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Help: For Moon/Planets, 8" SCG or 4.7" refractor?

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#1 Hazel


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Posted 02 November 2012 - 10:33 AM

Hi -- I would greatly appreciate some advice as I am rather new to astronomy. I am mostly interested in lunar and planetary observation and from a budget perspective I can afford a maximun of about $1200. After much research I think I have boiled it down to two scopes which seem to be decent performers and a good value: the Celestron NexStar Schmidt Cassegrain 8SE or the Celestron Omni XLT 120 mm Refractor. I know the SCG has a bigger apperture and that apperture is so important in astronomy, but it of course also has the large secondary mirror which I imagine could impact the image quality for the moon and planets (where contrast is so important). The 4.7 inch refractor is smaller but has no secondary (it is an achromatic so there will be some undesirable color fringing). It has an equatorial mount for which I would purchase the motor drive. Which one should I get? (again assume I am fine with spending about $1200 for the NexStar -- the Omni XLT refractor is less -- about $625). I provide the key specifications below including resolving power (I include the secondary mirror obstruction spec for the NexStar because I wonder if the resolving power really can be 0.57 arc seconds with such a large obstruction?). Thank you in advance for your thoughts and recommendations!
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

#2 Midnight Dan

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Posted 02 November 2012 - 10:52 AM

Between those two, I would go with the 8SE. The 120 only has about 40% of the light gathering capability of the 8" SCT. Even though you don't really need that so much with planets, it will make the scope more versatile and perform better on other targets.

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.

#3 JasonBurry


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Posted 02 November 2012 - 10:54 AM

Central obstruction affects contrast, not resolving power, which is down to aperature.

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.


#4 Maverick199


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Posted 02 November 2012 - 11:38 AM

I have the 6" version as well as 4" achromat. The 6" SCT wins in the battle of planets. The achromat has its own charm but given what I have seen in the 6" SCT, the 8se would be my preference.

#5 jgraham



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Posted 02 November 2012 - 11:49 AM

Welcome to Cloudy Nights!

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. :)

#6 Jon Isaacs

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Posted 02 November 2012 - 11:57 AM


Hello and Welcome to Cloudy Nights... :waytogo:

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.


#7 Pinbout


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Posted 02 November 2012 - 12:11 PM

I love my genesis for planetary, I can really push the mag on it. but being a 4inf5.4 I'm staring down a 4x barlowed 5mm eyepiece on good seeing to get a really nice size saturn.

Posted Image

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.

Posted Image

buy used and save lots of money.

an 8in sct ota can be bought for ~ $350.

a genesis should be ~ $1k

#8 REC



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Posted 02 November 2012 - 12:23 PM

The 8" SCT is an all around great performer. FYI, in the classifieds there is a Meade LS-8 for sale $1000 + shipping.

That's about 1/2 price!

Welcome to CN:)

#9 doctordub


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Posted 03 November 2012 - 06:33 PM

I think a 7" Mak would be excellent for the Moon and Planets. I have an Intes M703 that performs better on Planets than my 8" Meade SCT.

#10 Mike4242



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Posted 03 November 2012 - 06:51 PM

Between those 2 choices I would probably go with the 8se. Refractors are nice, I have 2, but I think if you only have one scope it should be a light bucket. Have you considered a dobsonian at all? $1200 can buy a very nice setup with more left for accessories.

#11 SeattleScott


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Posted 04 November 2012 - 12:41 AM

While I think refractors are cool scopes with great features, I personally wouldn't recommend getting a refractor as your only scope. They do better at complimenting other scopes. Here's the problem - a 100mm ED will do great on planets but not so great on DSO's, while a 6" achro will do well on DSO's but not so well on planets. Of course a 6" apo will do well with both but not many of us can afford one of those. So I would have to chime in with the others and recommend the 8" SCT since this is your first scope. Much better on DSO's, probably better on planets than the achromat, it is just a bit higher maintenance and won't do wide fields.

#12 drewp


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Posted 04 November 2012 - 01:44 AM

often see quoted obstruction specs way out of whack, by diameter is not the way to compare, by area is the correct way imo as there is a big difference in area obstructed by diameter vs area. i think in the case of the c8 the obstruction was shown as 31% when shown correctly by area its a hair over 6%. similarly the 120mm scope is not 86% of the c8 its 36%. factor in the obstruction might get you close to 40% this is a huge difference not a minimal one. theres almost a magnatude of difference between a c8 and c6 let alone a 4.7" acromat. i think if you go with the omni you will be wishing you had gone with the c8 in a short time. the omni was my first refractor on a gem. it was a good learning experiance but ive had a few c8`s now and theres really nothing to compare, i dont care what or how you want to stack up the numbers. the only thing that should push you to an omni 120 is price and there you would be better off with a 8" dob anyway.

#13 Ed D

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Posted 04 November 2012 - 08:02 AM

If your main interest is lunar and planetary I would strongly suggest considering a scope with a mount that tracks, which I think you have. It's nice to sit and observe planetary detail without having to bump/move the scope every minute or so. The NexStar tracks, the OMNI tracks with slow motion knobs, and you can get Orion XTg Dobs which track. At a later date you can add a motor drive to the GEM, if you so choose.

Ed D

#14 Achernar



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Posted 04 November 2012 - 08:52 AM

I think you are better off with the SCT, despite the fact it is really a jack of all trades and master of none telescope. The larger aperture will be a asset even on the moon and planets because of the much greater resolution. While the secondary mirror does lower contrast, enhanced coatings today make up for the most part it's effect upon the image. It can be used with just about any eyepieces you can get your hands on. There are focal reducers available for wide field observing with it, and the light grasp of an 8-inch will also show you thousands of galaxies, nebulae and star clusters from a dark site. You can also get a camera to couple to and focue with this telescope, even through the mounting is not made for long exposure work. You can still do some lunar and planetary work with it. The Nextstar 8-inch is lightweight, compact, sturdy and portable, if you have a small vehicle or cannot do heavy lifting due to age, injury or healt issues this telescope is a good choice for you. It is also easy to use. There will also be no color fringing or chromatic abberation that originates in the main optics. The only drawbacks is the corrector plate is prone to dewing over and it does require collimation from time to time. The first is addressed with a dew cap and heater, the second only applies to the secondary mirror, which the instructions details how to accomplish properly. The secondary mirror does not reduce resolution unless it is defective. If this is going to be your only telescope, you'll want a general purpose all around instrument that will work well on both Solar System and deep sky objects. If you are like most folks into astronomy, you'll want to check out everything. Clear skies!


#15 coopman



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Posted 04 November 2012 - 11:46 AM

Another vote for the 8SE over the 120 achro. I had some great lunar & planetary views with mine before I sold it to help pay for an apo refractor. I do miss my 8SE sometimes, but the cool-down time problems tried my patience severely.

#16 Hazel


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Posted 05 November 2012 - 10:15 AM

I wanted to thank you all for the very helpful advice -- I really appreciate it. Seems like the SCT wins! Thanks again and I look forward to being a member and contributing to this great web site!! -- Hazel

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