Re: [-BAS-] Choosing a beginner's telescope
Akarsh Simha
> [1]http://www.tejraj.com/kspace-7.html
>
> I am looking for a telescope which will help me start off exploring the
> night sky. I request you to please tell me if this is worth the money? I
> want to buy one ASAP, and get started with the exploration.
The particular telescope you have posted is a refracting type 2.4"
f/15 (900mm focal length) telescope on an equatorial mount. I notice
that the equatorial mount has slow motion controls (which are used to
move the telescope with precision on both axes). The advantage of this
telescope is you can:
1. View planets at good magnifications (the 900mm focal length gives
fairly good magnification - 100x on a 9mm eyepiece and 200x on a 4.5mm
eyepiece)
2. View bright objects from the city, pretty good number of objects
from dark sites.
3. Track objects as the earth rotates. With an equatorial mount, you
can track objects as the earth rotates by moving the telescope on only
one axis (called the RA axis or polar axis) and be sure that the view
field will not change.
4. Do piggy-back astrophotography. This means that you can mount a
camera on your telescope and track the sky through the telescope, so
that the stars do not trail on the photograph. [Google on this]. This
is what we were trying to do with the "other" telescope when you came
to Hosahalli.
5. Mount the refractor on a motorized equatorial mount and take
good-quality photographs of bright objects.
However, this is probably not what you want. You probably want a
telescope to do observations of planets and deep sky objects,
both. Here's why the telescope might not be useful to you:
1. This telescope is equivalent to a typical binocular (10x35 ~
10x50), but offers much higher magnification. While this might be good
for viewing open clusters and planets, it might not be good for
nebulae and galaxies etc.
2. The light gathering power of the refractor is low, because it has
only an aperture of 60mm. It only has high power (magnification). This
almost means that you can observe only planets with this telescope -
but you'll soon get tired of viewing planets :D
3. Equatorial mounts are very unconvenient for viewing, as you might
have realised during the observing session at Hosahalli. The eyepiece
can go to very awkward positions and you may have to stand in weird
postures to view through it. (I was moving the polar axis to avoid
this). Besides, there are counterweights which you will have to carry
around.
4. Eyepieces for this refractor are 1" barrel. The standard barrel
sizes are 1.25" and 2". This means that most eyepieces will not work
on this telescope. This eliminates the possibility of borrowing a
better eyepiece from someone at a starparty, or trying out one of
their filters.
I instead recommend these products for your interests, assuming that
you will not be as interested in piggy-back astrophotography as you
will be in observing:
http://www.tejraj.com/d100.html
http://www.tejraj.com/d125.html
Try to go in for the second one if possible, as it catches 50% more
light than the first one.
The advantages of these telescopes are:
1. They are very portable and light-weight. My friend used to carry
the first telescope with one hand, fully assembled to his terrace!
2. The first one (for Rs. 6.6 K or so) has a light gathering power of
2.75 times of the Konuspace-7 telescope that you linked us to.
3. Altazimuth mounts are easier to use.
4. Unlike the refractor, this comes with a standard 1.25" barrel
focusser, which means you can try out better eyepieces or try out
standard filters.
Disadvantages:
1. Image quality will not be as good as refractors. So objects may not
appear very clearly at high magnifications. Tejraj's single-lens (?)
barlow is not of good quality, so you might require better eyepieces
to achieve clarity at high magnification.
2. Alt-azimuth mount. Tracking will require you to move the telescope
on two axes.
3. No slow-motion controls. This might make the telescope difficult to
use at high magnifications.
4. Eyepiece lenses provided are not achromatic, and will suffer from
chromatic abberation.
Why I recommend the latter is because I feel that investing in big
aperture is better. I might be wrong here, because achromatic
refractors like the one you linked us to offer far better image
quality than reflecting telescopes. But the lack of image quality in
reflectors is mostly at the eyepiece (The eyepieces supplied with it
are very low grade, but ok for beginners) if the mirror is made with
care. Eyepieces are something you can buy anytime and change freely,
so you should probably try to boost the performance of your primary
optics. In this case, I feel that a 2.4" aperture is too low for a
long-term investment. (You'll soon get tired of viewing planets!)
Instead, you should probably compromise on the eyepiece quality,
because you could always buy excellent eyepieces later. This is if you
want to do observing.
If you want to do astrophotography, the refractor is probably a better
choice than the reflector, because it gives you an equatorial mount,
but it is still not a good choice. You can start taking piggy-back
photographs with it. But be warned that astrophotography is a
particularly expensive affair - you will need a fully manual camera,
mostly an SLR for piggy-back astrophotography itself. You can easily
get tired of piggyback astrophotography, and might want to move on to
prime-focus photography. At this point, you will have to buy a precise
motorized equatorial mount (which is really really expensive). Even
then, an achromatic refractor designed for a beginner will not provide
an image quality good-enough for prime focus photography. If you are
interested in photography, you should talk to Hemant (+91-98452-97169)
I hope this helped. I highly recommend that you take your time
investigating other possibilities and learn more about telescopes
before jumping to buy one, as Naveen said. I think it would help if
you went through each term in the specification of a telescope and
understand what that means.
For instance, it really helps to be able to understand answers to the
following questions:
1. How does aperture affect the capability of a telescope? When should
I go in for high aperture? (The answer is always! :D)
2. What is the advantage of a refractor over a reflector? Why should I
go in for a reflector at all?
3. What is an equatorial mount? What is an alt-azimuth mount? When
should I buy either?
4. How does focal length of a telescope affect its capability? When
should I go for long focal length, and when should I go in for short
focal length?
5. Should I buy a 'GOTO' telescope?
6. What are the kinds of eyepieces available? How does eyepiece focal
length affect viewing? What are the various eyepiece designs
available? Which one is better for me?
You might find something useful here:
Although I amn't sure.
HTH
Regards
Akarsh
Akarsh Simha
I forgot to add one thing. This is probably an important guideline
that will help select your telescope.
Telescopes advertised in terms of (magnifying) power (like 20x or
200x) should be critically reviewed. Dealers usually advertise in
terms of power to "lure" customers, especially when their aperture is
small, although this is not always true! More important than
magnifying power is light-gathering power (in most cases) and you
should look at that more critically, always. The real power is not the
power.
This is because you can increase the power of a telescope at will, by
replacing the eyepiece, but you can't increase the aperture at will!
Besides, it need not be true that the image is still clear at the
highest advertised power. For instance, Tejraj's 4" (that I linked you
to) advertises a power of 333x. While it is true that the highest
power obtainable with the eyepieces he supplies is 333x, this is
bogus, because the image quality is pathetic at this power.
Regards
Akarsh