sgp autofocus and galaxies

[iwols]

just about to start down the autofocus route but just wondered how sgp handles autofocus when trying to focus on an image built mainly of galaxy than stars,cheers

[iapa]

As far as I have read It does FWHM calculations on a largish  number of stars 

galaxies are just groups of stars as fast as SGPro is concerned 

[iwols]

thanks all and what about a close up of the moon for auto focusing

[spillage]

I could be really off the mark here but was under the impression that autofocus is not an actual autofocus like using a lens on a dslr but a way of focusing the camera remotely using a motor to replace your focus wheel on the scope.

[cjdawson]

1 hour ago, spillage said:

I could be really off the mark here but was under the impression that autofocus is not an actual autofocus like using a lens on a dslr but a way of focusing the camera remotely using a motor to replace your focus wheel on the scope.

You are right that it makes use of an electric focusser.  What the software does is take an image, then run an algorigm to calculate the current focus position.  It then shift the focus a bit and repeats the operation.  It does this iteratively whilst the numbers get better - indicating better focus.  Once the number gets a little worse, it backs the focusser off a little and gets back to place of the best figure.   It's a completely automated routine.    and actually works very similar to how the autofocus on a DSLR works, only slower and it's not built directly into the camera.

[spillage]

@cjdawson Thanks for the info..just another bit of expensive kit I now want. Please god when will this madness stop.

[ollypenrice]

Stars in galaxies are likely not to be resolved so it will use field stars from the MW which also lie in the frame. It won't recognize the galaxy as a group of stars though it may just possibly find some and use them.

Can deep sky autofocus routines work on the Moon?  They normally measure the Full Width Half Max of stars. (That is the diameter of the star at the half way point between its faintest extremity and its brightest point in the core.) Scroll down to the third image on this page for a FWHM graph. https://gemini-lights.com/pages/lux-and-lumens

Daytime camera autofocus works quite differently. The two popular methods are phase detection and contrast detection. In phase detection (I think!) the camera makes a series of measurements at different focal positions, recognizes when it is either side of focus by comparing two images and calculates the optimal position in between. Contrast detection is simpler: small scale contrasts (differences in pixel values) increase as focus improves.

Olly