Refractor aperture concerns

[michael.h.f.wilkinson]

you can see here :

http://www.gaherty.ca/rogers/achrefract.htm

i just converted it from inch to millimeter

Important caveat (well explained in the link): This is for achromats. An ED or triplet will have much better colour correction and thus approach the theoretical limits.

Regarding high performance in a small package: a Mak or SCT can deliver stunning views. My C8 OTA weighs in at only 4.7 kg, and I have yet to see the 4" achromat that outperforms it on planets or DSOs (except for a few wide field ones). I have compared it to a 4" F/9.8 (Bresser branded) and while the latter is a capable little scope, it is far bulkier to handle, and requires at least as much in terms of mounting as the C8. On really good nights, I can readily reach 290x with the 7mm, and have gone beyond that on Mars and the moon. No 4" achro will do that.

[Tomjo59]

you seem to be suffering from analysis paralysis. There is not an Astro scope of any type that is sealed all are open as soon as you remove the eyepiece.

All mirrors degrade slowly over time but the aperture gain over that time is substantial for the same cost.

There's no such thing as the perfect scope for everything not a free lunch. Somewhere you have to compromise.

I recommend attending a star party or club meeting before you buy

'Analysis paralysis' - the perfect phrase to describe my own prolonged contortions trying to decide which scope to get!

[michael.h.f.wilkinson]

'Analysis paralysis' - the perfect phrase to describe my own prolonged contortions trying to decide which scope to get!

Trust me: you want both (whatever two you are wondering about), and once you have them, you will want a third

[x935418]

Important caveat (well explained in the link): This is for achromats. An ED or triplet will have much better colour correction and thus approach the theoretical limits.

Regarding high performance in a small package: a Mak or SCT can deliver stunning views. My C8 OTA weighs in at only 4.7 kg, and I have yet to see the 4" achromat that outperforms it on planets or DSOs (except for a few wide field ones). I have compared it to a 4" F/9.8 (Bresser branded) and while the latter is a capable little scope, it is far bulkier to handle, and requires at least as much in terms of mounting as the C8. On really good nights, I can readily reach 290x with the 7mm, and have gone beyond that on Mars and the moon. No 4" achro will do that.

yes i know , but as flanker said at start of topic, his problem is choosing between two achromats .

SCT are great, some month ago , when i want to buy second telescope, i resisted the temptation to buy  SCT, as they are four time more expeinsive  in compare with normal Newt.

my budget just permit to choos between   5" SCT or 8" Newt , i preferred  8"  and

now i am not sorry for my selection

just some nights when i sleep,i see a 8" SCT in my dreams !

[michael.h.f.wilkinson]

yes i know , but as flanker said at start of topic, his problem is choosing between two achromats .

SCT are great, some month ago , when i want to buy second telescope, i resisted the temptation to buy  SCT, as they are four time more expeinsive  in compare with normal Newt.

my budget just permit to choos between   5" SCT or 8" Newt , i preferred  8"  and

now i am not sorry for my selection

just some nights when i sleep,i see a 8" SCT in my dreams !

SCTs are quite a bit more expensive. In a choice between a 5" SCT and an 8" Newtonian I would go for the Newtonian, unless I needed to carry it a lot. If you can simply set up in your garden, an 8" Newtonian is no big deal. If you need to transport it a long distance, it is quite a monster. I used to have a 6" F/8 Newtonian. Razor sharp, easy to keep collimated, but hopeless in terms of transport. I saved up for the C8 a long time (and got many contributions from friends and family as gift for getting my PhD), and the BIG advantage is that I could take it to France in 1999 for the eclipse in a little Peugeot 106 with plenty room to spare for the tent, all the camping gear, camera gear (oh, and the missus and her shoes ). I could also lug it quite a distance from our house to a nice open space in a park in evenings. This was possible with the 6" F/8, but much harder.

[timwetherell]

Having owned a couple of short focus achromats, I personally would be a bit wary of assuming that if you stop them down to 70mm the performance will be the same as a "native" 70 mm f9 scope. These scopes are built for wide field low power viewing (which they are superb for) but the lens figure is sometimes not that great. In a perfect lens yes, it's the edges that contribute most to the chromatic aberration halo but a lens built down to a price for a wide angle low power scope, the entire figure can be a couple of wavelengths off. At their best, refractors are fantastic telescopes but they are expensive and bulky. For portability and all round performance I would agree that something like a 6" SCT may be worth at least considering  

India being a hot humid climate may also influence the best choice of scope?

[F15Rules]

That's great and sound advice Tim:-)

Dave