Scope questions - a novice needs help!

[Zeffer]

In S@N magazine it counts a large telescope in the 'Stargazing Guide' as Reflector >200mm and refractor >90mm.

Firstly, why the huge difference - why are refractors able to have smaller optics?

Secondly, where does that leave my Mak?

Also, I know from photography that Apos are great but what is the equivalent size achro. For example, what size achro would be needed for it to be as good as an 80mm apo or does it not work like that? Most people say aperture is king but surely an Apo can be a bit smaller?

Thanks in advance for advancing my knowledge and please forgive any profoundly stupid questions!

[GordonCopestake]

A large aperture is 8" and up. A 90mm refractor has a small aperture no matter what S@N says. Refractors are VERY heavy and expensive when you get to large apertures so you dont see them very often but you can get 8" refractors (and larger). Reflectors can come in small sizes too but arn't very practical given there central obstruction.

Your mak is 5" which is average sized and a good compromise between portability and light gathering. Dont forget that the more aperture you have the harder these things are to carry around, especially at long focal lengths. SCT's and Maks go some way to folding the light path but bigger than 8" and even SCT's are heavy and bulky.

The comparison between APO and achro isn't an easy one and to answer your questions a 4" APO is exactly the same as a 4" achro in light gathering and resolution. BUT the APO brings all the colours to the same focus point and therefore appears to have a higher contrast. The achro will have a purple fringe around bright objects which detracts from the asthetics. However some people dont mind this and live happily with cheap refractors costing a 10th of an APO's price.

An 8" newt will ALWAYS beat a 4" APO, especially at the longer focal lengths. APO's are not the "best" telescope just another type of refractor. Many people argue that newts are better than APO's simply because APO's can't compete on aperture and cost. I agree. However for widefield stuff APO's are great. Being small they are portable and light. For high mag stuff you can't beat a newt. SCT's are the jack of all trades and master of none. Maks are similar to SCT's but tend to be a little better.

Sorry for the ramble, i've been drinking and my brain is shutting down for the weekend hehe

[Greg]

You won't get any false colour with a reflector.....And you'll get more for your money.

Downside is ....Heavier, bulkier and need more cool down time...

Personally....Even though I have both types of scope, I prefer my refractors...I't a personal choice.

[Brass n glass]

I used to own a 4 inch Vixen achro and now have a Televue 76 achro. The Vixen was a worthy and entirely adequate instrument but the Televue is in a different league for clarity. It's not only the absence of false colour, there's what some observers call a "bite" that the best achros can't match. It's like the difference between home videos and DVDs. Or, as somebody said in relation to the difference between achros and apos for watching wildlife: "it's as if a thin veil you didn't know was there has been pulled aside."

[GazOC]

In S@N magazine it counts a large telescope in the 'Stargazing Guide' as Reflector >200mm and refractor >90mm.

Firstly, why the huge difference - why are refractors able to have smaller optics?

The 200mm reflector/ 90mm refractor S&N thing is a very bad comparision IMO, on deep sky objects a 200mm Newt will show more than a 150mm refractor let alone a 90mm one.

[FLO]

Regarding the Achro v APO comparison:

As Gordon says, the light gathering ability is the same. However, the APO's higher quality glass/coatings will produce a cleaner and more 'pure' look to the image (if that makes sense) which can also appear brighter.

An APO also has higher acutence (edge separation) - an achro' does not bring all the colours together at the same point of focus so inevitably, the edges (mostly of brighter objects) appear less defined, which has a knock-on effect on resolution, which is why APOs can be 'pushed' to greater magnifications.

After saying that, some people are more 'sensitive' towards false colour than others. Also, there is no industry defined 'standard' for apochromatic...

Perhaps most importantly, there are some superb, well-corrected achromats available for a fraction of the price of an APO.

Hope that helps

PS: I agree with Gaz - the comparison between 90mm and 200mm is a poor one.

[Zeffer]

Thanks for all the help. It clarifies things a great deal - I wonder how S@N came up with that comparison?

[The Warthog]

I think it is a kind of cultural definition. Most people can't afford much more than a 4" refractor, so if you show up at a star party with a 5" or 6" refractor, people say, "Holy Rubbish, what a big scope!" But if you come with a 6" or 8" reflector, either dob or newt, people say, "Uh-huh, nice scope." They save the "Holy Rubbish's" for 12" and bigger reflectors.

Compared to the 60 and 70mm scopes you see in department stores, a 102 is honkin' huge.

I must admit I am a little puzzed by people, some of whose experience and advice I value highly, running out and buying 80mm and even 66mm APOs. A 66mm scope is still a 66mm scope, and has the limitations of arcseconds of resolution of any 66mm scope. I'll have to reserve judgement until I get a proper look through an APO at Saturn or Jupiter, though. The GRS and Cassini's division are the gold standard in planetary observation for me, and I haven't seen them yet.

[GordonCopestake]

You should be able to see cassini in your 6" scope easily WH, and your new 4" should show it well too.

The advantage of the little APO's is mostly there portability and ease of use. No cool down, no colimation and crisp high contrast views.

I'de like a nice 5" APO but a 8" newt well colimated would show you more.

[gsjag]

Hi Zeffer,

I posed the same question to S@N a couple of week ago and (I hope the editor won't mind me sharing this) they are reviewing that whole section. I found the editor to be a very helpful and open guy who positivly welcomed feedback on the publication.

Hope it helps

Gary

[Zeffer]

I'll keep my eye on it - it's good to hear they are so open about their publication.

Interestingly, in the birding world, a pair of Leica bins can be used into the evening long after other same-sized pairs are far too dark to bother using. Does this not mean the glass is superior etc. and lets more light in? Or are they just focusing the light better and SEEM to be letting more light in when they are not?

Thanks for all the answers!

[GazOC]

That section of the S@N magazine is one of the consistantly good parts of it, but the 200mm refractor/ 90mm bit needs looking at. More of a tweak than a overhaul.

[FLO]

Does this not mean the glass is superior etc. and lets more light in? Or are they just focusing the light better and SEEM to be letting more light in when they are not?

A bit of both

[GazOC]

Also what tends to get overlooked in achro/apo discussions is that the light thats smeared is not just focussed in the "wrong" place, it's also lost from where it should be. So you lose both ways....

[gsjag]

"Interestingly, in the birding world, a pair of Leica bins can be used into the evening"

Agreed. In fact it was using my 8x32 that re-ignited my interest.

[FLO]

The closet 'twitchers' out there might like to know that eyepiece adapters are available for Swarovski and Zeiss fieldscopes that allow some astro eyepieces to be fitted. I think Zeiss also have an adaptor that will convert their eyepieces to 1.25" astro fitting.

[gsjag]

Steve - yes, Zeiss do have an adaptor and also do one for their microscope eye pieces to fit a telescope. Would love some of their kit but not sure what the compromise would be if you did use an adaptor.

Regards

Gary

[MartinB]

I think the main reason for the popularity of small apos is imaging. My ED80 is very limited as a visual scope compared to my 8" SCT. However for imaging it has a number of advantages. Low focal length fast refractors not only provide a wide field of view they are much more tolerant of tracking errors. This often more than overcomes the problems of requiring longer exposures esp when guiding. Add to this the lighter weight making less demands on the mount and you can see why small apos are such a hit with imagers. Aperture is much less of an issue for imaging than for visual work.

[FLO]

Steve - yes, Zeiss do have an adaptor and also do one for their microscope eye pieces to fit a telescope. Would love some of their kit but not sure what the compromise would be if you did use an adaptor.

I have successfully used plossels in a Swarovski 80HD (larger, wide-field eyepieces do not work because there is not enough back-focus). I also discovered that some Nikon Fieldscope eyepieces fit directly into a 1.25 focuser- their zoom works a treat 8)

Haven't tried Zeiss eyepieces for astro but I imagine they are very good. I did read somewhere that the Leica Zoom eyepiece works very well so I guess they must also offer an adaptor.

[gsjag]

Thanks Steve. Certainly Zeiss and Leica will be on my near term list as will a Williams Ewan.

Not sure they will be suited to my ETX but I guess that even if they're not, they'll last for as long as I have 'scopes!

Gary

[Brass n glass]

Leica birdscope eyepieces are supposed to be just about the best you can get, but the very simple 1.25 in adaptor for using them in astro scopes costs £39. The two Zeiss equivalents (one for zoom eyepieces and one for the rest) cost more than £80 each. Ironically the adaptor that enables 1.25 in astro eyepieces to be used in Zeiss birdscopes costs about £25.