Focusing issue

[Tony1967]

Hi everyone. 

I have I meade lx850 14"sct which I have just started  using  for astrophotography,  I use a bahtinov mask to do my focusing when I get the central spikes centred my images are slightly out of focus . Anyone got any ideas how to overcome this issue. 

Thanks

[ollypenrice]

5 hours ago, Tony1967 said:

Hi everyone. 

I have I meade lx850 14"sct which I have just started  using  for astrophotography,  I use a bahtinov mask to do my focusing when I get the central spikes centred my images are slightly out of focus . Anyone got any ideas how to overcome this issue. 

Thanks

Let's check a few things. The most important is that you are are using the camera's images of the B-mask diffraction spikes to focus? In other words, you're going straight from the B-mask through the camera to the images through the camera? No eyepieces in the story?

Olly

Edited September 3, 2020 by ollypenrice

[Tony1967]

I'm not using  an eyepiece, I'm using my asi 533 camera and live view in apt to do my focusing. 

[newbie alert]

Is it collimated?

Are you storing in the house and using it straight away.. a 14inch is a huge scope, it may not even reach ambient on a hot summer eve..

[Tony1967]

It's permanently set up in my observatory. 

[michael8554]

Are you saying your long exposures are out of focus, or the live view with the mask removed ?

Do you have a zero image shift focuser in the setup ?

Do you have the mirror locked and only use the above to focus ?

Are you focusing on a star near your target ?

With these big SCTs you have to avoid main mirror movement when you make big moves in Dec, which will affect focus.

And long exposures may be bloated due to guiding inaccuracies.

Michael

[Tony1967]

The collimation is ok. My images are out of focus,  I've  locked the mirror up and I'm only using the zero image shift focuser to focus,  my test exposures are 25seconds but when I view my images and zoom in everything is slightly out of focus,  I've been  using vega to focus then I went over to the andromada galaxy took a couple of images when I checked them they were out of focus. 

Edited September 4, 2020 by Tony1967
Spelling mistakes

[michael8554]

I think you'll have to post a typical sub.

You are considerably oversampling at 0.22 arcsecs/pixel, which can lead to bloated stars.

And any exposure over a about 5 seconds at 3556mm focal length is going to exhibit guiding errors - is your guiding error only 0.22 arcsecs ?

Michael

[Tony1967]

Thanks for the reply. Don't know what the guiding error is, I don't get to use the scope much due to cloud cover, so grab few shots  when i can and see how they turn out and make a few adjustments , done a few 10 second subs last night before the clouds rolled in some looked ok others were a bit bloated