Anyone owns a 12" or larger DOB?

[weixing]

Hi,
Ok... how about this scope... 45" f4.5.

Have a nice day.

[acc]

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

A detailed response from Mag1 that I extracted from the Portaball Yahoo Group

"I'd like to correct a misimpression many have about the difference the between the 14.5" and the 12.5" as far as light gathering ability. Light collecting ability is a function of area, which as you know from high school geometry is computed by the radius of the circle squared, times pi. Apparent brightness at the eyepiece is also subject to the limitation of the relative sensitivity of the receiver, in this case the brain. Our brain does not process sensory changes arithmetically, but rather geometrically, which is the basis for the magnitude scale, as well as the decibel scale. To perceive a difference in brightness, loudness (or any sensory input for that matter) the brain requires a significant change. One magnitude is defined as the increase/decrease in brightness by the fifth root of 100, or roughly 2.5 times the previous one. That well agrees with Aristarchus' sytsem of visual magnitudes - of five equal steps from one to six. To achieve a magnitude change, one must increase the collection area by 2.5X. Since the radius squared of the 12.5" is ~40. To get a one magnitude increase 40 x 2.5 =100, and since the square root of 100 is 10, means you must have a mirror of 20" diameter to get a one magnitude change. Note that you can skip multiplication by pi for these calculations, as it is a constant. The area relative area of the 14.5" is only 53, which is about a 30% increase over the 12.5's. If we assume brightness perception increases on a straight line between magnitudes (it doesn't) then 30/250 that translates into less than 1/8th magnitude gain!

Telescope sizes have developed since Galileo's time following those one magnitude increments. Using his ~1" glass as a benchmark, steps of one magnitude result in the following sizes 2.6" (60mm), 3.1" (80mm), 4"(100mm), 6", 8", 12.5", 20", 30", 1 meter (~40"), 2 , 3, 4, 6, 8, etc. meters. Do those sizes sound familiar? That also explains why most people are decidedly underwhelmed going from a 4" scope to a 5", or from an 8" to a 10".

From a practical standpoint the 12 and the 14's have nearly an identical image brightness at the eyepiece. Since they have the same focal length, any given eyepiece will deliver the same magnification. What does change is the slope of the light cone, which goes from f/5 to f/4.3 and that results in an important change in the curvature of the image plane. At f/5 the entire field will appear in relatively good focus,and be mostly free from visible coma. Not so a f/4.3... there a coma/field curvature corrector (like the ParaCorr) should be built into the optical train. That means additional cost, and because of the additional air-glass surfaces, some light loss. Adding such a lens assembly also means an increase in weightin the counterweight is required (adding a total of 4-5 pounds to the weight of the telescope). Surprisingly, it also decreases the size of the true field. For example, a 35mm Panoptic delivers a
1.4 degree FOV in the 12", but only 1.2 degrees in the 14".

So what does this mean to you? Assuming you would have to buy a ParaCorr lens, that adds $225 to the price differential. Comparing base costs/inch of aperture that works out to as follows: $4,500/12.5 = $360 versus $5625/14.5 = $387.00. Is the "mines bigger than yours is" difference worth the money? That's your call. I'm not biased by what someone buys, as my prices are set so I make the same amount on any size. I would also add this, for the price differential of $1,125, you could put the12" on our drive platform ($800) and not only be more comfortable observing, but you would have a lighter total package, and some significant change in your pocket.

So you may be asking why offer in-between sizes like the 10 and 14? The answer is simple marketing. Because this is a male dominated hobby, catering to their size bragging rights perceptionshas engendered that, as well as offering different price points. Now someone with a 10" needs 16" to get a one magnitude gain, so to in order accommodate those folks a good marketer needs to have something approaching the odd sizes. I have always championed the standard sizes, at least when asked I can then tell as Paul Harvey puts it "The rest of the story..."

So there you have it... truth, and right from the better end of the horse! ;)

Pete Smitka

[Canopus Lim]

Yup this informative. I chose 12.5 inch too because it is signficantly better than an 8 inch and I have looked through a 12.5 inch once and it was really wow. Also the price difference between a 12.5 inch and a 14 inch/15 inch can allow me to buy other accessories and I also need not buy the paracorr which not only adds weight, big cost and light loss. Still waiting for my scope though.

[jermng]

Thanks CC!
I was just wondering why 12.5, as in why the 0.5 and not just make it 12 like some GSO and Synta Dobs ... now I know why ... And I also know not to "waste" money on a 15" ...... So maybe a 20"??? HAHAHA

[ariefm71]

I can definitely see the difference between 12.5" and 15"

[VinSnr]

I can definitely see the difference between 12.5" and 15"

with atmospheric limitation, I don't think it will be what is shown on the picture

[acc]

Wow, I think the simulated images on the obsession website are quite an exaggeration; in fact, I looked at M13 through a 12" (Orion) dob and a 16" once at identical magnifications at a star party and the difference wasn't that dramatic. I think such advertising websites muz be taken with a pinch/pound of salt

cheers
cc

[weixing]

Hi,

with atmospheric limitation, I don't think it will be what is shown on the picture

Agree... also, it depend on individual eyesight and the observation site...

Have a nice day.

[Tachyon]

Does this also applies to imaging?

Now I have to save up for a 20"! *ARGGGHHH*!!

[ariefm71]

Does this also applies to imaging?

Now I have to save up for a 20"! *ARGGGHHH*!!

If you have a good mount that can track for hours, a 2.5" scope will give better photo image than a visual image from a 25" scope.
http://www.singastro.org/viewtopic.php?t=2258