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Thread: DOBSONIAN vs SCT

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    Default DOBSONIAN vs SCT



    Thanks for adding me to the forum! I live in a suburb of Indianapolis, so it considered light polluted (transition from urban to suburban on the Bortle scale). I am interested in viewing deep sky objects from my backyard which is well shielded from street and yard lights.

    I purchased an 8" SCT last week (used Meade LX200 GPS), but I am finding the views are quite limited (the FOV is small as it takes only 1.25" diameter eye-pieces). So I am thing of getting a 12" Dobsonian which has a 2" eyepiece barrel. Will my viewing be dramatically better especially when seeing deep-sky objects?

    Last Saturday when it was new moon night, I went to a medium dark sky site (Bortle Class 5) nearby where there was public star party. They had a big 28" DOBS and I had an opportunity to look through it. It was good but not fantastic, and a little less than what I expected.

    So I am not sure if a 12" Dobs would do much for me compared to the 8" SCT I have. I am still a beginner with the SCT and I have a long ways to go to use it to its full potential, but I am also learning its short comings (awkward viewing position, hard to navigate the skies manually, small FOV). Hence the desire to try a Dobs.

    Any advice you can give me is greatly appreciated!

    Thank you!

    Raghu

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    Default

    If you have the funds to acquire a 12" dob, go for it. Twelve inches is always better than eight, but unless you're going under dark skies and dropping high end eyepieces in it, you probably won't get a whole heck of a lot more out of the dob than a wider FOV. I have essentially the same scope as you, but a Celestron and the FOV is adequate with the proper eyepieces. On that note, SCTs most certainly CAN be fitted with 2" equipment. All you need is a 2" diagonal and the appropriate equipment for mounting it on the barrel. There are a few ways to go about this. You can purchase a 2" diagonal intended for a refractor scope and a 2" visual back adapter that accepts it, or for a but pricier but cleaner method, you can get an SCT specific diagonal that has the thread for the back of your scope already on the diagonal. OR you can get an SCT specific crayford focuser to act as the 2" visual back adapter and pair it with a refractor diagonal. This upgrade will solve your 2" compatibility issue as well as give you a more precise focusing element VS the standard SCT dial. This has the effect of reducing the image shift inherent to stock SCT focusers as well as giving you a much more incremental focuser to fine tune your viewing. I would highly recommend this method for going 2" since upgrading your stock SCT focuser dial itself is a good bit more expensive than just getting a dual speed crayford specific for SCT use. It kills two birds with one stone.

    This is a good 2" dielectric diagonal with the refractor barrel. This is what you'd need if going the crayford focuser route.
    https://www.bhphotovideo.com/c/produ...BI%3A514&smp=y

    Being that you have a fork mounted SCT, if you choose to go with the crayford method, you'll need a short travel focuser. This one is one of the two shortest on the market. https://skygazeoptics.com/products/j...4aAuoIEALw_wcB

    If you'd rather not upgrade your focuser and just want a self contained 2" diagonal, this should suit your needs. Just keep in mind, upgrading the SCT focuser knob itself is a good bit more costly than the above method. https://www.highpointscientific.com/...MaAjeoEALw_wcB

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    Last edited by Bataleon; 08-04-2019 at 11:55 PM.
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    Default Re: DOBSONIAN vs SCT

    The only problem with going to 2" on the 8" SCT is the rear port has a baffle tube that is only 1.5", so you only get an extra .25" from it.

    You can significantly widen your view with the SCT if you get a focal reducer for it bringing the effective focal length from 2000mm to a much wider 1260mm which ends up being shorter than the 1500mm of the 12" Dob.

    I like my 12" Dob, but it is a HOSS and I really need to be determined to hunting down those faintest of targets to have to deal with the sheer bulk of the scope. My 8" SCT (or even the 8" Dob) is much easier to lug around.

    Totally off topic, but do you have a decent pair of binoculars? Don't disregard these for lots of fun and convenient DSO observing!

    A variable height comfortable observing chair is a MUST HAVE!!

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    Quote Originally Posted by Ozman13 View Post
    The only problem with going to 2" on the 8" SCT is the rear port has a baffle tube that is only 1.5", so you only get an extra .25" from it.

    You can significantly widen your view with the SCT if you get a focal reducer for it bringing the effective focal length from 2000mm to a much wider 1260mm which ends up being shorter than the 1500mm of the 12" Dob.

    I like my 12" Dob, but it is a HOSS and I really need to be determined to hunting down those faintest of targets to have to deal with the sheer bulk of the scope. My 8" SCT (or even the 8" Dob) is much easier to lug around.

    Totally off topic, but do you have a decent pair of binoculars? Don't disregard these for lots of fun and convenient DSO observing!
    Good call on the focal reducer, hadn't thought of that. I've been so spoiled with the wide angle in my Meade 5k that I haven't given FOV a second thought lol. Back on the dob question though, it certainly would be nice to have such a beast of a scope, but as you said, I would weigh the practicality of deploying such a contraption VS just slapping a few optical modifications on the existing kit.

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    Default Re: DOBSONIAN vs SCT

    I use a 0.5x screw-in focal reducer and it works really well.
    Equipmentally challenged.

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    Default Re: DOBSONIAN vs SCT

    Quote Originally Posted by Clouded Out View Post
    I use a 0.5x screw-in focal reducer and it works really well.
    It's kind of a toss up on this one. Some people get good results and some do not. I've tried three of them and wasn't pleased with any of them. Just last week, a newer member here was having the same bad results I got. And then there are several members here that use one frequently and report very pleasing results, sometimes even with imaging!

    Couldn't hurt to try one with any particular scope as they are so affordable and if it works well in that situation, certainly worth it.
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    Default Re: DOBSONIAN vs SCT

    A lot of it depends on one's motivation and expectations. I've found the increase in field of view is more like 1.8 times than 2. Also, on fast 'scopes, the exit pupil becomes very large and brightness is lost, rather than gained.

    My original use was afocal shots of deep sky objects! Yes, really and I continued with the method until I bought my first DSLR.
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    Default Re: DOBSONIAN vs SCT

    If you are not impressed with a 28" Dob then I recommend you sell your equipment and try online telescopes or view at professional observatories. I have a 12" Dob and it performs well enough on clear nights and dark sites considering it cost me only $630 but if your standards are higher, forget about it.
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    Default Re: DOBSONIAN vs SCT

    Thanks Bataleon for taking the time to respond with very good suggestions.
    I had looked into upgrading to the 2" SCT Diagonal before, but I was told that the LX200 8" SCT has only a 1.5" tube / bore so this upgrade will only give extra 0.25". So probably not worth doing the upgrade I think.
    SunRag

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    Default Re: DOBSONIAN vs SCT

    Interesting idea with the Focal Reducer! I should get one.
    So you think 12" DOB would be impractical to carry around even if it only from the garage to the backyard?
    I have an old binocular which is 7x35, but easy to handle. I do like looking through a binocular as it gives me the
    "space walk" feel :-).

 

 
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