I have compiled a list of common questions which arise from time to time in amongst amateur astronomers and may be of assistance to beginners also. I will continue to add more questions and answers along the way.

Astronomy magazines and books publish pictures of brightly coloured nebulae and galaxies. Will I see these in my telescope?

I received an email saying that this August, Mars will be as big as a Full Moon. Is this true?

What is the difference between Pyrex mirrors and BK7 mirrors?

What are "department store" telescopes? 

How do I know which store is a good one to buy a telescope from?

Astronomy magazines and books publish pictures of brightly coloured nebulae and galaxies. Will I see these in my telescope?

 

No. You will not see colours depicted in photos published in Astronomy books and magazines. These images are taken with digital cameras with long exposure photography and sometimes special light filters attached. Sure, you can so this sort of photography yourself but it does cost a bit of money to set up and learn.

 

There are some exceptions to this rule such as the planets within our own solar system, and some distant stars. The brighter planets will display different hues. Mars for example, will be a pale red to the naked eye and through a telescope or a pair of binoculars. Some stars will appear to have different colours, such as red and pale blues.

 

Here are two examples of comparisons between what is often published in magazines and books, and what you will actually see in a telescope under good seeing conditions and dark night skies.Special thanks to Ken James for permission to publish these images.

 

I received an email saying that this August, Mars will be as big as a Full Moon. Is this true?

No! It is a hoax and email spam. In fact this email has been circulating the internet regularly for the last few years. You can safely ignore and delete it, and don't propagate it by forwarding it onto people. This image to the right is what is included in the email. Unfortunately this image was taken by a NASA spacecraft approaching the Red planet.

To the naked eye, Mars is about as bright as one of the brighter stars in the sky, albeit it may have a little red hue to it. More information can be found at:

http://www.iceinspace.com.au/63-537-0-0-1-0.html

1. What is the difference between Pyrex mirrors and BK7 mirrors?

These days, primary mirrors for Newtonian reflectors are available to amateur astronomers made from two materials, either Pyrex or BK-7 glass.

Construction

BK7 is one of the types of glass that can be manufactured in large quantities and is readily available in China at very low cost. It can be melted in large crucibles and that lowers the per unit cost of the glass.

The basic factors of the per unit cost of glass is how much you make at a time and how often you melt that mixture. BK7 and Pyrex are two relatively large production glass mixtures. The higher sand (SiO2) content makes Pyrex higher melt temps. 

Finishing

Mirrors can be made from just about any stable material. The most important specification is not what type of material used, but how well the mirror is polished and figured.
Hardness of the two materials is similar with BK7 being slightly softer. Both take excellent polish equally well. Pyrex is easier for the optician to figure to a high quality level.
 

Thermal properties

BK7 has lower thermal properties than Pyrex.

There are two main retail sectors to buy your astronomical equipment. I highly recommend that you buy your telescopes and binoculars from specialist telescope outlets who deal with the major quality brands. In nearly every case, the staff behind the counter are amateur astronomers themselves and therefore will have an in depth knowledge of the equipment they are selling you. Your sale is their bread and butter and by supporting a good telescope store, you are also supporting the local astronomical community as a whole.

Telescopes which are sold in department stores and some on-line auction sites are mostly rubbish. Apart from the poor quality manufacturing, you will almost never receive any after sales support from the seller, and you can bet they don't even know the product they are selling you. Please, please avoid them at all cost.