Meade wedge and imaging

[Albireo380]

I have an LX90 and have been "dabbling" with prime focus imaging over the last year. The LX90 is in AltAz mode - and this seems to be okay for 30-45 second exposures. Then I get field rotation and nice star trails or "seagull" shapes set in, instead of star points.

In a previous thread, people told me to go out and get a wedge. This costs about £150 and I am wondering how much effect it will have on my ability to take longer exposures - how much "bang for my buck" do I get?

So, before I shell out £150 cash (unless anyone has a secondhand wedge going???), can you give me an idea of the improvement I would get?

Would I get 60-75 second exposures okay? 75-90 second exposures? Or what?

Hope you can help - as I would hate to spend the money and find marginal improvement.

Tom

[GordonCopestake]

The wedge just stops the field rotation as only one motor is moving. The rest is up to your polar alignment and how good your periodic error is. It's not really an ideal situation for photography (thats what gems were invented for) but some people get very good results. Put a wanted ad on astro buy/sell im sure someone has a wedge that they dont use. You could also make one yourself out of wood as all it is is an interface between your tripod head and the mount base at the angle of your latitude. Using a protractor and a saw you could make a slope that is 53 degrees or whatever your location is at. Most edges are expensive because they have to have a range of motion, but if you arnt going to move then a fixed wedge is much cheaper. If you wanted it out of metal get your local engineering company (or arthur) to quote you for one.

[MartinB]

Hi Tom. I have a nextar 8" which I have just started using on a wedge. It really is the only way to go for imaging except maybe when using the very low F2 provided by the Celestron Fastar system which isn't an option for the Lx90. I have a meade 3.3 F/R but even with this can only get 45 secs before rotation starts to become an issue.

Both the celestron and meade wedges come in for a lot of flak from serious astro imagers for not being rigid enough and having poor adjustment controls. Adjusting the celestron wedge certainly is a pain event with celestrons upgrade but I find the wedge very stable with the 8". 11" would probably be more of a problem.

One of the problems with a wedge is getting decent polar alignment. There are alignment scopes available but they aren't as satisfactory as those on gems. Drift alignment is best but thats a pain in the neck especially in the summer.

Once set up a wedge is a joy to use, much much more convenient that a gem with no problems around the meridian. It is also free of the problems fork mounted scopes have around the zenith.

I would recommend going for a wedge Tom. Like gordon says, they often appear second hand.

If you want a really decent piece of kit give Arthur (ambermile) a pm. But high quality wedges are £££££