Star trails

[rajdarge]

I have tried unsuccessfully to create star trails with an accurate starmap.
I obtained the accurate star map from https://svs.gsfc.nasa.gov/3895 -- thanks NASA
and used the spherical map of the stars
I used this image and did the following:
Used a standard 3dsMax BitmapTexture and applied it to the enviroment map
then animated a rotation in the UV. Apart from creating a headache with exposure it created the star dots (with Moblur on on the Physical Camera) but no actual motion blur visible to render. I used a variety of shutter speeds, and all the foreground objects were blurring nicely. But the background was always in focus (dof) and always sharp. Also the UV rotation didn't rotate it - though I didn't thoroughly test it
I then applied it to a VrayHDRI and used that in the environment map - it was nice and bright now, but rotations didn't blur - though they were now accurate.
I then put this as the texture map of a Dome light, made it a fully spherical light and locked it to dome orientation. Did an animated horizontal rotation of the Dome light and set the shutter speed to 12s
again sharp dots (no dof ) and no moblur at all of the sky.
I even applied it to a vray sphere as both a texture map and vray HDRI and still no blur
So what gives?
Is the Background of the Dome light always going to be sharp and never motion blur?

[Raven]

Use a sphere and self illuminated texture and it works.

Correction, appears to work on motion blur subdiv of 2, but anything higher and you clearly see the image brighter than the motion blur so the effect is lost.

Correction number 2. Look like it works, I had forgot to continue animating the sphere past the end of the timeline.

[Raven]

This is a 30 minute exposure.
I have not included the map as it exceeds the maximum upload size.
I had to play with the gamma and overall multiplier to get a look.

[rajdarge]

Thanks for the sample scene, and it does work -- Yay!
A few things I did differently: I made changed the HDRI to RGB output:RGB color
Star trails are usually in colour, though this may not be always apparent.
i also experimented with the Vray Light material as it has the autoexposure mode which makes life a lot easier
but I couldn't get this technique to work with a Vray Sphere primitive at all. the stars where always fixed
if you apply a copy of the image without the colour data and a gamma of 1.0 to the opacity map of the vraylightmtl you get a nice alpha channel too, looks awesome if DOF is on too.
Its solved the problem, but perhaps not as elegantly as I would have liked, not sure why the environment map/dome light doesn't animate: this would be a real pain if you had an animated HDRI as the lighting for a scene.

[rajdarge]

Having photographed star trails in my yard, I know that the sky appears to rotate around the pole star and will appear directly overhead only at the poles
http://astro.unl.edu/naap/motion2/starpaths.html

I tried your technique of rotating the sphere but with a tilted axis
I couldn't seem to get a smooth rotation unless I changed controllers for rotation to TCB but then I lost control of keyframes.
I was thinking of a maxscript solution (using quaternions - when I work out how to apply it ) but I was getting weird results with non smooth lines all over the place.
I will upload an example once I settle on a material for the sphere.
Have you tried this and had any luck?

you can see the funny linear streaks around what should be a nice pole star
This had a shutter speed of 147s. If I make the shutter speed faster, more stars become visible, and also the mottion becomes smoother. OF course the trails also become shorter
I fixed the rotation problem by parenting the sphere to a dummy object and rotating that by the tilt angle. horizontal rotation then became offset.

[Raven]

Your linear streaks look like what you get with the default motion blur samples of only 2.
I'll do a test of a tilted axis one.

[Raven]

Worked fine on my side.

Again I have left out the star field map due to upload limit.

[rajdarge]

You were dead right ,it was the motion blur samples. So I ramped them up to 35 on the Sphere.
But they still look grainy . Any advice on how to clean them up ?
I used the 8k map with 44 geometry samples on the "camera" rollout and 14 prepass samples
This image looks similar to what I have shot at night with my wideangle lens. It has DOF on, but the grainyness shows throught that. I used the "archviz interior pre-settting with gi quality 62% 48% and 48%

Thanks again for your help, its been a long time since I played around with vray settings. Been photographing for the last few years, so I'm very rusty.

[Raven]

Unfortunately the only way I have found to clean them up is to increase your min samples in the image sampler.
One of the devs would have to tell you if it is possible another way.