Field derotators and dobsonians

[Gem (Grant)]

Hi all,
Does anyone here use a field derotator to do casual imaging with a dob? Does it work ok? Is it a viable option for the casual imager?

[mswhin63 (Malcolm)]

I am not sure about visual observing, but for Astro photography the problem experienced with using a field de-rotator is that it will increase the focal distance for a camera. Dobsonian already have trouble with focal distance with the standard Crayford focus. It will have an effect on eyepieces but I don't know whether it will affect the focal distance or not.

Not sure why you need a de-rotator anyway.

[Gem (Grant)]

Just trying to get a feel for all the options available. A dob is easy to use, easy to move, etc... I do not have a permanent place for a scope, so I need to move it in and out all the time. I managed this with a 10 inch dob fine for many years, but its focal length (1400mm) made it too large to fit in the car. I am wanting to get another scope mainly for visual observing but would like to have the option for imaging later down the track. If a derotator works fine, then maybe a 10 or 12 inch collapsible could be an option. Otherwise, I would consider a 200mm reflector on a HEQ5 Pro. Just need to find the right balance of portability, ease of setup, visual pleasure and feasibility of imaging.

[Gem (Grant)]

I should add that the 10 inch is not with me anymore... it is interstate.

[gary]

Originally Posted by Gem

Hi all,
Does anyone here use a field derotator to do casual imaging with a dob? Does it work ok? Is it a viable option for the casual imager?

Hi Grant,

Gary Kopff here from Wildcard Innovations.

We get to correspond with a very large number of scope owners from around
the world so have a good feel for what is out there.

Field de-rotators on Dobs are very rare and those we hear about tend to be
installed on larger, premium Dobs. On Dobs in the 10" to 12" range, they
are rarer still to the point I have never corresponded with an owner that has
mentioned owning one.

Taking a step back, even before one uses a field de-rotator, one still needs to
drive the scope in Alt/Az as well.

Tracking Dobs are now commonplace. For example, for scopes in the 14" to 30"
class, servo controllers such as the ServoCAT and Sitech units used in
conjunction with our own Argo Navis Telescope Computer are commonplace.
One IIS member recently installed a ServoCAT/Argo Navis on a LightBridge 12".
Generic tracking Dobs in the 12" and 14" class are also shipping out of the Far East.

However, as mentioned, it is rare to hear of owners adding field derotators
as well.

As I posted here on IIS some time back, despite the limitations of field rotation,
there are plenty of enthusiasts out there achieving stunning results with
autoguiding on Alt/Az mounts using short exposures and software derotation.

For example, check out these images by Johannes Brachtendorf
using an 18" Obsession, Argo Navis, ServoCAT, SBIG ST 9E and software
derotation. As Johannes says, "no platform, no derotator, no adaptive optics".
http://homepages.uni-tuebingen.de/jo...r/Galaxies.htm
http://homepages.uni-tuebingen.de/jo...er/Nebulae.htm

Also be sure to read Johannes notes on the subject -
http://homepages.uni-tuebingen.de/jo...dorf/index.htm

So you wouldn't be breaking any new ground if you were to use short
exposures and software field derotation. However, on smaller apertures scopes with
long focal lengths, deep sky objects would still be challenging and so a Dob
in the 12" to 14" class may be restricted to brighter DSO's and the planets for
practical purposes.

A key phrase you used is "casual imager". The results using the above prescription
may not be of a caliber to put one in the running for a David Malin astrophotography
competition prize, but they can be immensely personally satisfying.

Equatorial platforms are another solution but have become less common
in recent years.

Hope the above might be helpful.

Best Regards

Gary Kopff
Managing Director
Wildcard Innovations Pty. Ltd.
20 Kilmory Place, Mount Kuring-Gai
NSW. 2080. Australia
Phone +61-2-9457-9049
Fax +61-2-9457-9593
sales@wildcard-innovations.com.au
http://www.wildcard-innovations.com.au

[Poita (Peter)]

Just for anyone else perusing this thread, Johannes links can be found here:
http://homepages.uni-tuebingen.de/jo...dorf/index.htm

[Merlin66 (Ken)]

De-rotators (for non-Meade SCT) were discussed a few months ago.
They are NOT cheap (I think around $3500) and need some sort of Alt-Az signal/input to do the necessary calculations/ corrections.
So you'd need a Alt-Az readout cum controller before you could even consider adding a de-rotator.
Not an easy solution......

[bojan]

Originally Posted by Merlin66

De-rotators (for non-Meade SCT) were discussed a few months ago.
They are NOT cheap (I think around $3500) and need some sort of Alt-Az signal/input to do the necessary calculations/ corrections.
So you'd need a Alt-Az readout cum controller before you could even consider adding a de-rotator.
Not an easy solution......

Mel Bartels' software (freeware, stepper motor design) supports de-rotator.
http://www.bbastrodesigns.com/BBAstroDesigns.html

For the mechanically oriented DIY individuals, de-rotator design and build is not such a big challenge.
I remember seeing some interesting designs on CN forum.
There are also some discussed on Yahoo scope group.

However, the eq platform for dobsonian may be the better (cheaper?) option, as it needs tracking along only one axis (polar), compared to 3, required for dobsonian.

[Satchmo]

Theres nothing particularly complex about driving a derotator- at least of the Pixsis ( ? ) variety. You can download the software to run one from the website and look for yourself.

From memory you just enter into the software the hour angle ( RA to or from the meridian ) of the object you want to image , and the software drives the rotation motor at the correct variable rate as the exposure progresses. This little program runs in the background to whatever you are using to drive your 'scope.

[Zubenel (Wes)]

I’m not planning on a field de-rotator but just tuned my servocat using appendix C of the servocat manual so that instead of an object drifting off my cameras chip in 25-30mins , last night I centred in PN NGC6807 and stayed there using the 12mm Nagler @211x /30arc min fov , for 2hrs and10mins before disappearing behind the neighbours Gum tree. Now I’m wondering how long I’ll be able to push exposure length ( av. to now was 3.2sec) before rotation will he an issue ? I’m thinking an auto guider might be a good ( not for de-rotation) for increased tracking accuracy however I don’t run a computer atm so probably not with the current set up .. thx Gary for the info. Very helpful as usual!!

[bojan]

Depends on where you are on Earth and where the object is on the sky....
See here.