FIRST LIGHT NOTES

This is a first light image of M101 & Vicinity in LRGBHa using my new Teleskop Services 130/910mm APO Triplet. Overall I am impressed with this scope, particularly as it serves for me a suitable upgrade from my older model Explore Scientific 127mm APO. My ES127 was badly color-corrected, particularly in the blue portion of the spectrum, producing unsightly blue halos that were difficult to process and correct. The TS scope has a good reputation and is spoken of highly as an excellent APO for its size and price point. I ordered it back in November 2019, and it did not arrive until the end of March 2020.

I have also deployed the TSFLAT2,5, which is a 2.5" flat field corrector. I have spaced my image train at the recommended 88mm for my 910mm focal length. The stars are not satisfyingly round, rather a bit slanted at the edges of the frame and perhaps a bit triangular as well. It is possible that I will need to play around with the spacing some more.

I state that this was my first light. That's not entirely accurate as I collected several night's worth of data prior to what is shown here, but I was plagued by too many internal reflections. I fitted a Moonlite 2.5" CFL on this scope, and unfortunately the interior color of the draw tube caused some surprisingly bright reflections that were of even more concern for me in a light polluted environment. I added flocking inside the tube, and this made a notable difference. The data used in this image was taken after the flocking was added.

TARGET NOTES

M101, the Pinwheel Galaxy, is a large, face-on spiral galaxy in Ursa Major, discovered by Pierre Mechain on March 27, 1781 and soon added by Charles Messier to his catalog. It's an iconic grand design spiral, one of my favorite targets, especially for testing out new equipment. It is only 6.4 Mpc distant, hence it's large angular size of 28.8 x 26.9 arcmin, physical size of about 170 kly. The number of stars contained within M101 are on the order of 1 trillion, twice as many as in the Milky Way, about equal to that of the Andromeda Galaxy. Of note are numerous Hydrogen regions, many of which I have been able to draw out in this image with the acquisition and processing of Hydrogen-alpha data, the details of which I note below.

Of equal fascination to me as M101 are the numerous companion galaxies and background galaxies present in this image. I will list the more prominent companions:

NGC 5474 (lower left): A peculiar dwarf galaxy, its disk, with hints of spiral structure, is strongly offset from its nucleus most likely due to past interaction with M101. Close inspection will show small regions drawn out in the Ha data.

NGC 5477 (center left): A dwarf galaxy with no discernible structure.

I will not list the various independently annotated HII regions within M101, but these are annotated in Version H for those interested.

Some of the more distant galaxies are listed here:

NGC 5422 (top-right): This is obviously seen edge-on, likely S0, estimated distance averaged at 27.3 Mpc.

NGC 5485 (top-left): This is a SA0 pec galaxy, estimated average distance of 28.4 Mpc. Close inspection in my image will reveal the presence of a dust lane encircling the core.

NGC 5473 (above and left of M101): This is classified as SAB0, but not very discernible in my image, with average estimated distance of 27.3 Mpc.

I will not list the annotated PGC galaxies here as they are too numerous.

ACQUISITION AND PROCESSING NOTES

Combined with my FLI ML16200, this scope produces images with scale 1.366 arcsec/px. This is suitable scale for large galaxies like M101 and image of its vicinity. It would not be my choice for closer work on galaxy detail, however I was still able to resolve adequate detail on this galaxy and was even able to employ a modest amount of deconvolution.

This image is also my first attempt at including Ha data to LRGB combination. One thing that had kept me back from trying this was wanting to figure out how best to combine Ha to the Red channel but to do it in a more organized way that did not enhance the red where I did not think there ought to be a sizeable Ha contribution. Recently, John Hayes posted a nice image of NGC 4631, the Whale Galaxy, in which he applied processing techniques from Vicent Peris and Martin Pugh, along with a link to the technique as compiled by Edoardo Luca Radice. I encourage those interested to follow the links for more information. I'll mention a few details here.

Rather than to add the Ha data directly to the Red data, I modified my total Ha data by subtracting a scaled amount of red as follows using PixInsight's PixelMath:

Ha2 = Ha - Q*(R - med(R))

Q is scaled so that all that remains in Ha2 is the prominent Ha data of the various nebular clusters in the arms of M101 and not the broad signal seen throughout the galaxy, most prominently the core. In this manner I also singled out small HII regions within several of the satellite galaxies. Version D shows a stretched image of the original Ha channel. Version E shows the result of this scaled subtraction with just prominent HII regions remaining. You can see that most of the stars are now gone.

Having already prepared a color calibrated RGB image from the broadband R, G and B channels, I then extracted these channels once again into separate monochrome images. Again using PixelMath, I combined the RGB+Ha channels as follows:

R = R + X*(Ha2 - med(Ha2))

G = G

B = B + X*0.2*(Ha2 - med(Ha2))

The addition to blue was something I had not considered before, but the reasoning is that the Ha data can be used to express both the H-alpha and H-beta data by including the factor of 0.2 as proper spectral balance.

The X factor, as explained by Radice, allows the imager to adjust the prominence of the added Hydrogen data, and he suggests something from X = 0.2 to 0.4. In my processing, I found that I preferred a more subdued contribution, however one that is still quite prominent, with value X = 0.15. However, looking at the uploaded image here on AstroBin, I now wish I had increased X a bit more. I will leave that to a future revision.

With this RGBHa image, I performed my preferred LRGB(Ha) combination by extracting the channels in Lab space, replacing the L channel with my own L image separately processed, and then tweaking it all to final taste. A cropped image of M101 that better shows the Hydrogen regions is Revision F.

CONCLUSION

Well, that's a lot to read through, and I appreciate those who hung in with me. This was a mostly successful first light of my new TS 130/910 APO with added FF, and I performed my very first LRGBHa combination.

I hope you enjoy, and as usual your comments, critiques and suggestions are greatly appreciated!