Help: For Moon/Planets, 8" SCG or 4.7" refractor?
Posted 02 November 2012 - 10:33 AM
Key spec for both scopes:
- Celestron NexStar Schmidt Cassegrain 8SE: RESOLUTION: 0.68 arc seconds; RESOLVING POWER: 0.57 arc seconds SECONDARY MIRROR OBSTRUCTION: 2.5 in (63.5 mm) SECONDARY MIRROR OBSTRUCTION BY AREA: 10% SECONDARY MIRROR OBSTRUCTION BY DIAMETER: 31.30% FOCAL LENGTH: 2032 mm (80 in) FOCAL RATIO: 10.01
- Celestron Omni XLT 120 mm Refractor Telescope: RESOLUTION: 1.19 arc seconds RESOLVING POWER: 0.97 arc seconds FOCAL LENGTH: 1000 mm (39.37 in) FOCAL RATIO: 8.33
Posted 02 November 2012 - 10:52 AM
The 120 also has 60% of the resolving power of the 8SE. The contrast between the two should be close enough so as to not notice the difference. Based on aperture alone, the 120 has 86% that of the 8SE, however other factors like mirror scatter comes into play which degrades the contrast of the 8" SCT. In fact, the refractor may end up with more contrast, but I think they will be close enough to be a wash.
While the 120 is a good scope, being an achromat it will have false color which the SCT will not. Even putting CA aside, achromats tend not to be made to as high a standards as Apochromats or ED/Apos. Overall I think think the 8" SCT will provide better quality images in most respects, and be a more versatile scope.
Posted 02 November 2012 - 10:54 AM
I think you'd be hard pressed to go wrong with either scope for your stated use. Refractors are reknowned for their high contrast images and relatively quick cooldown times. SC scopes are compact and relatively easy to move about, but cool more slowly.
Most nights, the difference in resolving power will be obscured by the atmospheric seeing.
I do mostly planetary and deep sky (galaxies and planetary nebulae mostly) observing myself, and have been very well served by my 8" dob. A much less expensive option, though one without any tracking.
There isn't a really clear winner here. Choosing from those 2, I'd probably lean towards the SC scope, just because it'd give me a bit more light grasp should I decide to persue some deep sky objects. Not that a nearly 5" refractor would seem particularly small to me either....
In truth, with that sort of budget, personally I'd be looking at about a 12" dob and some accessories.... I could get 4.5" refractor performance from such a scope (unobstructed!) using an aperature mask, and outperform both the scopes you're looking at on deep sky.
Just some thoughts. Variety is the spice of life.
Posted 02 November 2012 - 11:38 AM
Posted 02 November 2012 - 11:49 AM
Ohhh, tough call. I happen to have both an 8" SCT and a 6" achromat. Both have their advantages and if I'm going out for a quick look I generally grab the refractor even though physically it is a much larger scope. However, as much as I love my biggo refractor I'd recommend the 8" SCT. The 8" packs a lot of scope into a small package. The larger aperture will give you sharper views and you won't have to deal with the color issues of a refractor. The 8" SCT is also a great general purpose scope. A couple of relatively minor issues is that you will have to learn how to keep the SCT in alignment (not a biggy, but it takes a bit of fiddling to master) and you will have to give it time to thermally settle down once you take it outside (30-45 minutes usually does it). If budget is an issue the refractor would work well. It will settle down quickly (I don't even think about it with my 6") and there's nothing to align (I think, some refractors have adjustable cells). The challenges with the refractor are color (which can lower contrast) and with only 4.7" of aperture you'll be limited in the upper end of magnification as the image will dim as the magnification increases. That won;t be a problem with the 8". However, the refractor will perform much, much better at very low magnifications.
Sooooo, if budget is an issue, the refractor will serve you well. However, since you mentioned that lunar and planetary is your primary interest I'd go with the 8" SCT if it were within your budget.
Have fun shopping around! That's the fun part.
Posted 02 November 2012 - 11:57 AM
Hello and Welcome to Cloudy Nights...
If the choice were between an 8 inch SCT and 4.7mm apo refractor, it would be a close call. But I have owned a couple of the 120mm F/8.3s and the chromatic aberration definitely affects the contrast and sharpness of the planets.
I am partial to Newtonians for planetary viewing but they have their issues as well.
Posted 02 November 2012 - 12:11 PM
If the stupid frankenstorm would leave I'd get a really nice view of jupiter setting in the morning, as well as venus rising.
there's always tomorrow [no singing "there's got to be a morning after"].
my 8in sct is sometimes mushy on planets.
buy used and save lots of money.
an 8in sct ota can be bought for ~ $350.
a genesis should be ~ $1k
Posted 02 November 2012 - 12:23 PM
That's about 1/2 price!
Welcome to CN:)
Posted 03 November 2012 - 06:33 PM
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