Preview screen question

[Johnboy]

Hi folks,

Last year I bought a Celestron NexImage Solar System camera which I have enjoyed using to capture the moon and planets. I use Sharpcap to view the images and any changes I make to exposure or focus are instantly updated and displayed on my laptop. Because I enjoyed this so much I decided to buy a CCD that allowed me to image some of the brighter DSO's. The camera I bought came with ISCapture software. The problem I have is that when I pointed the camera at M42 I could see the image on my laptop, but any changes I made to exposure or focus were not immediately updated. I had to wait around three minutes for the screen to update and show any changes to the image. For instance if I tweaked the gain I would then have to wait about three minutes to see what changes had been made to the image. Once it updated and I tweaked the gain again I would have to wait another few minutes for the screen to update. I tried two different laptops (one using Win XP and the other with Win 7) and both were the same. Is this normal for DSO's or should the preview screen update instantly in real time like it does with planets and the moon? I felt it was impossible to take any images due to the length of time I waited for the preview screen to update between exposure and focus changes.

Any help would be great.

Thanks.

John

[Freddie]

What duration exposure were you using? The screen should update at the same rate as the exposure duration.

[Johnboy]

Hi Freddie,

The shop where I bought if from recommended I start with an exposure of about 30 seconds as I am currently using a fork mount without a wedge. I assume then making small changes to the exposure and focus will be a long job if I am waiting at least 30 seconds each time. Is there an easier or quicker way?

Thanks for your reply.

Best wishes,

John

[Johnboy]

Hi folks,

Would it be a case of pointing the scope at a DSO, getting a picture on the laptop, setting the exposure time and then taking a picture regardless of what it looks like on the screen? Then repeat this procedure for the amount of images I want to take? Presumably it would be over to DSS and PS then for processing?

It looks like the instant change in display is for brighter objects where the exposure time is shorter.

Cheers,

John

[Freddie]

You can use a shorter exposure to get focus on a bright star so that the screen updates quickly each time you make an adjustement. Once you are happy you are in focus and have the DSO in the correct position on the chip, start to capture images. You may want to check the first few to ensure you have the correct exposure length to avoid elongated stars if your tracking is not spot on for long exposures.

[Johnboy]

Brilliant Freddie, thank you. For some reason I didn't realise the update to the preview screen was connected to exposure length. I thought they were separate things.

Thanks once again.

John