wimvb

Diffraction spikes are multi coloured because light of different wavelengths are diffracted to different positions. This is most notable in narrowband images. Have a close look at the brightest blue stars in this image https://www.astrobin.com/full/6x98s9/0/

This is a bit of an experiment. The discussion that followed my posting of an image a few days ago, brought a very faint target to my attention (thanks @AKB ) . Galaxy ngc 3359 has a small and very faint companion, called the Little Cub (yes, that's the name it actually has in the catalogues). The Little Cub is a blue compact galaxy, also described as an Extremely Metal-poor Star-forming Galaxy (EMPG) in the constellation Ursa Major (Great Bear, hence the name Little Cub). To quote from the original paper reporting the discovery: "[...] making the Little Cub one of the lowest-metallicity star-forming galaxies currently known in the local universe. The Little Cub appears to be a companion of the spiral galaxy NGC 3359 and shows evidence of gas stripping. We may therefore be witnessing the quenching of a near-pristine galaxy as it makes its first passage about a Milky Way–like galaxy." https://iopscience.iop.org/article/10.3847/2041-8213/aa821f Little Cub has a gravitational interaction with ngc 3359 and is slowly being devoured by this Milky Way class galaxy. Little Cub has a surprisingly low ratio of Metallic atoms (atomic number larger than 2), meaning that it consists mainly of Hydrogen and Helium, the first two elements that formed in the young Universe. These properties make Little Cub an interesting research subject. But it is a difficult little rascal to image. Here's my one night attempt (4 hours Luminance and 4 hours RGB) at catching this dwarf galaxy. Do you see it? No it's not the Big'un at the left, nor any of the small spirals. This image (inverted L) zooms in on it. Like I said, a difficult little rascal. According to a Swedish expert, the small star inside the circle (lower half) is a magnitude 21.9 red star, and objects down to magnitude 23 can be resolved. Capture data: Telescope/Camera: SkyWatcher 190MN and ZWO ASI294MM with Optolong LRGB filters 80 x 3 min L frames at 0 gain, 4 hours 20 x 4 min RGB frames per channel, 4 hours When the moon is out of the way, I plan to add more LRGB and a few hours of Ha.

Not so much of a whisper now anymore, eh? 10 times more data (70 x 3 minutes, 3.5 hrs). There are more features emerging below the large galaxy. Just a few minutes ago, my mount did a successful meridian flip and I am now collecting 4 hours of RGB data as well.

Wow

@AKB Despite the moon getting awfully bright, I've pointed my scope in the direction of this galaxy (ngc 3359) and am collecting LRGB data, the "old fashioned way". Here's the very preliminary result of 7 x 3 minutes L subs. I will post the final image in due course in a separate thread, but since we were having a discussion about it, I thought I would post a teaser here. 😉 Detail of "the cub", or Blue Compact Galaxy SDSS J104442.66+630602.0

As I see it, the guide graph is in pixels, but we don't see the Y-scale anyway. The RMS figure in the screen grab shows total RMS = 0.2 (px) but also 0.55". So, unless the OP uses the wrong settings, guiding should be ok. Total guiding RMS is 0.2 px, DEC guiding RMS is 0.14 px and if RA and DEC are fairly orthogonal, RA guiding should also be about 0.14 px. But I agree that short unguided images are best to evaluate star shapes.

I use the blink process to weed among images. Then I let subframe selector assign a weight to each image, deprnding on star fwhm and eccentricity. I do this for lum but seldom for rgb. Seems to work well.

Your report on ngc 3359 looks interesting enough. I may have a go at it and try to catch the BCG ("blue cub"), depending on the weather the coming month or two. But I'll use traditional AP for it. To fill the field of my telescope/camera, I will frame it with a distant galaxy group http://aladin.unistra.fr/AladinLite/?target=10 44 29.645%2B63 19 25.67&fov=1.71&survey=P%2FDSS2%2Fcolor As far as EEA/EEVA is concerned, Ekos has very little support for live stacking. But I'm always interested in trying new techniques. As long as they are compatible with my current setup.

Haven't I already?

One hint would be that this is the deep sky section, not the Lounge. 😉

Time to put your 190MN on the mount perhaps? 🙂

Not me. 😉

This is just a little fun trial I did before my main object cleared the observatory wall. I pointed my telescope at LDN 1388, a dark nebula outside the galactic plane. One might wonder how one would go about detecting a dark nebula in this region at all, but apparently somebody did. Anyway, I spent 15 minutes in this area and collected 5 x 3 mins L subs. Calibrated, stacked and stretched in PixInsight (histogram transformation and curves transformation). Then plate solved and annotated. It seems you can catch quite a lot in just 15 minutes, including quasars and galaxies that are 1.5 billion light years distant. Just not dark nebulae outside the galactic plane.

This afternoon I had a discussion with Göran ( @gorann) about finding extended objects (ic. supernova remnants) in the online service Aladin ( https://aladin.u-strasbg.fr/AladinLite/ ). I think that this information may be relevant to more people, so here's a short write up. The easiest/fastest way to browse catalogues and find objects is (for me at least) to just open Aladin Lite, write an object ID into the Target box on the left, for example M51, and press enter. I then activate the SIMBAD catalogue (right hand side of the sky map), which shows a small marker around all the objects in the SIMBAD catalogue. Zooming in or out is easy with the mouse wheel. I use the bullseye tool at the left of the sky map to get more information on any object. Just click on it and move it to any marker and click again (don't move over another control, because this will inactivate the bullseye tool). Clicking on the object name will open an information page with a summary from SIMBAD. Usually this is all I need. On a few occasions, I can't find the information I want, and I have to go into the database VizieR ( https://vizier.cds.unistra.fr/viz-bin/VizieR ). The link to VizieR is at the top of the Aladin page. VizieR is a catalogue service, which provides a library of on line astronomical catalogues. Today I wanted to find information about Supernova remnants. There are three drop down lists just over the orange/red sky map symbol. I scrolled down the one on the right and chose SuperNovae_Remnants. Then I entered the approximate RA and DEC coordinates (J2000) plus a search radius, and pressed the button "Find Catalogues". The search gave a list of 11 catalogues. The latest catalogue by Green (VII/284, from 2019) looked the most promising, so I continued with that. Note the Search Criteria box on the left. My initial coordinates are still active. The catalogue is activated with my initial search criteria. I just hit the Submit button at the bottom of the page. The search gave me one single hit. I clicked on the object identifier (SNR column) to get to an information page about this supernova remnant. This opened a new window with several interesting references, all available on line.

Thanks, @Sp@ce_d. The galaxy is still a late night object, and new data will probably have to wait until after the coming full moon.

Just 4 hours of RGB data with minimal processing. But I did create a synthetic luminance for deconvolution. I plan to add at least 4 hours more of L and Ha before I'm satisfied. As usual: imaged with my SW 190MN and ASI294MM. 20 subs each of 4 minutes R, G and B. processed in PixInsight.

ngc 2964, 2968 and 2970 form a small galaxy group in Leo. The two smaller of these galaxies are seemingly joined by a tidal stream. It is unclear whether this stream is the result of an interaction betwen ngc 2968 and ngc 2970 or if ngc2968 in the past has merged with a fourth galaxy. The dust band that crosses the galaxy might confirm this latter possibility, because its velocity differs from that of the galaxy itself, and it may therefore be the remnant of a galaxy merger. The galaxy group is located approximately 60 million light years from the Milky Way in the upper part of the constellation Leo. Also in this image are various truly faint fuzzies. The most distant galaxy (towards the upper left corner) is approximately 8 billion light years from us. Acquisition details: Telescope: SkyWatcher 190MN on a SkyWatcher AZ-EQ6 mount Camera: ZWO ASI294MM with Optolong 31mm LRGB, and Baader 7 nm Ha filters Exposuretime: Ha: 320 min, RGB: 520 min, L: 528 min (total integration time just below 23 hours) Processed in PixInsight. Inverted annotated luminance image (with reflections from two nearby stars). Quasi Stellar Objects (QSO) and active galactic nuclei are indicated in red, with their redshift (from Vizier). Distant galaxies are indicated by their LEDA/SDSS identifier.

Beltingonline.com is your best friend for these matters. The belts are T2.5 if I recall correctly. It's good to have a spare belt or two, just in case. You might also want to check why there was a lot of swarf.

Even in Bortle 4 (and Bortle 2 for that matter) you still have "light pollution", just not man made. Recently we've had spectacular light pollution, aka Aurora Borealis. 😁 In all seriousness, very weak Aurora acts as light pollution and can give a green cast to images. You also have the sensitivity curve of your camera to consider. Most sensors are more sensitive to green than to blue, and will never give a truely neutral background. Btw, here's an old article showing how to apply masked stretch in PhotoShop/Gimp (p 86) https://rasc.ca/sites/default/files/publications/JRASC-2012-04-hr.pdf https://rasc.ca/jrasc-2012-04

The 10 s period is caused by a belt that is too loose. You just need to remove the RA belt cover and tighten the belt a little. I had the same with my AZ-EQ6 and this fixed it. Btw, have you tried guiding without PPEC (does the EQ6-R have mount stored PPEC?). The Predictive PEC guiding algorithm in PHD, together with multi-star guiding usually gives good results.

What is the colour of the core in the stacked but not stretched image? Does the colour change when you do colour calibration or when you remove any light pollution? What Steve ( @teoria_del_big_bang) is referring to is called masked stretch. In essence, you apply a mild stretch to the image, extract luminance and use that inverted as a mask to the image when you apply a second mild stretch. This can be repeated a couple of times. Pixinsight has a process called masked stretch, which applies this procedure up to 100 times to the image. It keeps colour in the bright areas but at the cost of producing a flat image.

I reckon he's not an astrophotographer? 😉

If you use blink on the images before image registration, then you shouldn't see the stars drifting in any one direction. If they do, then your dither step is too smal. You can use a few layers of white cloth (t-shirt) over the scope to dim the light from the flat panel. Try to get an exposure time longer than 0.1 s, preferrably close to 1 s.

The bright edge and dust motes indicate that your flats are over correcting. This is most likely caused by faulty calibration of the flats, or by bad darks. The flat exposure time of only 6 millisconds is very short. I suggest that you reshoot the flats with a longer exposure time (cover the scope with white fabric or paper). As for your calibration/stacking process, try with the wbpp script. Adam Block has made an excellent tutorial about this. Available on Youtube. The second issue is walking noise. You avoid it by dithering between exposures. Make sure you use a large enough dither step. Btw, with the ASI533 and the 150PDS, you don't gain anything by drizzle integration. Your image pixel scale is 1"/p (3.76*206/750). Drizzle works on undersampled images, which you most likely don't have.

What you need is pixinsight subframe selector