FAQ for those buying your first telescope

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FAQ for those buying your first telescope

Postby acc » Thu Oct 02, 2003 1:30 pm

Hi all
I have attached here a very informative article written by Kay Heem on some commonly asked questions related to a first-scope purchase.

cheers


Brief Explanation On Scope Types
This comparison is meant to help new comers (newbies) understand the different telescope types with their strengths and weaknesses so as to choose the right scope for their use. The factors considered include: cost, portability and image quality.

Bear in mind that the comments generally refer to the most commonly available brands/models for their range and there can be exceptions. Mounts also come in all sizes and prices, and the comments below are also meant to be a guide only.

You should go for viewing sessions to see and try the different types of scopes/mounts in action before deciding on your purchase.

The best scope for you is the one you will use most often.

Telescope Type
Achromat – Short refractor e.g Celestron or Skywatcher 80, 102, 120 or 150mm
• Portable in smaller apertures
• Affordable
• wide-fields
• Bad chromatic aberration (CA)
• not good for high power viewing


Achromat – Long refractor e.g. Skywatcher,
• Less portable than short achros
• Affordable
• Better for high power fields compared to above
• Needs sturdy mount
• Still has some chromatic aberration, but better than above


Apochromat (Apo) refractor e.g. Tele Vue, Vixen, Takahashi
• Excellent images
• Can push up to very high powers
• No CA or negligible CA on the brightest objects only e.g. Venus
• Most expensive for a given aperture


Reflector (Newtonian) e.g Skywatcher, Discovery
• Cheapest design for a given aperture
• No chromatic aberration (CA)
• Usually larger and not so portable (except for truss dobs – see below)
• Needs collimation for best performance.
• Short focal ratio (<=F5) reflectors are also more prone to coma

SCT e.g Celestron NexStar 8, Meade LX90
• Most compact given the long focal ratio (typically F10)
• Popular models have many accessories for astrophotography
• Good all-rounder scope, no CA
• Slightly lower contrast compared to apos and reflectors due to larger central obstruction
• Corrector lens prone to dewing
• Needs collimation for best performance


MCT e.g. Meade ETX series, Skywatcher
• Usually comes in longer focal ratio e.g. F12
• Images slightly better than SCT
• Portable, but slightly longer than SCT of similar aperture
• No CA
• Not so good for widefield
• More expensive than SCT for the same aperture
• Needs more cool-down time due to thick meniscus lens
• Corrector lens prone to dewing


Mak-Newt e.g. Intes MN61, Intes Micro MN66
• Very good images, almost on par with apos of same aperture
• Good for both wide-field and high power views
• Moderate cost
• No CA
• More expensive than reflector and SCT/MCT of same aperture
• Needs collimation for best performance

Semi-Apos fill the gap between achromats and apochromats in terms of pricing and performance.

Miscellaneous Comments

• Truss dobs are large aperture reflectors mounted on simple Alt-Az mounts. Instead of having a long solid tube, they usually have an upper cage and a lower mirror cell assembly, separated by truss poles. This enables them to be taken down to ease transport.
• Catadioptric scopes are scopes that make use of both refractive and reflective elements. Examples will be the Schmidt-Cassegrain (SCT), Maksutov-Cassegrain (MCT) and the Maksutov-Newtonian designs.
• Portability, Cost and image quality are all relative to the individual. Most of the time, you can only choose 2 out of the 3. If portability and cost are your main considerations, you may want to choose a short achromat refractor or a smaller (5”) SCT or MCT on an alt-az mount. If cost and image quality matter to you, you can choose a simple reflector or the Mak-Newt. These will give the best images for the dollar.
• If you want to do long-exposure astrophotography, the mount is the most important element in the setup and will probably end up being more expensive than your scope. Popular choices for astrophotography include apos and SCTs. Common reflectors tend to have insufficient in-travel in the focuser to accommodate the camera and may need modifications for them to be used.
• Avoid ‘Department Store’ scopes that state 900X magnification or something to that effect. Most viewing of deepsky objects (i.e. objects outside the solar system) need only 50-150X of magnification.

Scope Maintenance

Most scopes do not need regular servicing unlike a car as there are no moving parts that wear out quickly. However, some do require occasional tweaks to make it perform at its maximum potential. All scopes will require some degree of tender loving care (TLC), in that you do not treat it like a pair of army boots. Cleaning the outside of the scope is a must after a session, especially if there is dew. Use only a soft dry cloth for the exterior.

DO NOT ATTEMPT TO CLEAN THE OPTICS OF A SCOPE IF YOU ARE INEXPERIENCED!

This is especially true for mirrors. Telescope mirrors are ‘first-surface’ mirrors where the reflective coating is on the side of the incident light. This is opposed to the common bathroom mirrors that are ‘second-surface’ mirrors, where the reflective coating is at the far end of the glass.. Telescope mirrors need to be first surface mirrors as second surface mirrors will give you a ghost image from the first surface. Although most telescope mirrors have a silicon dioxide (SiO2) protective coating, they are still prone to scratches and you must resist all temptation to clean them like your bathroom mirror! There are lots of references on the internet on how to clean telescope mirrors.

Refractors, MCTs, SCTs and Mak-Newts are closed-tube scopes and there is usually no need to clean the inside. Cleaning of the lenses are more straight-forward than cleaning mirrors. Most times, all you need is a powerful manual blower and lens cleaning paper and solution. If in doubt, do consult someone more experienced as the optics are the most important part of the scope. Once damaged, it can be quite costly to service or replace. There are also good references on the internet. No window-cleaning solutions, please!

Reflectors, on the other hand, are open-tube designs and dust can and do get in. When not in use, keep the openings covered viz, the top, bottom and the focuser opening. When examined with a strong light, most mirrors in reflectors will reveal some dust. There is no need to try to remove these as they will have a negligible effect on the image.

The top reason for poor performance in reflectors and catadioptrics is mis-collimation. Collimating a scope simply means fine-tuning the position and direction of the optics to yield the best image. This actually sounds much more difficult than it really is. It can actually be fun, believe it or not! New users should not let this procedure scare them from purchasing reflectors. The different ways of collimating a scope is beyond the scope of this article and can be obtained from the internet. Once you get the concepts right, it will be a breeze.

Reflector mirrors come in normal reflectivity (averaging 88%) or enhanced reflectivity (about 94%). The latter type tend to lose the reflectivity faster than the former. However, there are many companies that can strip the old coating and recoat for you; just don’t send it to your neighbourhood mirror and frame maker for the service.

Popular models of SCTs and MCTs from Celestron and Meade employ moving primary mirrors for focusing. This means that the mirror actually moves forwards and backwards to achieve focus. This may result in slight image shift during focusing. Over time, the grease may dry up and the shift becomes more apparent. You are strongly advised to get it serviced by a trained technician instead of attempting a DIY when this happens.

In summary, refractors need the least maintenance, followed by SCTs / MCTs and reflectors.

Country of Manufacture
It used to be that the best optics come out from USA and Japan and that those from China and Taiwan should be forgotten. However, times have changed, and pretty good optics in mass produced scopes are coming from Taiwan and China. Good reviews on scopes from China and Taiwan are received regularly e.g. Orion 127mm MCT. However, there is still a lot of QC issues and you can get a great scope now and a lemon the next time. If you have the cash and do not want to play ‘Scope Roulette’, you should stick with the better known brands e.g. Tele Vue, Astro Physics, Discovery, Teleport, Portaball, Takahashi, Vixen etc; or buy a 2nd hand scope with established quality.

Mounts:

German Equatorial Mount (GEM)
• Usually heavy due to the counter-weights
• Usually can be fitted with motors +/- computers
• Needed for long-exposure astrophotography (mount must be polar-aligned first)
• Has steep learning curve for those using it for the first time


Alta-Azimuth mount

• More intuitive to use (left-right, up-down)
• Unsuitable for long-exposure astrophotography
• Higher end mounts can also be fitted with motors +/- computers
• A simple Alt-Az mount is usually fitted to large reflectors and this becomes known as a Dobsonian mount and the whole setup is known as a ‘Dob’

Computerized / Motorised Mounts

• These may be GEMs or Alt-Az mounts
• The computer helps in the location of your celestial objects
• Motors are fitted to certain mounts for automatic tracking or even auto-pointing to the object
• Newer models are equipped with GPS systems that help in the intializing of the computer.

-- There is no ‘Best Scope’, only the most suitable scope based on your budget, usage requirement and portability --

Binoculars
• Try to avoid zoom binoculars as the field of view (FOV) tend to be smaller and images are not as good as non-zoom ones.
• Most binoculars tend to be too heavy to hand-hold for long periods once they go 70mm or larger.
• Don’t look down on smaller binoculars e.g. 8X40mm as almost any optic is better than nothing!
• Go for BK4 prisms and multi-coatings where possible.

If you can only spare a couple of hundered dollars and you really need to get a scope, do consider a pair of binoculars or a 60-80mm spotting scope.
Last edited by acc on Tue Dec 30, 2003 12:33 pm, edited 2 times in total.

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