Star tracking

Glabella

When imaging DSO’s how often (? Milliseconds) does the S50 move to track the stars? I am hoping to use this information to help in planning when best to image objects. Thank you

wvreeven

Glabella

Support@Seestar

First of all, stellar parallaxes are so incredibly small that they do not influence celestial tracking at all. It took astronomers over two centuries to measure the first stellar parallax after the telescope was invented, that's how small they are! For solar system objects, notably the moon, the parallax is important because that can be as large as a degree or more. Also the motion of the moon needs to be taken into account when tracking that object.

Secondly, yes the celestial sphere rotates at 15 arcsec per sec. But with an AltAz telescope that rotation is nearly always over two axes at the same time. Only near the meridian (when the object passes south when in the northern hemisphere and north when in the southern hemisphere) almost only the Az(imuth) axis is used. In all other cases both axes need to be uses and that will lead to field rotation.

Because of that, you almost always want to image object from, say, an hour before until an hour after the object passes through the meridian. Almost always? Yes, AltAz mounts perform bad near the zenith so avoid a region of 5, or better 10, degrees around the zenith.

Support@Seestar

I need to ask the relevant engineer about this problem.
In general, during astronomical imaging, the speed of the telescope's movement is usually related to the parallax of the star and the rotation of the celestial sphere. For standard astronomical observations (such as stellar or celestial imaging), it tracks at a constant speed, typically 15 degrees per hour, equivalent to an angular shift of about 4 seconds per second. This equates to a shift of about 0.0011 degrees every millisecond.

Glabella

Support@Seestar Thank you for your response however, I am not sure I understand your response correctly. Just wondering, I think what I am asking is… what is the rate at which the stepper motor moves the scope… I am asking this question because I have noticed that there are a lot of times that I get failed subs and I only image at 10 seconds and times when it doesn’t even indicate whether it was a failed sub of any sort after completing 10 seconds and continuously for maybe even a minute, what should be six exposures it doesn’t give any explanation at all, and I’m just curious if this has anything to do with the position of the scope and the way or the accuracy of the horizontal calibration and levelling. my leveling is usually within 0.5 and horizontal calibration are usually done just on request and I always have my scope parked in the same location on the same angle or I should say feet at the same location so the level doesn’t change and it’s on a solid surface. I apologize for going on and on, but I’m just curious as to how often the stepper motor moves. It may be variable, perhaps based upon the scopes position and whatever algorithm is being used, but generally speaking, would there be a range and what determines that range? Thank you for any clarification you can provide

Glabella

wvreeven thank you for your insite. To continue this and please correct me where I am wrong if the camera sensor pixel size corresponds to approximately let’s just say 2.5 arc seconds per pixel then that would mean that in a 10 second exposure, a star literally would move 60 arc seconds. given that the celestial sphere rotates at 15 arcsec per second times 10 second exposure thus 150 arc seconds divided by 2.5 arcsec per pixel gives us 60 pixels so 60 pixels in 10 seconds. Or 24 Pixels per second. So, 1000 ms divided by 24 is let’s just say 40 so every 40 ms the scope would need to move in order for the light to be maintained on the same pixel. so I guess I would be wondering if the stepper motor is moving at a rate of 40 ms per step or less i.e. 20 ms per step or a step in every 20 ms. Of course, The size of a step(s) would also need to be adjusted based upon the Alt/Az algorithm used to calculate the horizontal and vertic Steppage. …. Sorry for my lack of knowledge in this I’m not an engineer just trying to think this through with my limited experience.

wvreeven

Glabella I don't follow your calculation.

When observing at the meridian the alt motor needs to rotate 15 arcsec per sec. With 2.5 arcsec pixels that is 6 pixel per sec or 0.06 pixel per 10 msec. The altitude motor will not move at all or only a tiny bit that may be ignored.

For the Moon its orbital motion needs to be taken into account. The sidereal period (from star to star) is about 27.5 days or about 13 degrees per day. That's 0.55 arcsec per sec. A little bit but it is necessary to take into account. Since the Moon moves more or leas along the ecliptic, its orbital motion most of the time is not only oriented in azimuth so that value is a maximumfor the azimuth direction.

When not observing at the meridian, the 15 arcsec per sec motion will be split up over both axes. How much depends on the location on earth, the azimuth and the altitude. Note that there are points in the sky where the motion in azimuth is zero and the motion in altitude is 15 arcsec per second.

Support@Seestar

Glabella
Based on your calculations, I think you're on the right track. Yes, the stepper motor needs to be adjusted frequently and slightly to keep the star centered. If the stepper motor can step every 20 milliseconds or less, it should be able to handle precise tracking adjustments.

However, I haven't received exact information regarding the stepper motor's movement frequency when the Seestar S50 tracks stars. It might be between 10 to 50 Hz.

When the tracking frequency is below a certain threshold, it will definitely affect image quality, especially during long exposures, high-magnification imaging, and time-lapse photography.

For Seestar S50, if the tracking frequency is insufficient, it could cause star trailing or misalignment in planetary imaging, especially at high focal lengths.

Perhaps you could improve the situation by adjusting focus, shortening exposure times, or adding a more stable mount. If you need more precise control, I will provide feedback to the team. However, there are currently no plans to address this specifically.