Anyone owns a 12" or larger DOB?

ariefm71 · Post by **ariefm71** » Tue Apr 11, 2006 3:38 pm

what are the waiting times for a telekit these days? VinSnr? anyone?

i presumed you're asking about the kit, not the finished telekit. i checked with randy last month, the 10" f/6 kit leadtime is only 1 month, a few months for bigger structure.

Post by **rcj** » Tue Apr 11, 2006 3:43 pm

hi arief,
thanks for the quick info on rough lead time.... yes it is the kit itself.

Tachyon · Post by **Tachyon** » Tue Apr 11, 2006 5:15 pm

I have looked at the Obsession pre-July prices. Seems like the sweet spot in terms of cost per inch is the 20" @ US$ 5,995. Siaheng - are you thinking of this one?

Tachyon · Post by **Tachyon** » Tue Apr 11, 2006 5:16 pm

acc wrote:Hi Andrew
As a rule of thumb, you need 2x the light gathering capacity in order for the eyes to notice a significant difference. So the normal upgrade path is from 8" to 12", or from 10" to 14". For me, the cost and weight of the 14 is really too much, so went for the 12

cheers
cc

Thanks for the info! The largest scope I have is 8" so I have null experience with anything larger than that!

Cheers!

Canopus Lim · Post by **Canopus Lim** » Tue Apr 11, 2006 5:21 pm

Ha the prices are rising. Anyway back to that discussion before about the 8 inch SCT compared to 15 inch Obsession... I think that is really outdated as Dave said that the old website was many years old in the 90s. Then that comparison was probably true. He just recently updated the website.

Post by **VinSnr** » Tue Apr 11, 2006 7:51 pm

Canopus Lim wrote:I will probably get the Obsession 12.5 inch in May. I have read an article from Thomas Back. He said that there is a big difference between optics of 0.8 and 0.88 strehl ratio. However above 0.95 they are considered excellent/perfect and it is really difficult to tell the difference between a 0.95 and 0.99 optics unless you have the perfect sky condition, perfect eye, perfect collimation and whatever all things perfect; hence limited by seeing.

I have also simulated the Point Spread Function in office (for 12.5 inch, F/5, 17 percent obstruction case.. which is my Obsession scope case) and 0.95 to 1 strehl is really very close. 0.88 and 0.8 strehl has a big difference which is true. I have chose the Torus mirror and by interformetry (by paper) it gives stats of 0.97 strehl which is excellent enough for me since I don't have perfect eyes and probably there will not be perfect skies, perfect cool down mirror and perfect collimation.

In Singapore, I think VinSnr is quite right but if the Meade Lightbridge mirror is the 'standard' 0.8 strehl so called minimum diffraction limit, I think there will be a difference in optics when viewed side by side. There should be as during the Mars watch, the 8 inch Portaball does show a bit more detail on Mars than my 8 inch Orion Dob. However, for strehl above 0.95, I think I won't be able to tell the difference at all especially for big mirrors which are more limited by the above cases. Maybe you guys who have keener eyes can tell the difference. We can have a test later when I get my scope.

If I were you, I buy a 12.5" Astrosky, put in a Steven's Optical mirror, put in Servocat GOTO, and I think I still have some loose change.

Actually it is interesting to see how Singastro evolved. Few years back, no one would even be interested to think about an 8" dob. Most are looking at 3-4" refractors. Discussions on newtonians are usually a big turn off.

Now, I see many people talk about 12" scopes and beyond....

The sky got brighter, but the scopes got bigger too. Not bad.

ariefm71 · Post by **ariefm71** » Tue Apr 11, 2006 8:18 pm

Canopus has already bought the Obsession 12.5", May is the expected arrival time.

Yup, large optics is the "in" thing now.

jermng · Post by **jermng** » Tue Apr 11, 2006 8:27 pm

I'm just thinking out loud, since I use mainly an f/10 SCT now, I can use cheaper EPs that give me pretty good results.

Even my f/6 scope gives ''ok' results if I'm not fussy ...

If I go down to an f/5 or 4.5 dob, I would have to upgrade all my EPs right? Wow, it doesn't just stop at the telescope!!

Post by **weixing** » Tue Apr 11, 2006 9:13 pm

Hi,

If I go down to an f/5 or 4.5 dob, I would have to upgrade all my EPs right? Wow, it doesn't just stop at the telescope!!

Cheaper eyepiece doesn't mean it can't use on a fast scope... A good plossl or orthoscopics can still use on fast scope with good result. A coma corrector will also help your cheaper eyepiece to perform on fast scope.

Have a nice day.

siahheng · Post by **siahheng** » Tue Apr 11, 2006 9:35 pm

Actually it is interesting to see how Singastro evolved. Few years back, no one would even be interested to think about an 8" dob. Most are looking at 3-4" refractors. Discussions on newtonians are usually a big turn off.

Now, I see many people talk about 12" scopes and beyond....

There is a bird flu around but looks like we all caught the aperture fever!

Andrew, I was thinking of the 18 inch but my worry is the weight. I am not getting any younger and may not be able to lift it up to my rooftop after a few more years.