PHD Guiding using a De-Rotator

BlackWidow (Mardy) · #1 16-12-2010, 10:24 PM

I know that I am going to get people telling me not to use a De-rotator and get a wedge. I am getting a wedge as well but anm wanting to try some things with the De-Rotator. I have seen some good sites with very good results with De-Rotators and most comments are based on opinions rather than from usage. Bit like opals being bad luck I guess...

Anyway this is my question... I have seen and read about people using PHD Guiding in Alt Az to guide an LX200. I understand that no matter how good the guiding, in time rotation will still be a problem in the shot. Hence the idea of using my De-Rotator to fix the rotation. At first I thought I would use a WebCam in and off axis guider at the back of the Field De-Rotator to guide the Corrected non rotating image. Hmmm

Could I assume that a better idea would be to use PHD guiding on a guide scope attached to my LX200 using a WebCam as the software was intended to be able to do. Then still use the De-Rotator at the back of the LX200 with another Long Exposure WebCam or my 300d SLR to do the long exposure. The De-Rotator will then in effect remove or reduce the rotation of the guided shot.

I just would like somone who has played with this sort of thing to explain if my logic is logical. I know some are going to want me to explain why I would bother with this process?? Well I just want to know and have a go at it. I am in the process of having a good wedge made as the retail ones are over priced and under designed. I will then use the wedge for more detailed and longer exposures. I have a very small property and have to set up each time I use the scope so I don't want to use the wedge option all the time. I have tried to contact some of the websites that have use De-Rotation but have not had any replys at this stage.]

Somtimes doing the hard things make the easy ones a piece of cake ( I know I might be kidding myself, buts that's OK isn't it?)

Seeing No Stars at the Moment
Mardy

Merlin66 (Ken) · #2 17-12-2010, 11:30 AM

Mardy,
There's a thread somewhere.....I explained how I used the derotator on my setup to successfully guide a spectroscope.
I'm sure what you are considering can be done... the derotator uses the site position and the RA/Dec of the object to calculate the required re-rotation speed. This is independent of the guide tracking control.
I only used it with an OAG and worked with the spectroscope slit on axis.
The deadband in the zenith killed my chances with the spectroscope!
Now I use the 12" Lx200 on a HHD wedge and cutdown HD tripod.
Hope this helps.

BlackWidow (Mardy) · #3 17-12-2010, 12:02 PM

Thanks Merlin for your advice. You are the very Man that has started my line of thought. I have read some of your info on the StarGazers lounge. I contacted Craig Stark from Stark Labs about using PHD to do what I'm suggesting and his claim was that the De-Rotator will effect the Guiding of PHD after about 30sec. But I feel he may not be understanding that the Guiding will not be connected to the De-Rotation in any way. As you have mentinoned the Guiding will be done via a Guide scope so my thoughts are that it will go about doing that job. The image capture will happen at the rear of the scope after the De-Rotator and the De-Rotator will address the image rotation. The other idea was to fit an AutoGuider before the De-Rotator so that I could capture a guide star, and that way still be done seperate from the image capture on the back of the De-Rotator.

I am finding it hard to get past all the myth about De-Rotating and get the facts from people that have used them. It seems they were originally and expensive option and to start with did not work well. It has neow setup a vein of myth that in most cases is sread by people that have never seen one.

The first one I purchase a year ago did not work (From the USA). It has taken this long to get it replaced by Meade. I got a new replacement two days ago. They are still avaliable and after looking inside they have made some needed improvements. I am sure this will add to my options avaliable to capture DSO's. As soon as we get good weather the trials begin.

Regards
Martin

Merlin66 (Ken) · #4 17-12-2010, 12:14 PM

Most of the stuff about derotators seems to be "urban myths"!!!
99% of all the professional observatories now use Alt-Az mountings with derotators!!
The issue is that the field rotation corrections are ONLY made to the optical axis of the scope ie a piggyback scope with a camera will still show field rotation.....however if the guide scope target star is in the middle of the main scope FOV then there's NO relative rotation of this star - it can be used IMHO to provide a guide signal to the mount.
I know I had success guiding a 20 micron slit on the spectroscope...what more can I say????

BlackWidow (Mardy) · #5 22-12-2010, 05:38 PM

Just another question if I may. I have set the De-Rotator up on my scope and wanted to ask if yours gave a beep every now and again say every 30 seconds or so? also did you have PEC on or off as I have noticed that if I have PEC turned on it just keeps letting out beeps every second or so. I have a clear night tonight so I am going to give it a test. It seems to be talking to the scope as when I slewed towards the overhead position I got six short beeps to advise about hitting the mount, it's all the other beeping that troubles me as the old instructions suggest that can indicate no communication with the scope mount... Any assistance would be a great help.

Regards
Martin

Merlin66 (Ken) · #6 23-12-2010, 08:57 AM

Martin,
Not sure I can help...
I never used PEC when guiding; PHD seemed to make all the necessary corrections. Can't honestly remember any beeping from the unit....
Did you manage to get any further testing ??

BlackWidow (Mardy) · #7 23-12-2010, 10:31 AM

tried last night on Orion and it seems to be working at this point. I still get beeping. somtimes only once in a while and other times more often. It is pretty annoying. So I removed it, opened it and fitted a switch to turn in off once I have locked on to my object. Just took some single LX shots that overexposed as the moon was rising. however the image seemed pretty good with only some slight image shift. Was very windy so that could be what caused that. However the first test seemed pretty good. I can't guide as yet as I am waiting for my other scope to arrive to bolt on the side. I also had trouble getting a stare in the reticle so I just let the scope do the work using PEC..

Will keep you informed and might even share a picture when I get a good one..

Thanks for your help

Seeing Stars
Martin

Merlin66 (Ken) · #8 23-12-2010, 10:34 AM

Sounds good so far!
I'm sure you will get even better results when you start to guide....