Need Opinions on 8" SCT vs 8" F6 newt/dob

Sandeep10 · Post by **Sandeep10** » Sun Jun 03, 2007 4:56 pm

Hi VinSnr
how much did it (8" LX200R OTA) cost... :?:

Post by **weixing** » Sun Jun 03, 2007 4:59 pm

Hi,

I'm not refering to comparing the two designs. I just want to know whether light-pollution significantly affect the view of DSOs when using low power on a fast dob under Singapore condition. Thanks.

Base on my experience, with an exit pupil 6.4mm (31x for my scope), the sky will show as light blue... those DSO with low surface brightness will be a problem. IMHO, an exit pupil of 2.5mm (80x for my scope) and smaller work best for DSO observing in light polluted Singapore sky... regardless of scope type.

Have a nice day.

Post by **VinSnr** » Sun Jun 03, 2007 5:14 pm

Anyway, if you don't have any transportation issue, a decent visual Newtonian will always perform better than a decent SCT. I love my Newtonian, but I'll have big problem if I'm going to bring it to Mersing due to it's size...

Have a nice day.

I got a 8" f/4. Gosh, this thing is light! Very easy to bring around. But collimation is very challenging and laser will not do the job. I use another tool which gives very little margin of error. Will detail my collimation process in another post.

Post by **weixing** » Sun Jun 03, 2007 5:26 pm

Hi,

I got a 8" f/4. Gosh, this thing is light! Very easy to bring around. But collimation is very challenging and laser will not do the job. I use another tool which gives very little margin of error. Will detail my collimation process in another post.

Which 8" F4 Newtonian did you get?? What is the weight and central obstruction size?? The lightest 8" F4 Newtonian I know of is the R200SS at slightly over 5kg. Base on Perseid, the weight of the GSO 8" F4 is around the same as my Celestron 8" F5 Newtonian... around 8kg to 9kg.

Have a nice day.

Meng Lee · Post by **Meng Lee** » Sun Jun 03, 2007 7:20 pm

Hmmm, sounds likes its really R200SS, its a good astrograph also.

Btw, i think this is portable, top quality Dob that you can consider (with the captical of course):

The Teleport, featuring Zambuto mirror:

http://www.teleporttelescopes.com/

Post by **acc** » Sun Jun 03, 2007 7:32 pm

jiahao1986 wrote: Hi weixing,

I'm not refering to comparing the two designs. I just want to know whether light-pollution significantly affect the view of DSOs when using low power on a fast dob under Singapore condition. Thanks.

Jia Hao

Hi Jia Hao
Regardless of scope design, a larger aperture will show more regardless of light-pollution. For example in Sg, a small scope (eg 80mm refractor) will show the jewel box as a compact collection of dim stars on a dark background. A larger scope such as an 8" dob will show many more stars on a brighter background. (some may mistakently conclude that the smaller scope has "better contrast"). One way to achieve a darker background is to use a high power ultra-wide angle eyepiece. This higher power darkens the sky glow while the ultra-wide angle helps to preserve a decent field of view. Large aperture + higher magnification also helps to bring out the varied colours of the stars in this magnificent cluster. Hope this helps.

rlow · Post by **rlow** » Sun Jun 03, 2007 7:34 pm

Hi Kayheem,

Assuming the use of similar quality mirrors on alt-az mount for both:

8"f/10 SCT
Pro - more compact, more portable, less collimation/maintenance, simpler EPs perform well for wide field, easier for imaging
Con - noticable lower contrast, dew problem, narrower max. TFOV

8" f/6 DOB
Pro - better contrast, less dew problem
Con - more bulky, more collimation/maintenance, require premium EPs for good wide field view, gives wider max. TFOV, harder to setup for imaging

so it all depends on your budget, your style of setup & use, what you prefer to view, and how high a quality of view you want. Premium quality mirrors & aperture do also make a difference. My preference is for the dob, but your mileage may differ.

Post by **acc** » Sun Jun 03, 2007 7:42 pm

Agree with Richard except that an SCT is probably more troublesome maintenance wise as a newtonian is optically and mechanically very simple (diy repairs will do in the unlikely event that something goes wrong), while an SCT will have more parts to breakdown and require a trip back to the factory.

And you can wash a dob with a hose or a bucket of water but try that with an SCT... heheh

cheers
cc

Canopus Lim · Post by **Canopus Lim** » Sun Jun 03, 2007 9:34 pm

Yup I would choose the mass produced dob if size is not an issue.

My ex Orion 8" dob, although it is cheap, just beats optically any average SCT in sharpness, contrast and throughput; and it sets up faster than any scope out there...I have yet to see an exceptional SCT. Probably there are exceptional ones..... but it is hard to get one due to the complexity in making an SCT.

However, if one do not drive, the smaller SCT (5" to 8") are much transportable.

To answer the light pollution question, all scopes regardless of type have to go higher magnification in order to see DSOs well. Therefore, in theory the 8" f/5 scope can go very wide, in practise in Singapore, it has to go higher power and smaller field. If compared to a 8" SCT, it too has to go at higher power although one can use a longer focal length eyepiece due to the scope's longish focal length. Do note that at a dark site, the 8" SCT will be at a disadvantage for large extended objects (Veil nebula or anything with low surface brightness) because of its 4mm max exit pupil with a 40mm widefield eyepiece. If one wants to have 'bright objects' the 8" f/5 newt can go brighter and wider in field due to its bigger exit pupil capability.