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  1. #1
    Paul Murray's Avatar
    Paul Murray Guest

    Default 100mm refractor or 250mm dobsonian for occasional use?




    (I have posted this is a couple of places to reach a wider audience, so
    apologies if you see it twice)

    I am looking for some advice on choosing the best telescope for me.

    I had a Tasco refractor as a child, but living in London there was seldom
    anything to see.
    My interest in astronomy has recently been reignited by playing with a
    camera lens adapter which acts as a very small telescope. I dug the old
    telescope out of the loft, but the mount and tripod make it very
    difficult to make any small adjustments, and the eyepieces have such small
    fields of view that it is difficult to find anything.

    I've decided to get myself a set of 7x50 or 8x40 binoculars and a telescope
    for Christmas. Following my experience with the old telescope I want
    something which can be smoothly moved around, and with the capability to
    reach a reasonably wide field of view, which I've defined rather arbitrarily
    as 2 degrees.

    I'm not particularly interested in the moon or the planets. I'm drawn more
    towards DSO and *finding* things. There is something of the pokemon about it,
    the existance of things like the Messier, Caldwell, double star and such
    lists give me something to aim for.

    Of primary importance is that it has to be easy to setup, and not require
    too much maintenance. This is going to be a dip in and out hobby for me, I'm
    looking to use the telescope reasonably often, but probably not for much
    more than an hour, maybe two, each time, as I've got to fit it in around
    other things, and be up each day at 7am to goto work!

    As far as an observation site goes, what I've got isn't great. I'm going to
    be using it in my back garden or patio. Obstructions in most directions will
    block out the lower portion of the sky, there is a reasonable amount of light
    pollution, and anything looking NE-SE will be looking across house roofs, so
    have heat-haze issues. Naked-eye visible magnitude on a good night is
    probably about 4-4.5. Realistically I'm going to spend most of my observing
    time here, and not get much of an opportunity to go to darker sites. It is
    important that is easy to transport the telescope in and out the house, but
    not that important that it is easy to travel with it.

    The budget is about 300 GBP. That should be for a useable set of equipment,
    OTA, mount and at least one eyepiece, although I'm not averse to buying
    extras down the line.

    Telescopes I have been considering at the price range are as follows: (in
    order of increasing aperture)

    Meade ETX-80AT
    http://tinyurl.com/dvvo2
    80mm f400 achromat refractor
    Alt-Az GOTO mount
    25mm(?) and 10mm(?) SP eyepiecess (approx 3deg and 1.25deg)
    No finder
    x2 Barlow included

    Celestron NexStar 102SLT
    http://www.dhinds.co.uk/pages/fullProd.php?id=463
    102mm f660 achromat refractor
    Alt-Az GOTO mount
    25mm and 9mm (approx 1.7deg and 0.5deg)
    Red dot finder

    Skywatcher Startravel 102 (EQ1) with RA motor
    http://www.sherwoods-photo.com/sky_w...atcher_fs.html
    102mm f500 achromat refractor
    GEM mount with slow-motion controls and RA motor
    25mm and 10mm (approx 2.5deg and 1deg)
    Red dot finder
    (This is about 50 quid cheaper than the others)

    Skywatcher Startravel 120 (EQ3-2)
    http://www.sherwoods-photo.com/sky_w...atcher_fs.html
    120mm f600 achromat refractor
    GEM mount with slow-motion controls
    25mm and 10mm (approx 2deg and 0.8deg)
    Red dot finder
    (To add an RA motor would be another 80 quid)

    Revelation 10 Dobsonian
    http://tinyurl.com/7zlos
    250mm f1250 reflector
    Dobsonian mount
    26mm 2" eyepiece (approx 1.3deg?)
    8x50 finder
    (Or 200mm f1200 model at 200 GBP)

    I've pretty much ruled out the Meade. I think the only reason I would choose
    that is if I were certain I wanted to get into imaging with their DSI camera.

    I'm not that bothered about having a GOTO scope, in fact I think it might be
    too much of a temptation and take some of the fun out of the *finding*
    aspect of astronomy. However I thought that having automatic tracking would
    be useful, and the price of the Celestron with GOTO is comparable to a
    similar aperture scope with RA motor, so it seems a reasonable deal.

    The last scope is obviously the odd one out.
    After my troubles with the first telescope I had pretty much decided that
    any scope must be a wide-field one with slow motion controls, and given that
    I'm not interested in the planets a short-tube achromat refractor seemed a
    good match. My instictive reaction to seeing a dobsonian is that I couldn't
    possibly point it well enough, or track anything through the sky. The added
    maintenance of collimation is also off-putting, as is the idea of dust and
    bits falling into the open tube. However it seems that everywhere I look the
    advice is that dobsonians are very suitable for beginners and are actually
    very easy to control, and it is hard to ignore the fact that I can get six
    times the light-gathering capability of the Celestron for the same price.
    The focal length is longer than I had been looking for, and hence the field
    of view narrower, but with a 2" focusser I could get something like a Meade
    4000QX 36mm and have 2deg field of view with the 8-inch model for the same
    overall price of 300 GBP, and that is still four times the light gathering.
    I've seen a lot of argument about whether large apertures are better or
    worse in poor seeing conditions, but haven't really seen a good conclusion.

    So my questions are:

    - Is an 8" or 10" dob suitable for a dabbler who want an hour here, and hour
    there?
    - How often am I likely to need to re-collimate the scope, and how long
    would that take?
    - Is it *really* possible to smoothly track objects with dobsonian mounts?
    - How much deeper, and how much extra detail would I see with the
    200mm/250mm reflector over the 100/120mm refractor.
    - Is the larger aperture going to be suitable for my viewing conditions?
    - Am I going to make the right decision? (Ok, that one isn't fair

    Any help gratefully accepted.
    Paul

  2. #2
    Chris L Peterson's Avatar
    Chris L Peterson Guest

    Default 100mm refractor or 250mm dobsonian for occasional use?

    On Tue, 22 Nov 2005 19:07:44 GMT, Paul Murray <paul@murray.net> wrote:


    Very much so.


    Figure on fairly often. It usually takes me about a minute to collimate
    my Dob, assuming just the typical touch up. It can take a few minutes if
    things are really out of whack (not common), or the first few times you
    do it. Don't worry about collimation- it is very simple with a
    Newtonian.


    Yes, except near the zenith. It depends on the quality of the mount, but
    even lesser ones can be pretty easily tuned up with a handful of cheap
    hardware store kit.


    I can't quantify that, but it will be significant. Given your primary
    interest in DSOs, you want to look at getting as much aperture as you
    can afford, and as you are physically comfortable handling. Fortunately,
    even rather large Dobs are easily managed.


    Yes. When you are dealing with light pollution, more aperture is always
    good.


    You already have: you've decided to get a telescope (and to ask some
    intelligent questions first) <g>.

    _________________________________________________

    Chris L Peterson
    Cloudbait Observatory
    http://www.cloudbait.com

  3. #3
    Nick Theodorakis's Avatar
    Nick Theodorakis Guest

    Default 100mm refractor or 250mm dobsonian for occasional use?

    On Tue, 22 Nov 2005 19:07:44 GMT, Paul Murray <paul@murray.net> wrote:

    [...]


    In that case, more aperture is better.

    [...]


    Sure. I have a 10 in Dob, and use it in that manner.

    [...]

    Nick
    I
    --
    Nick Theodorakis
    nick_theodorakis@hotmail.com
    contact form:
    http://theodorakis.net/contact.html

  4. #4
    Arnold's Avatar
    Arnold Guest

    Default 100mm refractor or 250mm dobsonian for occasional use?

    Paul Murray wrote:


    Yes, get binoculars, you won't be sorry if their quality is good. I
    bought a pair of Pentax's 8x40 PCF-WP and it is a joy to use them for
    quick scanning views of the skies.


    If you stay in the city, few galaxies will be visible. To really get to
    see more of them well, you need to get out of town.

    You will probably end up watching open & globular clusters and double
    stars from your home and don't write the planets and the moon off just yet.

    As for a 10" - the tube and mount are rather bulky and an effort to
    carry out and set up. I own a Meade Starfinder Dob. The extra
    collecting area is definitely worthwhile but be aware of it being
    cumbersome. Sometimes the amount of effort required to set everything
    up discourages one to view for short periods of time.

    I personally think you would get to use a 100mm or bigger refractor more
    often with less hassle. It offers you the wide fields, easy setup, no
    collimation, closed tube, short cooldown time and easy pointing. The
    question is alt-az or equitorial mount with tracking or not?

    Consider Orion scopes too.

    Careful, starlight is addictive!

    Arnold

  5. #5
    Arnold's Avatar
    Arnold Guest

    Default 100mm refractor or 250mm dobsonian for occasional use? - repost

    Paul Murray wrote:

    telescope
    arbitrarily

    Yes, get binoculars, you won't be sorry if their quality is good. I
    bought a pair of Pentax's 8x40 PCF-WP and it is a joy to use them for
    quick scanning views of the skies.

    more
    about it,

    If you stay in the city, few galaxies will be visible. To really get to
    see more of them well, you need to get out of town.

    You will probably end up watching open & globular clusters and double
    stars from your home and don't write the planets and the moon off just yet.

    As for a 10" - the tube and mount are rather bulky and an effort to
    carry out and set up. I own a Meade Starfinder Dob. The extra
    collecting area is definitely worthwhile but be aware of it being
    cumbersome. Sometimes the amount of effort required to set everything
    up discourages one to view for short periods of time.

    I personally think you would get to use a 100mm or bigger refractor more
    often with less hassle. It offers you the wide fields, easy setup, no
    collimation, closed tube, short cooldown time and easy pointing. The
    question is alt-az or equitorial mount with tracking or not?

    Consider Orion scopes too.

    Careful, starlight is addictive!

    Arnold

  6. #6
    Stephen Paul's Avatar
    Stephen Paul Guest

    Default 100mm refractor or 250mm dobsonian for occasional use?


    "Paul Murray" <paul@murray.net> wrote in message
    news:30Kgf.50209$Ys5.39036@fe12.news.easynews.com. ..


    Just some rambling thoughts out of my experiences....

    Realistically, under those sky conditions, galaxies are pretty much out of
    the question. The more ineteresting objects will be globular clusters,
    galactic clusters, double stars, planets, and the moon. For any of those
    (that is for all) objects, more aperture is better.

    The 250mm Dobsonian will definitely give the brightest images, draw in the
    dimmer stars, and provide the best resolution. However, a 250mm Newtonian
    OTA is not exactly "easy" to haul out to the yard (I know, I had one). I
    ended up with a small, two wheeled hand truck to roll the scope out to the
    yard in one piece.

    A 200mm Newtonian OTA is a lot easier to carry around, but even so, you will
    likely find yourself making two trips (at least), one for the base, the
    other for the OTA (and then there's the chair, the small table for the
    charts, and an eyepiece/accessory case). Although it is possible that if you
    put a carry handle of some sort on the OTA, that you might be able to carry
    the base in one hand, and the OTA in the other, cutting down on the trips.

    The 200mm Newtonian OTA will also cool down faster than the 250mm. Don't
    discount the significance of this issue for a casual week night observer.

    Of course ideally you would get a small, secure shed in which to store a
    larger Dobsonian (with cover) in your garden/patio. If you can manage that,
    then even a 300mm Dobsonian would be very nice and manageable. Being stored
    outside in the shed, it would require little to no cool down time, and the
    setup time would be as long as it takes to roll it out of the shed (get a
    hand truck), and to setup your table and chair.

    -Stephen




  7. #7
    g626700-groups@yahoo.com's Avatar
    g626700-groups@yahoo.com Guest

    Default 100mm refractor or 250mm dobsonian for occasional use?

    You can also attach a fan to the mirror cell, to cut cooldown to 20
    minutes or so. Dobs set up the fastest by far, and aperature rules for
    deep sky. They don't track, but at low magnifications, that's not an
    issue at all.

    I'd get the largest dob that you can easily take outside, transport by
    car to a dark site, and afford leaving funds for some premium
    eyepieces. I use a 200mm. I can set it up in about a minute, and it
    easily fits in the trunk of a midsized car. If you really won't travel
    to dark sites, leave some budget for a light polution filter. They're
    not cheap.

    I don't care for achromats, and a large apochromat could cost as much
    as a cottage in Wales!

    Greg


 

 

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