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 a

- 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

On Tue, 22 Nov 2005 19:07:44 GMT, Paul Murray wrote:

So my questions a

- Is an 8" or 10" dob suitable for a dabbler who want an hour here, and hour
there?

Very much so.

- How often am I likely to need to re-collimate the scope, and how long
would that take?

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.

- Is it *really* possible to smoothly track objects with dobsonian mounts?

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.

- How much deeper, and how much extra detail would I see with the
200mm/250mm reflector over the 100/120mm refractor.

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.

- Is the larger aperture going to be suitable for my viewing conditions?

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

- Am I going to make the right decision? (Ok, that one isn't fair

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

On Tue, 22 Nov 2005 19:07:44 GMT, Paul Murray wrote:


(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'm not particularly interested in the moon or the planets. I'm drawn more
towards DSO and *finding* things.

In that case, more aperture is better.

[...]

So my questions a

- Is an 8" or 10" dob suitable for a dabbler who want an hour here, and hour
there?

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

[...]

Nick
I
--
Nick Theodorakis

contact form:
http://theodorakis.net/contact.html

Paul Murray wrote:

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.

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.

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.

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

Paul Murray wrote:

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.

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.

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.

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

"Paul Murray" wrote in message
. ..


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.

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

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