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Thread: Cassegrain vs Newtonian

  1. #11
    Alan Sheppard's Avatar
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    Default Re: Cassegrain vs Newtonian



    Yuri,

    The SCTs with their large central obstruction are not the best for the planets because the diminished contrast.

    The refractor is the best I believe.

    Between Newtonian and SCT the Newt would win out due to the smaller central obstruction, in my opinion.

    I have not researched this recently so I could be off base!

    Alan

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    Default Re: Cassegrain vs Newtonian

    For any type of observing, including planets.... SCT, Newt or Refractor of equal dollar value I'll take the Newt any day, no question.

    Dave

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    Default Re: Cassegrain vs Newtonian

    SCTs have one nice feature.. the eyepeice is usually in a good spot nomatter what you look at.
    When I put my Schmidt Newontian (just a newt with a corrector plate instead of spider to hold secondary) on the mount.. sometimes the EP ends up in an un usable location and when I try to gently rotate it, I ruined alignment some times.

    After that depends on what you want to see.....

    I have spent nights on the 760mm Newt with a 40mm EP just enjoying wide star feilds...
    but my 9.25" SCT is king looking at planets and smaller objects.
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  4. #14
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    Default Re: Cassegrain vs Newtonian

    From my experience an 8 inch Schmidt Cassegrain performs on lunar & planetary targets about on par with a 6 inch Newtonian or a 5 inch refractor. A good rule of thumb as far as telescope contrast performance is concerned is to subtract the diameter of the secondary from the aperture diameter to arrive at the contrast performance aperture. An 8 inch SCT with a 3 inch secondary is about equivalent to a 5 inch unobstructed telescope as far as planetary contrast performance is concerned (8-3=5). A 6 inch Newtonian with a 1 inch secondary would be about the same (6-1=5). Of course all of this assumes equal optical quality. In the end however, there is no substitute for aperture assuming reasonable optical quality.
    olddogg60 likes this.

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    Default Re: Cassegrain vs Newtonian

    I have owned them all and I think the best way to answer this question is as follows: A decent reflector would usually out perform an SCT of equal size. I have yet to see an sct give as good of images as a decent reflector. The SCTs don't seem to give image quality that a newt does. I have had Orion, criterion, home made, Discovery Dobs, Meade and Celestron and not one failed to give cleaner images than any sct of the same size. The inherant complexity of the SCT robs them of the ability to produce the clean images a DECENT newt produces. I had 4 inch ED and apo refractors made by Orion, TeleVue and Takahashi that easily produced better and clearer images that 8 inch SCTs, but certainly the SCT was better on fuzzies, faint stars and galaxies. The decent 8 inch newt always out performed these high end refractors in all areas though! I would take a decent 6 inch reflector over a decent 4 inch refractor or decent 6 inch sct as well. I have searched for 58 years and have come to the conclusion after all those years and some 100 telescopes that a decent newtonian is very difficult to beat by by equal size decent SCTs or decent refractors a couple inches smaller. I will stick with a newtonian dob, no chromatic abberation, great color correction and priced below all the others inch for inch.

  6. #16
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    Default Re: Cassegrain vs Newtonian

    Even though I mostly use my GoTo 8" SCT, I do so because I want to maximise the number of objects I see in a short space of time and I use it for imaging (however I am an infant at imaging). My 8" Dob is a wonderful scope and the quality of what I see with it is better than my SCT as it is f/5. For pure visual use, an 8" Dob is great value for money. It also means you need to learn the sky/constellations which I see as a good thing. The Dob is portable and versatile. They aren't expensive in comparison and make a great starting point. Never underestimate the value of a decent pair of binoculars too. Maybe get both? Good for those nights when the clouds might roll in. A Dob is pretty fast to set up and pull down though.
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    Default

    From the scopes I've used over the years, I think a Newtonian has a slight edge over an SCT as far as image quality goes - but the SCT has other advantages - much more compact, a bigger focus range (handy for imaging, not all that relevant for visual unless you want to try a binoviewer), and no diffraction spikes. The slower f ratio also makes it easier on eyepieces, and the longer focal length is handy for high magnifications.

    But for visual use, a fork mounted alt-az goto SCT is about as good as it gets for convenience - the eyepiece stays in a convenient position and at the same angle, and doesn't move around much. This is hugely more convenient than the contortions an EQ mount can involve - and a far smaller height change than you get with a dob. the goto is handy if you're somewhere with a lot of light pollution, but the tracking (which you'd also get with a motorised EQ mount) really makes for more relaxed viewing (especially at high magnifications). You get to spend all your time observing, instead of alternating between observing and adjusting; you can change eyepieces, take a break, or switch observers without worrying about losing your target.

    (Not to say that you can't track things manually - you can, I have (and do, with the mount in my avatar pic), and you soon get used to it - and it's nice not to be at the mercy of electronics and batteries - but a tracking mount is certainly handy).

    At a given aperture, I'd prefer the views through a nice APO refractor - I'm a refractor fan, and it's hard to beat the sharpness and contrast - pinpoint diamond stars on a black velvet background - but given the expense and limited maximum affordable/portable aperture range, if you want a large aperture you're basically limited to something mirror based.

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    Default Re: Cassegrain vs Newtonian

    Get thee to a local star party and see for yourself. I am trying to decide between a 12" newtonian with a dob mount or an SCT 9.25" and after two summers of volunteering at a local star party and viewing through both types, I found I got the most visually pleasing views through the dobs. It was very close and much depended in how well the scopes were collimated and what we were viewing, but the dobs had the edge for reasons most others have posted.

    Is it the one scope for you? Depends on other factors like portablity, observing location etc. For me, I may go SCT as most of my observing will be from a deck or on uneven rocky ground, so having the optics up higher to avoid the deck railings, and being able to have a tripod for uneven ground pretty well makes the decision for me. Add my aging back to the equation and not wanting to be hunched over or kneeling to look through the dob when observing near the horizon and it's an SCT for me. But visually, I prefer a Newtonian, and seem to agree with members of the public at our star parties who tell me the view through my smaller Astroscan of Saturn 4" f/L of F4 is better than the 11" SCT.
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    Default Re: Cassegrain vs Newtonian

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    Default Re: Cassegrain vs Newtonian

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