My brand new 12nm H-alpha filter arrived this week and I was anxious to try it out. What more of a true test than trying to image during a weekend full moon. This was also the second night out using my piggy-backed guide scope and ASI120. After some initial clouds passed, transparency was great, and seeing was probably close to the best it has been all Winter.
I couldn’t guide to save my life. No adjustment of the gain or gamma in PHD2 gave me anything better than a gray screen with slightly lighter gray stars. My graph was jumping through a range of about eight arc-seconds every exposure. In about five-and-a-half hours of attempted imaging, I got exactly 45 minutes of usable data. I thought I got a lemon camera, or I just couldn’t do it during the almost full moon, nullifying one of the reasons I purcahsed the H-alpha filter.
Setting up the following night, ready for frustration, I had a cartoon light bulb explode over my head. My OTA is white, and my piggybacked guide scope setup sits a little further back to help with balancing on the center of the OTA. I thought the moon might be reflecting light off the OTA and effectively shining a flashlight into the guide scope.
Two simple and cheap ideas came to mind. I had what I needed on hand.
- Using a 63-58mm step-down ring, I attached a lens hood that was thrown in with a cheap 75-300mm lens to the end of the guide scope. I’m not losing any aperture or introducing any vignetting due to the small sensor on the ASI120 guide camera.
- Directly in front of the guide scope, I attached a piece of black felt to the OTA for light absorption.
Here is a section of the resulting guiding graph. Remember, this is on an LXD75 mount. I set up a little less than three hours ago, and I have a combined two hours worth of good data between two targets, including waiting 15 minutes or so for a patch of clouds to pass by.
In this hobby, it’s the little things that can make a great looking night a complete waste of time. We’re dealing with tolerances so tight there is no room for error. Two quick fixes that took me no more than five minutes with a cost of zero dollars allowed me to go 45 straight exposures that all look to be good data. The only thing that broke that streak was my guide star running into a tree branch. That blows away anything I’ve done since I started at this hobby again.
We can go through our checklists, and check everything twice, but we’re consistently thrown curve-balls and forever learning in this hobby. Make your setup and configuration as repeatable as possible so the little things stick out like an airplane going through your five minute exposure.
Clear Skies, Bleary Eyes – KA