Alt/Az + De-rotation = No Diffraction Spikes?

I’m gonna take a stab at this. I think you have the right idea in that with a alt/az mount the spider is in a fixed orientation relative to the sky while the sky and sensor rotate. That should give exactly the effect you’ve described. The best way to confirm the rest would be to ask for a set of subs and see what happens.

I suppose if you are already going alt/az this would make sense. If not there are easier ways to eliminate or diffuse the spikes in a reflector as I’m sure you know.

Back when I was starting out, I used to do extremely long exposures. It was the only way I could get an image with my first camera (POS Orion G3 color)

One night, it and my POS AVX mount were having a good night and I was doing a long exposure, (Because neither I, nor 5 other good imagers could stack the dog-doo subs that thing put out.)

That particular night I ran a 5400 second sub. Non-stop. 1 hour and 30 minutes.

It was a picturd, but it worked. One good friend told me “Sonny, you just stack with time.”

Ahh, those crazy days of Picturds. (And chitty equipment)

John Stone · Jan 21, 2026, 02:23 AM

Tony Gondola:
I suppose if you are already going alt/az this would make sense. If not there are easier ways to eliminate or diffuse the spikes in a reflector as I’m sure you know.

I'm thinking with my idea the spikes are completely rejected instead of diffused/blurred.   

The data in the stacked image will be just as sharp under the spikes as other places in the frame (except for the small loss in SNR), right?

Have you asked your friend with the alt/az setup? Maybe he can turn you on to a set of subs that you can test the idea out on?

Boy, can’t tell much from that single cropped image. With my newt I don’t get spikes on all the stars, just the very brightest ones so you can see why it’s hard to make a judgement.

📷 m32 final render-3.jpgThere must be someone on the bin that’s using an alt/az mount with a rotator that could comment.

andrea tasselli · Jan 21, 2026, 06:23 PM

The main argument is as folIlows:

An Alt-az mount would track a given patch of the sky by keeping the center of the field at the same position while the field would slowly rotate around it's center. The speed of the rotation would depend on both the object's coordinates as well as the latitude of the observer's location. From the point of view of the sensor thus the whole field experiences a rotation as if the whole OTA would rotate around its axis as well. Adding a rotator would counteract this and keep the image centred and free of rotation about its pivot point, thus being in fact equivalent to an equatorial mount, thus the spikes would retain the same rotation about the field as if it were on an equatorial mount. 

Or otherwise, check this image from the ESO VLT (amongst others). That is an Alt-Az mount with an optical de-rotator if I'm not mistaken:

The Helix Nebula* | ESO

I’m still not sure. With an Eq mount the spider rotates with the rotation of the field. With an alt/az the spider doesn’t rotate with the field, the rotator does. If we accept that as true then I think that’s a fundamental difference.

Yes but with an alt/az mounted ota, there is.

andrea tasselli · Jan 21, 2026 at 06:23 PM

The main argument is as follows:

An Alt-az mount would track a given patch of the sky by keeping the center of the field at the same position while the field would slowly rotate around it's center. The speed of the rotation would depend on both the object's coordinates as well as the latitude of the observer's location. From the point of view of the sensor thus the whole field experiences a rotation as if the whole OTA would rotate around its axis as well. Adding a rotator would counteract this and keep the image centred and free of rotation about its pivot point, thus being in fact equivalent to an equatorial mount, thus the spikes would retain the same rotation about the field as if it were on an equatorial mount. 

Or otherwise, check this image from the ESO VLT (amongst others). That is an Alt-Az mount with an optical de-rotator if I'm not mistaken:

The Helix Nebula* | ESO

That Helix nebula image was taken by an EQ mounted scope… https://en.wikipedia.org/wiki/MPG/ESO_telescope