How to make a field derotator

[Vroobel]

Good morning

After many months of theoretical preparation, and then the practical ones, I have finished work on my own field derotator.

As usually, I would like to share the effect of this big project making a video-report in YouTube.

I invite you to watch.

Best regards, 

Tomasz

 

[festoon]

This is fantastic - well done!

[carastro]

Fantastic and so clever.

Carole 

[Tommohawk]

Looks like a proper job! Can I ask what you intend to use it for?

[Vroobel]

As you could see in my video I have Dobsonian mount so I cannot perform longer exposures than 25 seconds. Every longer one captures star trails. Presently, with the derotator, I am able to leave my camera set up for thousand pictures and go home to watch TV or movie. The only problem is that my drives have strange small movements invisible without capturing the pictures. Short exposures like 5 or 10 seconds show nothing, stars are simply points. But longer ones make patterns like butterfly., so I think the error is looped. I ordered better quality stepper motor drives and want to replace them and check. If that is OK, I want to play with astro-photography. My F5 scope and IDAS LPS-D2 filter (which I gonna buy) can do it even in Liverpool 

[Tommohawk]

So I'm wondering how you control the derotator? I'm interested because I was also considering a DOB for astrophotography. Do you use some kind of software?

[Vroobel]

I wrote my system completely on my own. I calculate both Az and Alt and put them into equation shown in video. My Latitude is taken from GPS. Everything else is a calculation based on complicated algorithm developed on my own as well. Finally every picture presents objects with the same angle.

[Vroobel]

Look, the derotator is a kind of proof that it is possible to make it at home. It was like a bet with some people from the community. Since I finished this quite nice project I gonna prepare to performing of a big fork mount able to drive my 10" OTA. Today I made a deal and I nearly have strong harmonic gear box. That should be strong like the EQ8. I don't expect any problem with astro-photography at all, maybe only the light pollution... 

Edited May 8, 2020 by Vroobel

[Juicy6]

Wow, amazing project. I like the video presentation. Very exciting!

Christer, Sweden

[Ricker]

You should go into production. I would buy one. 

[Vroobel]

🤔

I cannot say that I didn't think about it. But how will you connect it to your exact stuff? The derotator needs 12V, that is easy. Beside that it gets Direction and Step signals from the Raspberry Pi. And GND of course. I made it this way to adapt it to ASCOM in future, but the future isn't now yet. I don't have mount compatible with ASCOM, so cannot test it anyway. 

Edit:

The LV8729 stepper motor driver can work with 6-36V, so the 12V is there only because i operate with this voltage in my system. 

Edited May 10, 2020 by Vroobel

[Ricker]

I would suggest that many ppl would want just a rotator and not necessarily a derotator, that would simplify things. 

 

Fantastic job. Well impressed. 

[Vroobel]

That is right. But that doesn't change the problem of positioning the rotator before every session. This feature requires the ASCOM as well. Moreover, I know that some indicator of "zero" position should be implemented there.

[rpineau]

Existing rotator use a small magnet with a hall effect sensor to home the rotator on power up.

[Vroobel]

Thank you for the information. 

Inside my derotator is simple universal board with only LV8729 driver and set of jumpers for micro steps set up. I can think about it and add a feature of the rotator when I make step towards the ASCOM compatibility.

[Vroobel]

Hi,

As I used a theory described in article of The Royal Astronomical Society of Canada as a base for my derotator acting, I decided to inform them about my project. An effect of the correspondence exceeded my vision: the author placed an information about my derotator in the article. 😲

http://calgary.rasc.ca/field_rotation.htm
 

 

[nmaron]

Are you willing to share the STL files you used to print the gear and housing?

[gazza]

On 11/05/2020 at 00:19, Ricker said:

I would suggest that many ppl would want just a rotator and not necessarily a derotator, that would simplify things. 

 

Fantastic job. Well impressed. 

There is a nice 3d printable field rotator here:

https://www.thingiverse.com/thing:4399875

cheers

Gary

[Vroobel]

Good to know. 

[photon08]

A nice job !

The other day, I came across now discontinued Meade field derotator.   It's brilliant and simple at the same time, IMHO. It does not require an external computer to control. Instead, it gets the current alt/az and the location (only latitude matters)  from the mount and calculates the rotation rate to counteract. I guess the downside of this simplicity (the direct connection with a mount) is that it can work only with a set of mounts it's designed for unless there is a standard to read alt/az and latitude from a mount. 

I wish there was one available for my mount (Orion XT10g;  Skywatcher synscan hand controller is used so that it'd be compatible with any skywatcher alt-az mount in terms of communication protocol, I believe).  Without that, my option is limited to getting a field rotator that supports the field de-rotation for alt-az to hook up to a computer.