar volcanoes, the Hortensius Omega cluster, also known as the Hortensius Domes.

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NASA photo

This is not so much of a "can you see this feature" sort of challenge, but one of teaching yourself how to distinguish run-of-the-mill-mountains vs shield volcanoes on the lunar surface. But note the individual domes are small, the largest about 12km in diameter, so it will be a challenge for scopes under 3" in aperture.

Unless you knew what to look for, if you happened to come across this cluster of volcanoes you would just dismiss them as just a cluster of mountains. Instead, there's a couple of distinguishing features that go to help identify these as shield volcanoes.

Because these are not particularly high features, timing of the lunar phase is important, two days after first quarter or a day or two after last quarter.

What is most remarkable about this region is that there are literally dozens of volcanic domes that dot the entire area. The above NASA photo shows how wide spread they are. Making it easier to identify this area is it lies due west of Copernicus.

Happy hunting!

Alex.
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  #33  
Old 12-10-2018, 09:27 PM
morls (Stephen)
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Great stuff, thanks Alex.

I've found some great moon maps, published by the US Geological Survey:

https://pubs.er.usgs.gov/publication/sim3316

Stephen
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