Revelation telescope

[astro man]

Hi guys 

I am thinking of buying a Revelation 8 inch dobsonian scope.. Are they any good has anyone used this make  before as I had never heard of them. 

Thanks 

Astro man

[rory]

I asked this recently . I think the skywatcher gets the nod on the mirrors being parabolic ,

But the revelations have slightly better focuser , being dual , and a cooling fan . The mirrors are bk not parabolic . That's the general idea I drawn from comments .

[triton1]

I haven,t heard anything negative about them,nearly bought one myself.They also sell good value eyepieces and barlows.There basically the same as GSO which are very popular.

Regards Jonn

[JED-E3]

I asked this recently . I think the skywatcher gets the nod on the mirrors being parabolic ,

But the revelations have slightly better focuser , being dual , and a cooling fan . The mirrors are bk not parabolic . That's the general idea I drawn from comments .

Hmmm, I'm pretty sure the GSO dobs have parabolic mirrors, could you mean they're not pyrex?

[rory]

Hmmm, I'm pretty sure the GSO dobs have parabolic mirrors, could you mean they're not pyrex?

ah, sorry , I meant pyrex glass yes.

[umadog]

Note that no mirrors are currently "Pyrex" since Corning have stopped making the stuff. If you see that a mirror has low expansion borosilicate, then that's essentially Pyrex.

I spent quite a long time reading up about mirrors, and as far as I can tell the take-home message is that the expansion index of the mirror substrate isn't something to lose sleep over. During cool-down the mirror is changing shape because different bit of it are different temperatures. So you may see some spherical aberration. A mirror with a lower expansion coefficient should retain its shape better during cool-down because it changes shape less. However, you also have the thermal boundary layer during cool-down and that's present no matter what glass you have. Once the mirror is cooled, it's a homogeneous temperature and its original shape returns. All mirrors (all glass types) will do this if they are able to reach thermal equilibrium. The critical factor in reaching equilibrium is mirror thickness, as cooling time is proportional to the square of the thickness. The type of glass is not relevant for cooling time. So as far as the user is concerned, expansion coefficient is only an issue during cool-down which in any case is a time when you don't expect to do high-power work. The mirror's figure matters much more than the expansion index of the substrate.

The time the expansion index matters is arguably not for the user but for the mirror maker. The mirror heats up even during the final polishing stages and in order to be tested it must cool again. A low-expansion mirror will change shape less per unit temperature rise and so this speeds up the testing process. Even more important, a well-annealed mirror changes shape more homogeneously, which also speeds up this process. It is easier to get a good figure on a mirror that is annealed well and is low-expansion. Thus, your odds of getting a good objective may be higher with low-expansion glass. However, as far as I know, the mass-produced optical shops don't check the anneal, which is at least as important.

In practice, if you're buy a scope with GSO or Synta optics, you're best bet is to buy based on scope specs and price then test the mirror and hope it's a good one. Worrying about substrate is not worthwhile, since this is just one factor amongst many that influences the final figure.