Hickson 88


Object: Hickson88, 4 galaxies in Aquarius.

Components, top to bottom:

Click here for a DigitalSkySurvey image, and here for the Sloan DSS image. Some catalogs refer to PGC65612 as NGC 6975, I believe this is an error.

Subimages: 23 frames x 600s (3.8 hrs) SD Mask combined after Bias and Dark calibration. 4 other frames were discarded due to changes in focus.
Scope: C11XLT with RLS65 reducer, EFL=1981mm (f/7 effective)
Mount: Losmandy G11 with Gemini L4
Camera: ST7EI (0.94"/pixel) set to -15C (first 15 frames) and -25C (last 8 frames)
Guiding: SX716 camera on a Taurus Tracker III OAG
Acquisition and Processing: MaxIm DL/CCD v4.5

Galaxy group imaged with over more than a month, 2006/08/16, 08/25, 08/30, 08/31 and 09/29. I started imaging at f/10 hoping to resolve the smaller galaxies in this group. After three nights spread over two weeks I had weak results with constantly poor seeing. Therefore I adding a focal reducer to the image train and obtained the image above over two nights. Unfortunately the telescope was now out of collimation making for some strangely shaped stars. These galaxies are quite faint and small and would really require much more integration time and good seeing to get a good image.

The above image was deconvolved with a diagonal kernel filter to remove the elongation due to miscollimation, then it was deconvolved with a Gaussian kernel (which essentially reproduces the Lucy-Richardson deconvolution routine). Finally a gamma stretch was applied (DDP with no filter). Compare this result with my previous attempts.

Below is an image from the data acquired at f/10. The image was deconvolved with LR, gamma stretched and then cropped to remove border effects from non overlapping subimages.

Subimages: 29 frames x 600s (4.8 hrs) SD Mask combined after full calibration. 26 other frames were discarded due to changes in focus, bad tracking or bad transpenrency.
Scope: C11XLT, EFL=2935mm (f/10.5 effective)
Mount: Losmandy G11 with Gemini L4
Camera: ST7EI (0.63"/pixel) set to -15C
Guiding: SX716 camera on a Taurus Tracker III OAG
Acquisition and Processing: MaxIm DL/CCD v4.5


Here is a direct comparison between the two images, the f/10 image on the right has been rescaled and rotated to match the f/7 image on th left. Both have been cropped to show the brightest three member galaxies.

In my opinion the f/10 image does contain a small amount of extra detail (e.g. near the core of NGC6976). However the lower resolution image seems to contain more faint details above the noise floor (e.g. spiral arms of NGC6976). I must conclude that when the seeing produces stars with FWHM>5.5 (3.5" at f/10) I am better off using a focal reducer to minimize read noise and gain field of view. Other objects I have imaged did respond to the extra resolution (see NGC6412 for example) but in those cases the FWHM<4 (2.8" at f/10).