motorised rotators are they worth it

[iwols]

hi after first time trying to work mosaics out just wondered how good rotators are and why are they so expensive,any comments suggestions appreciated cheers

Edited November 11, 2020 by iwols

[michael8554]

I thought Rotaters were used to compensate for the Field Rotation on Alt/Az mounted imaging rigs ?

So I'm not sure why you need one on your HEQ5 ?

Do you have slight Field Rotation on your adjacent images due to poor Polar Alignment ?

I would use the Rotate function in PhotoShop etc.

Michael

[gilesco]

Rotators are also used in remote observatory setups, simply to frame the target and perhaps avoid having to mosaic targets that fit in landscape but not portrait.

[tomato]

Assuming you have an equatorial mount for AP, rotators are used to automate the rotation of your camera assembly to facilitate framing, especially for mosaics. They are expensive as they are engineered to rotate a significant mass precisely.

A lot of folks rotate their cameras by hand or keep them in a fixed position once initially aligned, but they are useful especially on remote set ups. They do add to the all up weight that your mount has to deal with.

My own view is I would rather spend that kind of money on a camera, filters or telescope.

[vlaiv]

Maybe DIY can be a cheap solution?

Here is project for rotator for AZ mounts.

https://github.com/cytan299/field_derotator

I think it can be adopted fairly easily to serve as just framing rotator.

[sloz1664]

41 minutes ago, tomato said:

Assuming you have an equatorial mount for AP, rotators are used to automate the rotation of your camera assembly to facilitate framing, especially for mosaics. They are expensive as they are engineered to rotate a significant mass precisely.

A lot of folks rotate their cameras by hand or keep them in a fixed position once initially aligned, but they are useful especially on remote set ups. They do add to the all up weight that your mount has to deal with.

My own view is I would rather spend that kind of money on a camera, filters or telescope.

Including precious back focus.

Steve

[tooth_dr]

Moonlite do a nice focuser/rotator package

https://www.teleskop-express.de/shop/product_info.php/language/en/info/p9100_MoonLite-NiteCrawler-3-5--Rotating-Worm-Drive-Focuser-incl--Adaption.html

[vlaiv]

12 minutes ago, tooth_dr said:

Moonlite do a nice focuser/rotator package

https://www.teleskop-express.de/shop/product_info.php/language/en/info/p9100_MoonLite-NiteCrawler-3-5--Rotating-Worm-Drive-Focuser-incl--Adaption.html

And for that price, I would rather change my ASI1600 for ASI6200 and simply software rotate / crop away instead of mechanical rotating .

[gilesco]

13 minutes ago, vlaiv said:

And for that price, I would rather change my ASI1600 for ASI6200 and simply software rotate / crop away instead of mechanical rotating .

Yes, rotators do seem to have a hefty price tag, I appreciate they are often sub micron accurate, but for framing purposes that just isn't a necessary part of the requirements, I'd quite happily have some drift during rotation tasks on an EQ mount if it is just framing use - as long as there is no movement during acquisition and tracking.

[Jonk]

This is the reason for 'needing' a rotator when doing mosaics...

The camera position didn't change here, so when aligning adjacent images, they must be rotated in software in order to correctly overlap.

One problem of not having a rotator, is if you don't have enough overlap, you could end up with gaps.

I've wondered about it, but for this images scale on a small refractor (3.2"pp on a Star 71) I couldn't fit a rotator in anyway.

Just make sure you have enough overlap to avoid gaps bwtween, and also when you do a final crop, to avoid losing some of the target.

Note this is a test mosaic using 4 x 30m ha subs, just to see how the final mosaic would look.

This was taken using SGP's mosaic feature, which only shows a flat grid, so not a final representation of how the frames go together. Actually, I'm not sure if adding a PA (pointing angle) to the settings will show the rotation in the wizard, (I'll try this next time).

[vlaiv]

9 minutes ago, Jonk said:

This is the reason for 'needing' a rotator when doing mosaics...

Only on very wide field shots if your stacking / mosaic software does not cope well with lens distortion.

Above has nothing to do with rotation of the camera - as you pointed out, camera was at the same angle for each panel.

This has to do with fact that we are "projecting" sphere onto flat surface and due to this we have certain geometric distortion in our images. For very small FOV this distortion is negligible, but when you move to wide FOV, it really needs to be taken into account.

Similar thing happens with maps that represent surface of the earth - there is no map that will accurately show whole earth - one needs to use different projections and each projection method has some drawbacks.

In order to properly make mosaic without rotation in this case, software needs to project each panel onto the sphere - do stitching in that domain and then project back to flat surface to form a single image.

Btw, rotating in software is not problematic at all and can be part of stacking process - each sub needs to be interpolated in order to align it properly to rest of the stack and there is no reason why it can't be rotated in the process (and it often does - poor polar alignment will lead to field rotation over the course of imaging night even if one is guiding).

[Ouroboros]

Is plate solving able to rotate an automatic rotator as well as control telescope pointing? Correct me if I’m wrong, but I can imagine that if that were possible then an advantage of an AR is that your  computer will be able to precisely re-frame a target in different imaging sessions. 

[vlaiv]

2 minutes ago, Ouroboros said:

Is plate solving able to rotate an automatic rotator as well as control telescope pointing? Correct me if I’m wrong, but I can imagine that if that were possible then an advantage of an AR is that your  computer will be able to precisely re-frame a target in different imaging sessions. 

Yes, plate solving gives you angle as well and that can be used to rotate the frame to match between the sessions.

When doing manually - procedure is trial and error. It gives you angle, you adjust angle with manual rotator and check again until you are satisfied with how close you are. Same as focusing really - measure FWHM/ HFR - adjust focus, rinse, repeat ...

Automatic rotator does the same thing as automatic focuser - lets computer do all the steps automatically.

[andrew s]

I am not sure you need them for imaging.  You should have flats for each angle you use!

I have one for my spectroscopy to avoid the target spectra overlapping an other or stars in the field. As my kit is remote it's a necessary bit of kit.

Regards Andrew 

[iwols]

wow some great comments here guys thanks

[iwols]

3 hours ago, michael8554 said:

I thought Rotaters were used to compensate for the Field Rotation on Alt/Az mounted imaging rigs ?

So I'm not sure why you need one on your HEQ5 ?

Do you have slight Field Rotation on your adjacent images due to poor Polar Alignment ?

I would use the Rotate function in PhotoShop etc.

Michael

mainly for mosaics cheers

[iwols]

3 hours ago, tomato said:

Assuming you have an equatorial mount for AP, rotators are used to automate the rotation of your camera assembly to facilitate framing, especially for mosaics. They are expensive as they are engineered to rotate a significant mass precisely.

A lot of folks rotate their cameras by hand or keep them in a fixed position once initially aligned, but they are useful especially on remote set ups. They do add to the all up weight that your mount has to deal with.

My own view is I would rather spend that kind of money on a camera, filters or telescope.

fare comment

[iwols]

1 hour ago, tooth_dr said:

Moonlite do a nice focuser/rotator package

https://www.teleskop-express.de/shop/product_info.php/language/en/info/p9100_MoonLite-NiteCrawler-3-5--Rotating-Worm-Drive-Focuser-incl--Adaption.html

wow what a price thanks though

[iwols]

1 hour ago, Jonk said:

This is the reason for 'needing' a rotator when doing mosaics...

The camera position didn't change here, so when aligning adjacent images, they must be rotated in software in order to correctly overlap.

One problem of not having a rotator, is if you don't have enough overlap, you could end up with gaps.

I've wondered about it, but for this images scale on a small refractor (3.2"pp on a Star 71) I couldn't fit a rotator in anyway.

Just make sure you have enough overlap to avoid gaps bwtween, and also when you do a final crop, to avoid losing some of the target.

Note this is a test mosaic using 4 x 30m ha subs, just to see how the final mosaic would look.

This was taken using SGP's mosaic feature, which only shows a flat grid, so not a final representation of how the frames go together. Actually, I'm not sure if adding a PA (pointing angle) to the settings will show the rotation in the wizard, (I'll try this next time).

that explains it exactly thanks

[iwols]

this looks good but is the price justifiable,not sure.

https://pegasusastro.com/products/falcon-rotator/

[Ouroboros]

19 minutes ago, andrew s said:

Clipped 

You should have flats for each angle you use!

I always take flats because of dust bunnies on my sensor, which change from time to time.  But I have wondered whether angle matters much because as far as vignetting is concerned telescopes are rotationally symmetrical.  

[Jonk]

5 minutes ago, Ouroboros said:

I always take flats because of dust bunnies on my sensor, which change from time to time.  But I have wondered whether angle matters much because as far as vignetting is concerned telescopes are rotationally symmetrical.  

If you rotate the entire assembly, no it shouldn't matter as long as nothing moves or the dust inside doesn’t move.

If you rotate anything within the assembly, i.e just the camera, then flats absolutely should be taken for each scenario.

The same applies if a camera is taken off and then put back (following session for example) even if it’s in the same place.

[JamesF]

10 minutes ago, Ouroboros said:

I always take flats because of dust bunnies on my sensor, which change from time to time.  But I have wondered whether angle matters much because as far as vignetting is concerned telescopes are rotationally symmetrical.  

I find myself wondering if vignetting is symmetrical in practice even if it might be in theory.

James

[andrew s]

18 minutes ago, Ouroboros said:

I always take flats because of dust bunnies on my sensor, which change from time to time.  But I have wondered whether angle matters much because as far as vignetting is concerned telescopes are rotationally symmetrical.  

They only have rotational symmetry if the collimation is spot on so that the mechanical and optical axis are perfectly aligned.  They have to stay that way during any change of angle. If a Newtonian secondary is a little off mechanically (it does not have a unique optic axis and you can slide it in the plane of its reflective surface) you can have perfect optical collimation but the vignetting can be decentred.

Regards Andrew 

Edited November 12, 2020 by andrew s

[Ouroboros]

1 hour ago, andrew s said:

They only have rotational symmetry if the collimation is spot on so that the mechanical and optical axis are perfectly aligned.  They have to stay that way during any change of angle. If a Newtonian secondary is a little off mechanically (it does not have a unique optic axis and you can slide it in the plane of its reflective surface) you can have perfect optical collimation but the vignetting can be decentred.

Regards Andrew 

Yes, that makes sense. As do the last few posts. I had refractors in mind specifically and for which the sensor should be centred.  
I suppose I’m thinking about a situation in which you forgot or weren’t able to take flats at the time. You might at a pinch get away with ones taken earlier or later. Better than nowt anyway.