Swansea Astronomical Society Blog: The Sombrero galaxy and some tests on a low cost Barlow

For the Sombrero galaxy, 309 x 13 second exposures were taken with AstroDMx Capture on the morning of the 9th March with aQHY5 mono camera at the prime focus of an Altair EDF60. A good deal of the frames were unusable due to cloud or drive movement, so I used DSS to automatically choose all frames above a score of 200 for stacking which resulted in some 243 frames being selected by DSS (79%) of which 50% were used in the stack. This effectively gave a 53 minute overall exposure.

Click on an image to get a closer view

Sombrero galaxy

Barlow lens trials:

I did a comparison of “with” and “without” Barlow lens image captures for a budget Barlow lens bought from the web for just £15 which was advertised as giving 5 times magnification.

The Barlow lens comes with good quality protective caps top and bottom and allows the magnifying lens to be removed by unscrewing it to provide a useful extension tube, whilst at the other end an M42 thread allows an SLR to be attached via a suitable “T” adapter. The lens comprises just a single glass unit of reasonable optical quality. The build quality of the housing is good, being made of anodised metal, coated with anti-reflective black paint.

Tests were carried out for images of a planet, a star, a galaxy and a nebula using a QHY5 mono camera and an Altair EDF 60 telescope. Data sets were captured in AstroDMx Capture using the same, or comparative exposure settings for the Barlow/non-Barlow captures, with image size comparisons made on a selection of single frame or stacked images. Where stacking was required this was done in either Autostakkert!3 or DSS with light stretch processing made in the Gimp. The side-by-side views were compiled in Photoshop with no resizing being made of the images to allow like for like image size comparisons to be made.

Results tend to suggest that the magnifying capability of this Barlow lens is far less than that advertised; not only with respect to the relative sizes of the captures being compared, but also their relative brightness’s too. To check on this, raw frame data was submitted to Astrometry.net for field of view (FOV) assessment of non-Barlow and Barlow captures. The non-Barlow capture FOV was reported as being 1,579 arcmin, whilst the Barlow came in at around 527 arcmin. This revealed that the Barlow magnified the targets captured by around three times rather than five with the test arrangement used.

Whilst this Barlow lenses’ primary role is to be a five times magnifying lens well at best this is questionable, I feel that its versatility for SLR use and its ability to convert to being just a draw tube extension makes it a worthwhile buy however. Just don’t expect huge magnifications!

Image tests:

Planet: Mars