Field De-Rotators or Wedge LX200

[BlackWidow]

Hello all.. I am a new guy to this stuff and this is only my second post.. I am wanting to take photo's using my LX200 10" GPS with a Canon 350D. I am up to speed on what I have to do and have made all my cables software and the like. The only area that I am still unsure is the use of a De-Rotator or a wedge. I Understand that using a de rotator will not allow me to piggy back but still can't make up my mind. I have a small back yard so polar alignment is difficult due to objects being in the way. I also intend going mobile to dark sites to image. As I have never polar aligned I am only guessing that it can take a long time to do.. However I have not spoken to anyone that has used a De-Rotator. I want to know what sort of success they have or not had using this system. It seems to be a simple option for the situation that I am in. I still however don't want to spend the money on one instead of a Wedge if setting up is not that hard and MUCH BETTER results are obtained..


Input from those with experience in this subject would be greatly appreciated..


Regards
Mardy:help:

[bojan]

I would go for a wedge.
Derotator can not operate without encoders and computer (because the rotation rate depends on position of object above horizon).
Wedge can be build without much effort, and it will not cost you a fortune.

[rogerg]

Very few people use de-rotators because they're usually found not to be as good a solution as the wedge:
- no piggyback
- from what I hear limited to medium length exposures without trailing, at best
- increases length of imaging train (only an issue depending on what else you have in the imaging train).
- being alt-az rather than eq means you keep both RA and DEC motors going all the time. All of a sudden you're having to compensate for errors in the DEC motor/gears as much or even more than than the RA, where as with a reasonable alignment and EQ mounting you only need worry about PE and backlash in RA. DEC usually has a lot more backlash than RA too, which makes it harder to work with.

I also suspect FOV plays a part with rotators - thinking logically about the situation, if the rotator is rotating at a rate applicable to the current RA and DEC, and your camera has a very large FOV then surely rotation is going to show increasingly from centre to edge of image. Not positive, just a thought I had now.

Even with limited sky you should be able to achieve good polar alignment by using a combination of techniques. There's of course drift alignment, where you need the meridian and either E or W horizons of about 20 degrees (ideally, higher can be tollorable). There's also various methods where you utilise the goto, slewing between objects at different points in the sky and making corrections based on that. There's also software such as PolarAlignMax that you can use to simply map "random" 15 stars say, across the sky, then have it tell you your alignment error. .

Polar alignment with a temporary setup in limited horizons is more frustrating than where you have a permanent setup in the same situation. If you're setup is currently temporary, perhaps your best solution will be a permanent pier out in the garden with the wedge permanently attached (except when taken to dark sites on occasions) and make sure the wedge is well secured so you can take the telescope on/off without needing to make much correction to your alignment when you do so. A lot of people do this, and just put a tarp/bag over the wedge when not in use. The pier is relatively affordable - I had one made locally for $450 several years back.

Roger.

[BlackWidow]

Tanks Roger for your advice.. No place for a pier on my small yard and no sign of any horizons from my yard. Its really a little like an observertory with high walls all around me.. I can see Crux if I position the scope in the right place but its hard to get a balance between that and the best viewing spot.. I am thinking that a Wedge might be the go, or the combination of both.. Purchase a De-Rotator and try to make a wedge... Polar Aligning scares the hell out of me.. The more I read the harder it seems to get.. I am in a club but been to two meetings and have not fund anyone who can help.. Some think they can :-) Thnks for your advice, I would love to here from anyone who may have used a De-Rotator

[tonybarry]

Hello Martin,

I run a smaller LX90-8" in Alt-Az mode, using a video camera for image acquisition. I have found that field rotation is just what the Drizzle stacking technique likes, so in my case it's a feature rather than a bug.

I hope this helps.

Regards,
TB

[bojan]

Roger,
this statement is not correct. The rotation angle/rate depends only on duration of exposure / the direction the telescope is pointed at.
It will be fastest at the zenith, slowest at E and W points on the horizon

Have a look at this doc, it will clarify many issues that were mentioned in this thread.
http://autostarsuite.net/forums/storage/19/4981/Field%20Rotation%20V3.pdf

[BlackWidow]

thanks guys however I would like to keep this on subject. Does not the field rotator connect to the LX200 and the system then adjusts the rotation based on the object in FOV that is being tracked? I dont know this I am just asking as it's and assumption only...


Regards
Martin

[Merlin66]

Ok, I can help here....
I have a collection of Lx200 telescopes: a 12" Lx200 Classic on a wedge and until recently ( see below) a 10" Lx200 mounted Alt-Az with a de-rorator.
The de-rotator does what it says on the box....it works, and works very well!!
It uses the signal from the position encoders in the Lx200 to vary the rate of rotation of the eyepiece/ camera etc to give an excellent full field coverage for the duration of the exposure/ or observations. Everytime!
The performance was as good as my wedge mounted 12" Lx200.

Yes, there is a downside :(
You can't piggyback a camera on the OTA as only the main optics are being de-rotated; it adds about 80mm to the rear cell which doesn't allow large cameras etc to fit below and still go between the forks.

I sold my de-rotator and the Lx200 Drive/forks on the 10" and mounted it on a NEQ6pro..... why? Certainly not because of the de-rotator...
I found over here in the UK that a lot of objects I wanted to observe were in the Alt-Az "dead band" overhead and I couldn't get the spectroscope under the forks which severely limited me.

The move to the NEQ6pro saves my back in lifting the scope ( OTA only now only weighs 14.7Kg) and I get full sky coverage.

Ken

[BlackWidow]

Thanks Ken. That was what I needed to know. I was sure that the electronics would look after the speeds etc for correct rotation. my first thought was to get a de-rotator so that I had the speed of setup. in the summer here it gets dark a bit late so that would be a help. In the winter when the areas are lower in the sky and the night falls early I would have more time to setup and use the wedge... You are the first person that I have had information from that has used a De-rotator.

Thanks for your input


Regards
Martin