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MAS Frequently Asked Questions


MAS Frequently Asked Questions

Frequently Asked Questions about astronomy (see also MAS Beginners page)

This page answers many commonly asked questions about astronomy. If you have a question on a subject any not covered here, please help support this page by using our on-line enquiry form (requires Java Script to be enabled).

Much of the material here has been taken from topics presented at out monthly meetings. If any member has subject they would like to present for 15-20 miniutes as a 'second half' topic, please contact the Meetings Secretary (to contribute directly to this page, please contact the Webmaster

(+) 0001 How do I find a local Astronomical Society ?

(+) 0002 How is Star brightness measured ?

(+) 0003 What are the Constellations ?

(+) 0004 What is the shape of the Earth ?

(+) 0005 What equipment do I need to start astronomy ?

(+) 0007 What telescope should I buy ?

(+) 0008 How much Magnification can I get ?

(+) 0013 What are Equatorial and Alt Az mounts ?

(+) 0100 How do I use my first telescope ?

(+) 0109 What is a finder ?

(+) 0110 What is an eyepiece ?

(-) 0111 What is a Barlow ?

A Barlow is an adapter that is fitted between the telescope focus assembly and the eyepiece (i.e. it 'stacks' in front of the eyepiece). They contain a set of additional lenses which increases the focal length of the telescope, thus increasing the magnification achieved (with that eyepiece) at the expense of reducing the field of view (and thus the brightness of everything you see). It should be noted that the older single lens Barlow designs often added noticeable darkening and distortion at the edges of the field of view. The most popular Barlow lens is the 'x2' (which doubles the focal length of the telescope). Other types are the x2.5, x3 and (really only usable with web-cams for planetary imaging) the x5 (see below). Note that a Barlow often needs extra focus distance from your telescope. Whilst refractors have no problems with this, some Maksutov-Cassegrains and Maksutov-Newtonians have limited focus distance meaning you may not be able to reach focus with all Barlow / eyepiece combinations. One interesting type of Barlow was the 'Meade Series 4000 #127 2x-3x Variable' ($50), which allowed the user to vary the magnification between x2 and x3 by moving the lens along a sliding tube (with 'stops' at each x0.25 intervals). It was apparently not a success, as Meade has now discontinued it (perhaps because it didn't work with the Meade ETX-70/90/105/125 telescopes or any other telescope 'faster' than f/6). The 'nearest equivalent' is the Astro Engineering AC730i 1.5/2x/2.3x (£55, at scopesnskies), which achieves the 3 different magnifications using two screw-in extension tubes. Another way to change the magnification of your Barlow, if your telescope is a Schmidt-Cassegrain or a refractor (or any telescope that can be sucessfully used with a right-angle adapter, or 'star diagonal' with some 'spare' focus distance), is to place the Barlow in front of the diagonal. This adds an extra x1 to the magnification, so, for example, placing a x2 Barlow in front of the star diagonal gets you x3. Finally, when imaging using a Barlow in 'prime focus' mode (i.e. with no other lens between the Barlow and the camera sensor chip), it is possible to vary (increase) the magnification by moving the camera sensor further away from the normal focus point (at the 'top' of the Barlow). For example, with the Televue Powermates, adding a 10mm spacer / extension tube gets you at least another 50% = so a Televue x2 becomes a x3 (a x3 becomes x5 ... and a x5 is converted into a x7.7 !), although, once again your telescope focus assembly must have some extra travel. It should be noted that, the more you push a Barlow beyond it's magnification design, the more 'vignetting' (distortion) you are likely to see at the edges of the field of view. However, the center of the field of view will be fine, which is all you need for web-cam style imaging. The Barlow lens is named after Peter Barlow, mathematician and physicist (and not some character in a well known TV 'soap' series :-) ) Advice: Short focal length eyepieces (10mm and below) often have very small exit lens diameters (although modern 'planetary' designs are much better). As a result, you will often find it easier to see using a 20mm eyepiece with a x2 Barlow (so 10mm effective), rather than a small diameter 10mm eyepiece - and a 10mm with x2 Barlow will almost certainly always be better than a 5mm ! Note that whilst a x2 Barlow with a 10mm eyepiece gives you 5mm effective (so doubles your mag), unless you have a very stable mount and good 'tracking', this may well be 'too high a magnification' to actually get a stable image (it is, however, necessary to see things like the surface details on Mars) ! Don't forget that the magnification is the focal length of the telescope divided by the eyepiece .. and at x100 magnification and above, mount stability and tracking accuracy become more important x5 Barlows The x5 Barlow is only really usable for planetary imaging with a 'web cam' style of camera, where 'ultra-high' magnifications are required to see surface detail and where the tiny sensor chips only need the very center of the field of view (and where the atmospherically distorted 'frames' can be dropped by your stacking software) Even so, the maximum magnification should be limited to about 250x, since beyond this atmospheric turbulence (which is also magnified 5x) will typically make it impossible to get any decent image frames at all. Refractors and straight Newtonians with typical focal lengths in the order of 1000mm should thus be limited to 4mm effective at the eyepiece (i.e. a 20mm eyepiece with the x5 Barlow in 'eyepiece projection' mode). Note that in the atmospherically turbulent UK skies, the 'rule of thumb' for max. achievable mag. is 'x30 per objective inch', so, if you want decent images at 250 mag. you need to be using a telescope with a primary mirror of 8" or bigger. Visual observing with a x5 Barlow should be restricted to short focal length telescopes of less than 600mm or so (since they are the only ones that will gather sufficient light), and even then only with Apochromatic refractors and the larger reflectors. The 'gold standard' in x5 Barlows is the Televue 'Powermate x5' ($200 / £157(at Telescopehouse))

The Orion 8715 High-Power 1.25-Inch 5x 4-Element ($170) is another well specified unit, and possibly also worth considering is the 'Meade 5x Telextender' ($160 - now discontinued, but sometimes found on eBay) or Bresser SA-Barlow 5x (£145), however the very well specified 'Astro Engineering Supreme 5x photovisual (£90)' is said to perform rather poorly - a Televue 3x ($150) combined with an extension tube (giving 5x effective) is said to perform much better than the AE (and perhaps even has the edge over the Meade)
Somewhat cheaper is the 'Explore Scientific 5x' ($115), and, for planetary astro-photographers on a budget, other x5 Barlows are the 'Astro-Tech AT5XB 5X apochromatic ED triplet' ($50) and the 'Revelation Astro 5x' ($50 / £35), plus it's 'Made in Taiwan' equivalent, the 'GSO 1.25" 5x Apochromatic' ($55). Note that a search on eBay often turns up well-known brands 'direct from China' at less than half the normal price. However whilst these may be made in exactly the same factories as the 'real thing', it's not unusual to hear stories of parts 'salvaged' from scrap bins (or made by un-skilled workers during some 'phantom' night shift), so check the sellers 'feedback' very carefully before buying ! One final note for DSLR astro-photographers. For 'prime focus' imaging, ideally your Barlow needs to come with a standard 'T2 mount thread' at the 'top end' allowing you to screw the Barlow directly into your camera adapter (rather than relying on the Barlows thumbscrew 'clamp', which is designed to hold eyepieces weighing ounces and not DSLR's weighing kilos, and will dump your expensive DSLR body onto the ground at the first opportunity). On some designs (such as Televue), the T thread adapter is an 'extra', adding $50 to the cost.
This note last modified: 11th Jun 2019 10:58.


(+) 0112 What is a focal reducer ?

(+) 0114 What are Nebular filters ?

(+) 0116 How do I use Setting Circles ?

(+) 0120 What is collimation ?

(+) 0125 How can I safely observe the Sun ?

(+) 1030 How can I take photos of the stars ?

(+) 1033 How to take photos of the Aurora ? - (Northern Lights)

(+) 1035 How to calculate FOV for prime focus ?

(+) 1036 How do I calculate FOV for Eyepiece projection ?

(+) 1037 How do I use a Raspberry Pi camera for astrophotography ?

(+) 1038 What is Star trailing ?

(+) 1039 How can I take photos of Meteors ?

(+) 2100 What is Universal Time (UTC) ?

(+) 2114 What are AUs Parsecs and Light Years ?

(+) 2115 What is Bodes Law ?

(+) 3010 When was Neptune discovered ?

(+) 4000 How do I update Stellarium with new Comet data ?

(+) 5000 How To build the MAS (Raspberry Pi) photoframe ?