I think it might be time to call it quits on this image. With the weather again poor, and an upcoming trip, I dispair of capturing enough colour data to finish this subject this season.
This is my first image attempting to use an ONAG on my TEC140 with an Astrophysics 2X barlow. I think I am finally getting a grip on how to use this gear but the journey has been difficult. First my pointing model for the PMX was not good enough. Then I had to solve a complicated PEC issue with TSX, and finally learn the right way to guide using the ONAG. I think the TEC140 is a bit small in aperture to be entirely effective with the ONAG. My theory is the IR light used for guiding is borderline strong enough to guide effectively on dimmer stars without going to very long exposures, and I have had great difficulty focusing the guide camera (an ST-i). I believe the difficulty in focus is also related to lack of aperture and that IR focus may be a bit of an issue with this APO. In any case the ONAG guide camera focusing tube leaves something to be desired. It may be fine for SCTs and newtonians but I think focus is too critical (especially when there isn't a lot of IR signal) to manage very well by hand. I have just ordered a Borg helical focuser that ought to solve this issue.
I tried different configurations of the barlow and ONAG. The most secure connection is with the barlow on the telescope port of the ONAG and the camera (G2-8300) screwed in directly to the t-thread of the imaging port. Unfortunately this configuration is quite difficult to use. For one there is considerable illumination fall off from centre. This fixes up ok with flats but it seems excessive. The bigger problem, however, is that I'm then guiding through the barlow. This made finding stars much harder and reduced IR light even further. So. I've had to resort to putting the barlow after the ONAG and using 2" extension tubes to the camera. I am using a Blue Fireball t-2 to 2" nosepiece adapter with 2 set screws to hold the camera + barlow. Not ideal at all, but CCDInspector tells me the alignment isn't bad. Sadly the AP barlow does not have threads on the telescope side so there isn't (as far as I can tell) any other solution. AP indicates the next generation of the 2X barlow will have threads. The other major issue was that there was no way to use my IDAS 2" LP filter. If I attach it to the telescope port of the ONAG there would be no IR light for guiding, and without threads on the barlow it seemed there was no solution. However, I accidentally discovered that the 2" filter will screw in on the camera side of the 2" nosepiece adapter of the moravian camera. Obviously then the adapter would not fit on the camera any longer. I solved that problem by building a stand off sleeve out of 6mm aluminium. It works great! (not shown in the photo)
This image is 4 hr worth of 5 min subs over a period of May 6 - June 13 with the camera in many different configurations with and without the IDAS. The image scale varied from .54 to .575 arcsec due to the spacing of the barlow. I was surprised at how easily CCDStack combined all these disparate images! The stars are not perfect. Some is just poor guiding as I really struggled with this up until recently. Other distortion may be the barlow, though I've cropped off the corners which were not very good. I tried not to push this data very hard. I used decon in CCDStack, hipass and soft light and Noise Ninja in CS5. Hopefully I can get some colour data sometime, and I'd like to replace most of the stars with shorter subs and better focus. These are not as good as I want, but it's all I've got.
Thanks for looking!
Peter
http://www.pbase.com/prejto/image/151028296/original
smaller:
http://www.pbase.com/prejto/image/151028296/large
ONAG Setup:
http://www.pbase.com/prejto/image/151028536
