# Why do all the stars near this comet show up as four separate dots in a row?

After seeing this tweet about our not-so-new but at least newly-discovered "mini-moon" I scrolled down and saw the tweeted image below.

Since the object is the known short-period comet 114P/Wiseman–Skiff I'm guessing that the telescope guiding could have been pre-calculated and continuous, and yet the stars appear as four discrete and equally-spaced compact spots instead of continuous streaks.

Writing in the image says 2020 Jan 18.22 and 4x30s

What might be the explanation and reason for this?

Could these be stacked images resulting from a clever night of observing, perhaps bouncing back and forth between multiple objects so that each can be observed over an extended period of time?

Or do the four discrete spots per star allow for better astrometry than one long streak?

Or both? Or neither?

tweet

Comet 114P/Wiseman-Skiff tonight at the Catalina Sky Survey 0.68m Schmidt.

• The "4x30s" surely indicates 4 exposures, 30 seconds each. Even if the photos were taken sequentially, there would be a gap between the stars due to the time it takes to read out each image. What is interesting in this image is that each discrete star spot looks like the star is trailing horizontally, then the scope was moved vertically for the next image. If the scope were tracking the comet, the 4 star images would be in a straight line. K Wierzchos may be the only person who can answer! – JohnHoltz Feb 27 '20 at 17:38
• That's an interesting observation about the shapes of the individual spots! I know that astronomical CCDs are likely to be read out moe slowly than those used in commercial cameras and of course larger, wow I will be surprised if readout times are significant compared to the 30 seconds of each exposure. – uhoh Feb 27 '20 at 22:47