Which mount should I shoose-DIY

[pirate of skies (Rob)]

Good day there everyone,

As a new member of the IIS , I was hoping to find some clear cut answers to some questions I have on astrophotography. I am unable to afford the big dollars needed for a slewing eq6 mount or similar. I think my only option here is to make (DIY style) something that will track for photography.
I have an 8 inch Dob SW scope, with a few mods already done.
I was thinking of an equatorial platform, but have heard these only allow tracking close to the meridean, and mostly up to one hour. This may be an easy option, but I am not sure if I should limit myself to this. Has anyone made a Equatorial mount for an 8 inch or larger, and added motors (and motor drivers), to operate at tracking speeds ?
My inexperince in tracking, might be limiting me, as I may be missing something that is important.

What is the general consenus here ?

Please help, my brain is churning on the inside with too
much information.

[bojan]

Perhaps you should consider Mel Bartels solution, motor driven dobsonian.
http://www.bbastrodesigns.com/BBAstr...ted_Telescopes

For astrophotgraphy you will need field rotator (supported by mel's software).

[Davros (Lauren)]

me thinks i just found a new project, thanks for that link.

[pirate of skies (Rob)]

Thanks for that Bojan, I am still looking into it a bit further. It seems the eq platform is the simpler , less expensive way out. Man, astronomy is so expensive. I might have to sell one of my cars.
If I guide a dob (alt and Azimuth) and add a wedge for my latitude, isn't this the same a an german equatorial mount ?

[erick (Eric)]

Originally Posted by pirate of skies

If I guide a dob (alt and Azimuth) and add a wedge for my latitude, isn't this the same a an german equatorial mount ?

Sadly no, because your tube does not rotate in the same way a tube on a GEM will. Search for "field rotation". So you are limited to shorter exposures before the stars in the edges of the field start to leave trails.

[AstroJunk (Jonathan)]

Originally Posted by pirate of skies

I may be missing something that is important.

Yes, money!

Astrophotograpy will lead you to the door of the Devil's Bank Manager quicker than you can say "10 one minute subs"

[pirate of skies (Rob)]

OK, So with alt/azimuth mounts, the stars would be trailing on the edge of the lens as the field of view doesnt turn to compensate for the rotating earth.. So this means the field of view must rotate perpindicular to the axis of the starlight. This would happen on a German eq mount, and also on a eq platform, (to the extent of the size of the sector arc that was cut).

Is this right ? What does the rate of rotation of the field of view need to be ?

[bojan]

Originally Posted by pirate of skies

OK, So with alt/azimuth mounts, the stars would be trailing on the edge of the lens as the field of view doesnt turn to compensate for the rotating earth.. So this means the field of view must rotate perpindicular to the axis of the starlight. This would happen on a German eq mount, and also on a eq platform, (to the extent of the size of the sector arc that was cut).

Is this right ? What does the rate of rotation of the field of view need to be ?

Right. And wrong

The field rotation rate for dob is variable (depends on direction where dob is pointing) but Bartel's software supports field rotator.

BTW, if you add wedge (~37° for Melbourne, ~30 for Brisbane?) under the dob mount, it will be the same as equatorial.. and you will not need field rotator.
But the whole thing will be very unstable.. dobs are designed for vertical loads only.
The better way then may be the equatorial platform. Only the tracking time is limited with this approach.
Equatorial mount does not need field rotator as there is no such effect, telescope trails correctly.

Actually, dob is simpler and cheaper to build, and is better for visual.
And for GoTo, you have to drive both motors (both axis) anyway.. so..
But for photography, I think equatorial is better in terms of results (accurate tracking etc)..

[pirate of skies (Rob)]

Thanks Mr Bo-jan-gles ? Appreciate your help.
I think I will just build the eq platform for starters, and hopefully get a good hour or so for exposure. I will learn by experience as I go. Maybe sell my car later...

Thanks for your time, you've been very helpfull.

[bojan]

No worries mate :-)
The whole point of ATM is, you do not need to sell you car... and besides, you still need it for carrying your telescope around ;-)

[Geoff45 (Geoff)]

The fact that the dob on an equatorial platform will only track for an hour is not a problem, since most people only take 5-15 minute subs anyway. After the hour just reset the platform and continue imaging. Have a look at this site for some great pics taken on an EP.
Geoff

[Rod]

There is a very good yahoo group on equatorial platforms here:

http://tech.groups.yahoo.com/group/eqplatforms/

Good Luck!

Rod.