Is Bigger Always Better?

Post by **acc** » Sun Oct 12, 2003 7:34 pm

For those who would like to contract aperture fever, read the following extract from the Nov issue of Sky and Telescope:

... the difference between a 6" and 12" can be the difference between seeing just the Meisser galaxies in the Virgo Cluster and seeing an additional 30 or 40 NGC galaxies in the same field. It is the difference between viewing M51, the Whirlpool Galaxy, as two faint smudges rather than reliably seeing the spiral arms in the main component and the bridge connecting it with the component.

With a large telescope, globular star clusters start to take on individual personalities, as opposed to looking like the same grainy blob in the eyepiece. If you have sensitive vision, you can start to see a hit of red colouration in the stars of the globular clusters...

Yummy! 8)

Post by **VinSnr** » Sun Oct 12, 2003 10:15 pm

Yummy! 8)

Ooi....dun make me drool your 16.1" leh

chris shaw · Post by **chris shaw** » Wed Oct 15, 2003 12:14 pm

If you compare 8 inch and 11 inch - there are distinct resolution differences based on size alone.

But comparing a 7 inch with 8 inch - I don't think there is much difference.

Previously I asked a question concerning whether to buy a 7 inch Mak or 8 inch SCT and the reply I got was that barring their fundamental optical and focal length differences, their 1 inch difference in aperture does not really amount to much. In other words, the difference between a 7 inch Mak or 8 inch SCT has got to do with the difference between two STYLES of scope and NOT their 1 inch aperture difference.

Any comments?

Chris

Post by **zong** » Wed Oct 15, 2003 10:23 pm

Yeah, I kinda get the idea aperture isn't much to consider unless it's really a big difference. 3 inches is really big difference, man... but one thing i don't really catch is the difference between a mak and an SCT, aren't both catadiotropics? so what can be the difference in viewing between 7inch mak and 8inch SCT, 'cept that mak has to take more time to reach thermal equilibrium... That i'd like to understand, cos i don't have a scope and i can't test it out

Post by **weixing** » Wed Oct 15, 2003 11:38 pm

Hi,
The 7 Inch MCT are comparable to a 8 Inch SCT because the Central Obstruction of the SCT are bigger than the MCT. So the 1 Inch different is begin offset.
The main different between MCT and SCT are the corrector plate and the secondary mirror. The SCT use a thin aspheric Schmidt correcting and a secondary mirror. But the MCT use a thick meniscus correcting lens with a strong curvature and the secondary mirror usually is an aluminized spot on the corrector lens. As a result, the MCT secondary mirror is much smaller than the SCT, thus giving slightly better resolution for planetary observing and also since it is an aluminized spot on the corrector lens, MCT seldom need to collinate.
Anyway, most of the time you can't tell the different between scope with only 1 inch of difference. Also, the quality of the optics is also importance when the difference in aperture is small.
Hope the information is useful...

kcy · Post by **kcy** » Thu Oct 16, 2003 10:23 pm

Weixing

But the MCT use a thick meniscus correcting lens with a strong curvature and the secondary mirror

Erm a strong curvature lens means more bending of light. These causes refraction of light on all the different wavelengths right? Different wave lengths means different focal length?

I prefer sct..although for me a mct is better because it needs less collimation =)

Post by **zong** » Fri Oct 17, 2003 12:07 am

hm... so only difference between SCT and MCT are one need collimation more than the other, and one has better planetary viewing than the other. I'd like the SCT better tho, if you say like that... cos less time to reach thermal equilibrium. I wouldn't like to wait an hour for optics to reach best performance!

Post by **weixing** » Fri Oct 17, 2003 12:09 am

Hi,
Ha Ha... you got the idea without knowing it...

Yes, you are right - the meniscus lens has spherical aberration, but the meniscus lens could introduce positive spherical aberration to compensate for the negative spherical aberration of a spherical primary mirror, or system of mirrors, without introducing chromatic aberrations... Get the idea...

Post by **weixing** » Fri Oct 17, 2003 12:17 am

Hi,
Don't worry about thermal equilibrium, you won't have this problem...

Singapore is not so cold and the different between in-door and out-door temperature is not so great.
Anyway, the MCT should already reach thermal equilibrium after you set-up all your stuff and ready for observation...

Post by **Airconvent** » Fri Oct 17, 2003 12:48 am

chris shaw wrote:Previously I asked a question concerning whether to buy a 7 inch Mak or 8 inch SCT and the reply I got was that barring their fundamental optical and focal length differences, their 1 inch difference in aperture does not really amount to much. In other words, the difference between a 7 inch Mak or 8 inch SCT has got to do with the difference between two STYLES of scope and NOT their 1 inch aperture difference.

Any comments?

Chris

I was told that the rule of thumb when upgrading aperture is that the diameter must be AT LEAST 50% bigger for you to see an appreciable difference.
Hence, for your ETX125 (5"), you have to upgrade to minimum 7.5" and since there is no such thing as a 7.5", then go for an 8" LX90 or LX200 or LXD55 or Nexstar 8...

SingAstro

Is Bigger Always Better?

Is Bigger Always Better?

Re: Is Bigger Always Better?