Target Selection Tool: Img. Time

[theskyhound]

Its important to think about what the Img. Time column actually is. The purpose of this column is to tell you the maximum amount of time there will be to image the object on a single night. It looks at the time spent above the airmass you have set. But it also uses the best night of the year (see column). Using that night it also looks at how long the night is on that date (ignoring the moon). The airmass constraint is what usually matters but for your northern latitude, I believe the time between twilight is the limiting factor for summer objects.

So we should consider the possibility that it is actually working as intended, even though the results look off. This is all with the obstructed horizon turned off (please keep it off until we are sure things are working correctly).

I believe what may be happening is that the Messier list has a lot of summer Milky Way objects in it, which are best during your short nights. Many of these are extended objects that cover so many pixels (F/7?) that they are difficult to get a high SNR for, and are being filtered out because you can't image them in a useful way with the telescope and camera and filters that you are using. Those are the objects where the columns are blank.

I'd be interested to see what M42 looks like in your list. It should have a different Img. Time because it is a winter object. But before you check that be sure to disable the obstructed horizon and set the Max Airmass to a more reasonable number, like 2.0.

Maybe its time to show us your imaging system.