ZWO first light disaster

[Rick Spencer]

Yesterday, I tested the camera (ZWO ASI290MC) in daylight, on a house chimney approx a quarter of a mile away. This is the furthest terrestrial target available to my location. It worked, and I could get focus with the focuser roughly half in/out.

I tried with it for two hours last night on my 114mm aperture, F5 (update - my mistake, should be F8.77), 1000mm FL reflector and could not even find a single star!! I checked that there were stars visible, using a 40mm EP, although they were not centred. I tried random parts of the sky, around the Plough. I went through the full range of the focuser, using settings from 1 sec to 20 secs exposure, several Gain settings between 100 - 300, with and without a 2x barlow. I also tried with APT and Sharpcap 3.2, both produced a blank dark image (with a single dead pixel), obviously varying in intensity depending on the settings used at the time).

Surely I should have seen something, even in a sparsely populated portion of sky? Any ideas what I did wrong, or failed to set?

 

Edited June 27, 2019 by Rick Spencer

[Cornelius Varley]

A couple of things spring to mind. First, the Astromaster 114 has a focal ratio of f8.77 not f5 (1000/114 = 8.77). Try a brighter target such as the moon or a planet before trying stars., the ASI290 has quite small pixels so would be more suitable for lunar/planetary imaging

[cjdawson]

Hi @Rick Spencer

 

I've got one of those cameras and have a few suggestions.....

 

1. Even though the chimney was a distance off, it's still close in terms of telescope focussing.  It's likely that you need more travel on your focusser to achieve focus, when out of focus, you will very quickly loose the ability to see the smudges of out of focus stars.... which brings me to point 2.

2. Lengthen the exposure time make it 5 seconds to start with, and see if you get anything at all.   Also you might want to combine this with my next tip.

3. Change the Gain, on the ZWO ASI290's a gain value of 110 is known as unity gain, this is supposed to be some kind of magic gain that is both sensetive and low noise.   It's worth setting that in the camera settings for night time work.

That lot should be enough to get you to be able to see something.   Hopefully you can tweak things from there.

 

One final thought, once you have that working, it's work running some tests with a program like sharpcap.  There's another value called the offset that you'll need to adjust.   To do this put a cover on the lens and take a dark frame.  The minium output from the camera is a pixel value of 8, so make sure that the minium value in the image is above this, and I mean, so that when you take 1 from the offset you get 8.  That way you'll be getting a true reading from your camera and will be able to use dark and flat frames effectively.

[david_taurus83]

Did you stretch the previews? I can take 5 minute subs that look blank until you stretch them. Especially harder if your trying to find the focus point.

[Rick Spencer]

1 hour ago, Cornelius Varley said:

First, the Astromaster 114 has a focal ratio of f8.77 not f5 (1000/114 = 8.77). 

Of course, you are correct. I just got mixed up with the other 'scope.

[Paul73]

Good luck. Stick with it. Plenty of good advice there.

Paul