Do I need an apo?

[markystars]

Hi, I would like some advice on weather or not I should buy an Apo Refractor. I currently use a 8" skywatcher, a 120mm Achro Refractor and I have my little 80mm basic refractor that comes out when i wish to look at the moon. I read so many great reports on 80/100 ED Apo's that it seems they are the best telescopes to buy. To my little knowledge, I believe that the bigger the primary mirror, the bigger the image and the further you can see, therefore why would a 100mm Apo be so much better and much more expensive than my 8" Skywatcher? And what exactly IS Apo?

I have tried to find a post that explains this but can't so please accept my apologies if there is one.

Thanks

[roundycat]

Apo is short for apochromatic which means the lens(s) are corrected to bring light to the same focal plane regardless of whether it is red, yellow/green or blue light. Lack of it means your lens or scope suffers from sphero-chromatism. You will see a sharply focused image with either blue or red halos around the central star, maybe both. The degree to which scopes are corrected has a direct reflection on the price. The specification requirement differs from one maker to the next. The Tak FSQ for instance has a correction that goes from blue at around 400nm to near infr red at 1000nm. You will not see many of the cheap 80-100mm apo's reaching that standard but they can still be quite good.

Dennis

[jgs001]

That's not an easy one to answer... but...

Apo = Apochromatic. This means, that all colours of light come to a focus point at the same point. This is unlike an Acrhomat which doesn't focus blues as well as the others, thus the blue/violet fringing around brighter objects.

Image scale isn't defined by the aperture, but rather by the focal length. Larger aperture provides greater resolution and light gather. However, for deep sky imaging at least, light gathering is less of an issue as you can make up for it by using a longer exposure.

An 80mm ED scope is short, lightweight and easy to manage, providing well corrected wide field views/images. This makes it a great starter scope for imaging. I suspect the shorter focal length, also provides for a more tolerant system for tracking errors too.

If you are happy with your current setup, why buy a small Apo refractor ? The cost, btw, comes from the higher quality glasses used in the lens cell to provide that colour correction.

[Doc]

A lesser quality lens might be Achromatic, focusing 2 frequencies to the focal plane. You would get color fringing of the third primary colour.

An APO lens will focus three colours to the focal plane them being red, green and blue. In doing this it eliminates false colour in the object being observed.

However, today the term APO or apochromatic is used quite loosely by lensmanufacturers and designers. It supposedly means that the lens meets certaincriteria for focussing different colours.

[ombos]

OK the tech bits have been covered above so I'll leave that one, my Tak will give you clear wide angled views without coma etc, but APOs are not the be all and end all. What someone would view as an excellent view some would say no thats poor and visa-versa. unless your get 5" min you are very limited to what you can see. I use my 250mm newt for DSO at the end of the day a great pic of nothing with the tak is useless but the newt picks up DSO far far better and I also use it for local too, dont knock the newts SCTs etc, yes more work keeping them in order and they give you a bit of coma etc etc but its down to what you want yourself. I think everyone who has a nice apo also as a large aperture scope too. Ask doc if he would swap his 16" for 102 no. Yes easy to transport and pop on your mount and away you go, no false colour at all but your right, stick with your present scope until you've viewed though a few nice APOs ad see if they are worth it to you! You may be surprised. You may have read the blog below?? IMHO its Carp lol

I wrote to you some time back, perhaps 18 months ago asking for your help in deciding which SCT to buy. 8" or 10". f10 or f6.3. Blah, blah, blah... I started by looking at an 8", worked up to a 10" and finally came back to the 7" Meade MAK LX-200. You e-mailed back a short but very disappointing response. I mean, after all, I had done my homework and KNEW in my heart that the 7" MAK was the answer. To paraphrase, you said something like, "If you're gonna spend $3,500 to $4,500 why not look at one of the 4" refractors?" What nerve I thought. What arrogance or perhaps even ignorance, I thought. Why everyone who knows anything knows that aperture is everything. The puny 7" is a compromise for me to begin with. After all, I had my 13.1" light bucket and had the sky by the tail. (I had a 3" Gibson f10 reflector back in High School circa 1963 and I remember what Mars, Jupiter and Saturn looked like at 475X through a 3") A 4" for how much???

As good, no make that great, fortune would have it I bumped into a parent at my kids school who had just "upgraded" from a 14" SCT to a used 4" Takahashi FS102. Wait a minute... Upgrade??? The wheels spun and the brain tried to regroup.

Fast forward 6 months.

My Takahashi arrived. Before it even left the packing materials behind, I knew I was to be in for a major treat.

Fast forward 11 days. New "astro-purchase-curse-fog bank" lifts. Clear steady dark sky. Oh my God!

Today, 4 months post arrival. My Tak FS102 sits atop a Vixen GP/DX with Sky Sensor 2000. I have seen more "M" objects, detail on planets and NGC objects than in all my the previous years as an observer. I use the scope every "good" night. It takes 12 minutes to be on target from the decision to "GO".

Now to the point of this lengthy epistle.

First, please accept my apology for my initial reaction to your e-mail. Second, please accept my sincere and heart felt thanks for steering me away from that "mass-marketed SCT" and toward the path to my Takahashi.

I cannot thank you enough.

Ron, you are more than welcome, and thank you for letting me share this letter with other readers. And for a humorous (not to mention highly instructive!) perspective that compares the average Department store refractor with a high-quality refractor, click on this link

[Doc]

Would I swap my 16" for a 4" Achro then NO but for a 4" Tak I might be tempted.

At the end of the day my 16" Dob will beat a 4" Tak (APO) on almost everything except maybe splitting tight doubles as collimation then really starts telling and has to be really good.

In my dreams I would like the best of both worlds and have both.

Aperture rules so does dark skies put these together and you have everything.

[glowjet]

The basics of the different lenses has been covered and as has been said aperture rules, but not always. This Country is blessed with it

[Peter Drew]

Hello Markystars. You already have an "Apo", telescopes with reflecting optics are pure apochromats, chromatic aberration is a function of refracted light. The real question is, do you need a wide field high quality photographic or grab and go scope in which case an Apo refractor would be ideal

[glowjet]

Sorry about that , after cat jumped on key board. To continue This country is blessed with indifferent weather at the best of times, larger mirrors have to see through a larger column of air which often disturbs the image, smaller Refractor scopes can tend to overcome this as the column of air is much smaller and there is no central obstruction, giving a clearer image. This is probably why refractors are popular, especially the APO types . If Father Christmas offers you an Astro-Physics 130 Starfire

do not say no.

John.

[acey]

I believe that the bigger the primary mirror, the bigger the image and the further you can see, therefore why would a 100mm Apo be so much better and much more expensive than my 8" Skywatcher?

The bigger the aperture, the more light you gather, and you can see fainter (further) objects. Your 8" will give better views of galaxies than any 100mm scope.

Where refractors win is in contrast and sharpness. A good 100mm refractor will give better planetary views than your 8" Skywatcher.

The poorer contrast of Newtonians is due to their central obstruction (the secondary mirror). Refractors don't have this. But their lenses are apt to produce chromatic aberration (colour fringing round images), which reflectors don't suffer from. Apo's are a way of minimising those fringes.

You pay your money and you take your choice. Most important thing is to decide what sort of observing you want to do, then get the tool for the job.

As an affordable, general purpose all-rounder, the 8-inch Newtonian is hard to beat. So unless you can see a desperate need for another scope, I'd say stick with what you've got, and enjoy it.

If you're not happy with planetary views through your Skywatcher, there are many way of improving matters; e.g. baffling, flocking, replacement spider. And of course clean, well collimated optics.

Andrew

[dweller25]

You will need a 5" Apo to outperform your 8" reflector, so you are looking at a 120ED minimum. The Meade 127 triplet is also worth considering. Anything smaller will disappoint.

Hi, I would like some advice on weather or not I should buy an Apo Refractor. I currently use a 8" skywatcher, a 120mm Achro Refractor and I have my little 80mm basic refractor that comes out when i wish to look at the moon. I read so many great reports on 80/100 ED Apo's that it seems they are the best telescopes to buy. To my little knowledge, I believe that the bigger the primary mirror, the bigger the image and the further you can see, therefore why would a 100mm Apo be so much better and much more expensive than my 8" Skywatcher? And what exactly IS Apo?

I have tried to find a post that explains this but can't so please accept my apologies if there is one.

Thanks

[markystars]

Wow, thank you guys for the replies, I believe I now understand why the apo refractors are so popular. I will take all the advice given and I think for now at least I shall stick with what i have got. I am just beginning to consider getting into astro photography and it seems that the Apo's are best suited for this, however being new to it, I probably wouldnt benifit yet from the increased qaulity of the image. I think if santa does stop at my house then the Apo will be on my wish list, until then, I shall continue searching the cosmos for DSO's and other wonderful things.

Thanks again for the sound advice, when I do get an Apo I will now know what to expect.

Mark..