by Richard Crisp
There has been a bit of discussion recently on some other groups about widefield imaging with camera lenses so I decided to pull out the Pentax 150mm f/2.8 Takkumar medium format lens and adaptor I made a while back to do a bit of widefield imaging using the ST10XME again.
I was running it stopped down to f/4 and was shooting 7.5 minute subexposures using an AstroDon 50mm unmounted 6nm FWHM Ha filter.
The two objects I bagged were the widefield around the Cone/Foxfur/Christmas Tree Cluster nebulae and the region around Gum 1 which is near Thor's helmet.
The Cone/Foxfur area got ten exposures of 7.5 minutes while the Gum1 shot had to make do with only four such exposures before fog shut me down.
Both images, when processed, indicated they could use more exposure time. The imaging was a battle last night dealing with high humidity (had to use the hair dryer between shots) and threatening fog, but I did log 1.75 hours total exposure time on the two objects
Still they came out OK in my mind.
This was the first time I self-guided this setup. In the past I was using a Losmandy side by side saddle plate to mount the imaging rig with an AP80/900mm guidescope/ST7E for guiding.
This is also the first time I flat-fielded the results from the camera lens arrangement.
Like most things there's a learning curve for me using this particular setup but the results seemed good enough to show. I hope to add more data to these shots in coming days, skies permitting.
http://www.narrowbandimaging.com/gum1_pentax_ha_page.htm
http://www.narrowbandimaging.com/ngc2264_cone_foxfur_Ha_pentax_page.htm
One thing I noticed about this lens is that it is not perfectly color corrected. That meant the Astrodons weren't parfocal with this setup. But it doesn't matter because the narrowband filter only lets a small range of wavelengths through at a time.
It would only be a problem if you were to shoot a luminance frame but why bother with luminance when you have such a fast system? Just shoot R, G, B and skip the luminance if you are out to use RG&B filters.
So to me, even though it is nice to have well corrected lenses, so long as they give a flat field and you use colored filters, it doesn't really matter if they are corrected or not. The LRGB technique using binned color is really a workaround to compensate for imaging slowdown when using color filters, but if the exposure times are short anyway, then that workaround isn't needed in my mind.
Posted on sf-bay-tac Jan 13, 2005 12:55:55 PT
Converted by report.pm 1.2 Jan 27, 2005 21:23:34 PT