Pinpoint stars...

[ryandsimmons]

Last night was the first time in ages that I have been able to get out and do some imaging.  I didn't have long, so just did some test shots of the Ring Nebula which I was quite happy with and will try for a "proper" image tomorrow.

One of the things I keep meaning to ask however, is why are my stars always so globular as opposed to pin pricks.  Is this simply a matter of focus, or Focal Ratio?

Ignore the tear drop shape (which I know was due to a rushed polar alignment) but in all my photos I always get stars that are globes.  I was just wondering if this was because of their brigtness saturating the 30 second exposures, or whether it is is just that focus is out.

I am using a Bhatinov mask for focusing.

For reference the kit I am using is:

C8 (F10) with no focal reducer

CG5 Mount

Canon EOS 1000d camera (unmodified.)

Advice as always would be greatly appreciated.

Example test shot (stacked) from last night attached.

ring by Ryan Simmons, on Flickr

[Leveye]

The focus and tracking seem just barely out to me. Your on your way though. Nice image. Keep at it!

[dph1nm]

One of the things I keep meaning to ask however, is why are my stars always so globular as opposed to pin pricks.  Is this simply a matter of focus, or Focal Ratio?

Star images have a finite size due the effects of atmospheric seeing and the resolution of the telescope. With a C8 the former is likely to be the dominant effect.   It varies from night to night but is likely to be several arcseconds, so that is going to be quite a few pixels across.

The only way to get them smaller is to image from space (or use professional adaptive optics)!

NIgelM

[ryandsimmons]

Cheers for the help as always.  Good to know.  Actually thinking about it I should have realised that the atmosphere would be affecting things.

Well tonight is supposed to be much better from where I am, so I am hoping for at least an hour of exposures to see what I can get with better alignment.

[pyrasanth]

Focus is always critical & a fraction of a turn can make the difference between a good & a brilliant photograph. Seeing conditions will spread the light as said earlier. If you invest in an electronic focuser you can then use automated focus tools like Focusmax which will generate with the use of a CCD camera a light curve which corresponds to the true position of focus by calculating the FWHM median. The difference is substantial between doing this manually & using electronic tools. Focus becomes even more critical at shorter focal ratios and I cannot satisfactory focus my Hyperstar at F2 without electronic aids. Narrow band filters can help as well as they stop the starlight saturating the CCD cells & thus prevent blooming which gives you larger stars.

The ability of your mount to track accurately can also have a bearing on whether the stars will appear sharp & the collimation of your optics particularly with catadioptric telescopes.

Temperature also has a role to play & if you take long exposure photographs the focus may change as the optics respond to temperature changes. Some filters are not fully par-focal either so again a change of filter may require you to re-focus. All these points need to be considered in your sharp star deliberations & I still don't get perfect sharp stars!