Please compare between two Orion achromatic refractors?

Question

First Orion Explorer 90mm
http://www.telescope.com/shopping/product/detailmain.jsp?itemID=294&itemType=PRODUCT&iMainCat=4&iSubCat=13&iProductID=294&relateInfo=3&add=yes#tabLink
second Orion Ezplorer 80mm
http://www.telescope.com/shopping/product/detailmain.jsp?itemID=111184&itemType=PRODUCT&iMainCat=4&iSubCat=13&iProductID=111184&relateInfo=3&add=yes#tabLink
the terms of comparison:
if same amount of chromatic and spherical aberrations are present which image will be the more magnified one. OR:
if both images are equally magnified which will be the one with less aberrations.
I am asking because I learned that refractors' chromatic and spherical aberrations are depend on their focal ratios.

Guys, the price difference is considerable in order resolutions like "they are both equally well so go with the 90mm" to be aceptable!!!
Dear adolph,
the barlow lens will do nothing on the focal length of the scope - it will do on the focal length of the eyepiece.check the wikipedia article on barlow lenses
anyone out there?

Answer 1

The degree of chromatic aberration in an achromat is proportional to the focal ratio times the aperture. So the 80mm f/11.4 will have less chromatic aberration than the 90mm f/10.1. The difference will not be huge, and is probably outweighed by the difference in aperture. Both of these are fairly long by current standards and CA will be not bad for an achromat.

The minimum focal ratio for color-free performance for an achromat has been variously given as somewhere between 3 and 5 times the aperture in inches, so at least 9.4 for 80mm and 10.6 for 90mm, using the most lenient standard. Such a scope will still show a bit of purple haze around bright objects, as the red and blue ends of the spectrum will go far out of focus. In short, you would not mistake it for an apochromat. Note that the 80mm meets the minimum criterion, while the 90mm just misses.

As far as magnification, being of the same focal length, both scopes will have the same magnification with a given eyepiece.

If you go to the Yahoo tech groups you might be able to find some informed opinions about these particular telescopes. Try these:
http://tech.groups.yahoo.com/group/Refractors/
http://tech.groups.yahoo.com/group/telescopes/

Added:
Adolph has some good information, but he's mistaken about Barlow lenses - they have no effect on chromatic aberration. The CA is dependent on the focal ratio of the objective, and nothing you do downstream of that, short of some exotic color-correcting lenses, will improve it.

Answer 2

These two Telescopes are so close in optical performance, that you won't even notice the difference unless you are a very experienced observer. The type of eyepiece design selected will make more of a difference than the slight difference between the telescopes. Both are of very good quality and are sold under many brand names, Orion being my favourite. As has already been mentioned, Chromatic aberration decreases with an increase in F/Ratio. A 4" Achromat needs an F/Ratio of about F/16 or better to eliminate it.
Unfortunately Telescopes with these F/ratios are made only by a couple of companies any more, They used to be the norm!!
The barlow lens ( if it is a good one ! ) furnished with the Refractors is meant to take care of this. The 2X Barlow with the 80mm Refractor for example will bring it's F/ratio up to over F/22. Voila' no chromatic aberration. This was the original purpose of the Barlow lens, NOT the increase of magnification as is commonly assumed today.
A word about small telescopes in general.
Regardless of the brand name on the tube and on the box, The vast majority of these Telescopes are manufactured by a chinese company named Synta.
So, aside from the name on the box and the paint job, the biggest difference is made by the quality control procedures in place by the company selling them. ( In my experience, Orion being the best in this regard ) Price differences are due mainly to Brand name recognition ( you're paying for the name) or gadgets added to the basic scope, most of which may be " nice to have", but don't make the scope see any better. ( Meade loves to do this ).
BTW, this does not only apply to Refractors.
Meade and Celestron make their bread and butter from their Cassegrain Telescopes and their other products are mainly chinese imports sold under private label. There are actually two factories in china and one in Taiwan who make these types of Telescopes. They are pretty much equal in quality and performance. Which is surprisingly good and getting better all the time.
The thing to watch out for is when these scopes are sold under off-brand names. These are often the rejects from the reputable dealers.
Carefully check the detailed specifications and stay away when you see a scope with identical specs being sold for half price, it could well be a reject!
That's why I constantly advise newcomers against buying on ebay, amazon, etc. because this is where a lot of these end up.
Sorry for rambling. I'm off my soapbox now.

Adolph

Answer 3

Injanier has compared the two well. I've tested 70mm f/10 and 102mm f/9.8 refractors from the same manufacturer, and both had acceptable optical performance. In fact, if you can afford it, I'd recommend the 102mm f/9.8 over either of these scopes. It's available from SkyWatcher throughout most of the world, and from Celestron in the USA:
http://www.skywatchertelescope.net/swtinc/product.php?id=36&class1=1&class2=103
http://www.celestron.com/c2/product.php?CatID=63&ProdID=430

If you want the finest achromatic refractors on the market today, check these out:
http://www.antaresoptical.com/vixen-spec.htm

Here's an article I wrote on achromatic refractors:
http://www.gaherty.ca/rogers/achrefract.htm

Answer 4

5/9 of the radius of the Earth. This is assuming that points are chosen randomly according to the following algorithm: We take a cube which precisely encloses the Earth and choose points uniformly from within that cube. If the point chosen is outside the Earth, it is rejected and this process is repeated until a point inside or on the Earth is chosen. This is the first point. This procedure is repeated in the same way to select the second point.