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Max Exposure time for LX-90 in Alt-Az?


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Hi all,

I've decided to have a go at AP, I have a half decent scope in the Meade LX-90, I have a choice of DSLR - between a EOS300D and a 40D, I also have a netbook should I need it. I need to get the necessary adaptors between the DSLR and the scope but that shouldn't be a problem.

I will get a equatorial wedge for use with the scope when funds allow but in the meantime I'll have to stick with alt-az mode. My basic understanding is that because of field rotation, the exposure times I use won't be able to be that long. What I'm wondering is how long exposures could I get away until field rotation becomes evident?

As you can probably guess, I'm a total beginner when it comes to AP so I'm on the bottom of a rather big learning curve.

Thanks, Mark

Edited by marco
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Without auto-guiding, I would suspect inaccuracies in tracking will limit you before the rotation of the sky.

I use a 1000D on a goto alt-az mount + 660mm FL scope and I am limited to 30secs, not by field rotation but by the mount itself. And the LX-90 is a much longer focal length.


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You could try with a Focal Reducer in the system. and stick to high latitudes.

A Wedge would be an advantage too, but it's all extra money.

There is a Yahoo LX90 User Group that is a good source of help. Also the Meade 4M Forum is useful for technical problems you may encounter. There a loads of good Images been produced on AltAz mounted LX9O Scopes.

A couple of points from Matt Taylor here. He produced some videos for Meade a while ago.



Edited by barkis
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Thanks for the replies and there's some food for thought there. I will get the wedge

somewhen in the future when the funds allow, as for autoguiding and a focal reducer then I'll take these into consideration then splash out as and when the money is available. It'll probably take a year or to acquire all the bits n bobs but I'll get there in the end.

Cheers, Mark

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Its quite possible to make a wedge out plywood to get you going... especiallly if your goign to be use it at a fixed location or over a small rangle of "latitudes"...

A lot of these are made so the scopes at failry low level - you do away with the tipod and construct what is a box with an angled face to mount the scope on... you can use threaded inserts and "furniture feet" to adjust the level etc...

Ok it wont have the fine adjustments of a professionally engineered metal one but it can be made cheaplly, quickly and accurately with a bit of patience...


Edited by Psychobilly
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When I tried this I found that 5 sec was reasonable, any longer than that & blurring of the stars due to mount inaccuracies was likely. The LX90 is really recommendable only as a visual only mount. The LX200 is much more solid (and a lot heavier!)

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