Howto

How to do some common (and not so common) tasks in SharpCap.

Delete a Capture Profile

In the windows explorer address bar type in

%APPDATA%\rwg\SharpCap\CaptureProfiles

and press return. This will take you to the folder where SharpCap stores the capture profile files. Delete the profiles that you want to get rid of. Restart SharpCap. Job done.

Open SharpCap without automatically connecting to the most recently used camera

Holding down the Control key while SharpCap is starting will stop SharpCap from connecting to a camera on a one time basis. You can also turn this behaviour off completely in the Settings.

Focus on a Star using SharpCap's focusing aid

Getting focus just right can be one of the big challenges of Astrophotography. SharpCap provides three different focusing aids to help you, one of which is design for use on point sources like stars - its called FWHM measurement. FWHM stands for 'Full width half maximum'. Basically SharpCap calculates the maximum brightness of a star and then measures how far it is between half that brightness on one side of the star to half brightness on the other. The closer the star is to focus, the smaller the number will be, so you will be looking to get the smallest possible value of the FWHM value.

First, get a target star in the field of view of the camera. It doesn't need to be too bright, and indeed you don't really want it to saturate (give the maximum brightness value) the camera. Now click on the 'Magnifying glass' icon on the taskbar (right hand end) and choose 'FWHM measurement' from the drop down. What you'll notice is that a small red box will appear on the screen over the preview image. SharpCap is only going to measure inside this box, so you need to drag it over your star. You only want one star in the box, so change its size too if necessary by dragging the edges or corners.

Once you have your star nicely in the box, turn on the histogram quickly too see if star is saturated. You'll notice that the box stays on the screen - this histogram funciton is only measuring inside the box too when the box is visible. If the star is saturated then you will see a peak at the very right hand side of the histogram - if that's happening, turn the exposure or gain down until the peak goes away.

With the star brightness now adjusted, select FWHM again (the histogram will vanish to be replaced by a graph of the focus measurements). The next step is to adjust the black leve slider on the left hand side. Any pixels below the black level are ignored for the calculations and are shown with a red striped overlay inside the box. You need to set the black level so that almost all the black background around the visible star is being ignored (has red stripes). Set it so there is a black margin of just a few pixels on each side of the star that isn't in the striped area.

Finally we're ready to start focusing. The numbers on the right hand side show the current value of the FWHM and the best recorded recently. The graph shows a trace of recent values with the newest values at the right hand side. Lower values are better and they are shown in green colours on the graph, worse (higher) scores are shown in yellows, oranges and reds.

A good approach is once you have a rough focus, slowly work the focuser all the way through from a bit out of focus on one side to a bit out of focus on the other. The graph will show a shape with a dip in the middle (where you were closest to perfect focus) and the 'Best' score on the right hand side will show the lowest FWHM recorded. Now go back towards the perfect focus position, trying to match or beat the 'Best' value that you obtained in your first run through. Once you've done that, you're in focus.

On some telescopes (SCTs particularly), objects may shift in view as you adjust focus, particularly when changing movement direction on the focuser. When this occurs, you just need to drag the selection rectangle back over the new location of your target star.



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