Mosaic with different exposure lengths

[Atacamallama]

Greetings all,

I am gearing up for a project: an LRGB (poss +HA) of the Andromeda Galaxy. FWIW my gear is: AltairAstro Altair Wave 102 F7 Triplet Atik460 Mono operating in Spain remotely.

My question is, for the core of the galaxy I am presumably going to need a shorter exposure than for the rest of the object. How do I accomodate this when doing a mosaic? Would it be best to get the whole core in one frame and use a lower exposure for that. Or if the core is split over more than one frame, with some frames containing Core + outlying areas - how should that be acommodated? I have attached a putative capture screen (from SGP) if that gives any ideas on what I should do.

TIA for any input

 

[angryowl]

You don't mention which processing software you are using but I'll share my method of imaging high dynamic range targets via mosaics.

I take my long exposures the same length and integration time for all of the mosaic frames then only take short exposures for the core of the galaxy. If you happen to have most of the core in one frame, like I did, I just took short exposures for that frame, but if you plan on having the core in two or more frames I can't provide any advice as I haven't tried it. I suppose for simplicity if you can try and stick to having all of the core in one frame.

I then pre process and stack my two separate long and short exposure subs in PixInsight, then use the HDRComposition process in PI to replace the core area of the galaxy in the long exposure image with the short exposure image. It usually does a great job with the default settings. This will then produce a 64 bit HDR image which I then use with StarAlignment and GradientMergeMosaic processes to create the final mosaic image. 

I wouldn't know the procedure using any other software but I hope this gives you an idea of the workflow.

[angryowl]

Here's a sample of an unfinished 9 panel mosaic which is not yet processed past the GMM stage and slightly stretched to show the core area of the galaxy.

For the centre frame I used 30 30s and 21 600s exposures.

[michael.h.f.wilkinson]

Actually, I am not sure M31 needs that much in the way of variation in exposure time, the dynamic range is not nearly as large as that of M42. I have found that combining different exposure times in Astro Pixel Processor is very easy. I haven't tried in on mosaics, however.

[Atacamallama]

Many thanks for the input, and I can confirm that I shall be using Pixinsight. Looks like I can make things most simple for myself by putting the core in one frame.

 

[ollypenrice]

Like Michael, I'm not convinced that short exposures are necessary in shooting Andromeda. In the end I did extract a tiny bit more core detail by shooting it in a TEC140 when the main image was at lower resolution in a Tak 106 of half the focal length. The problem with short subs is that they are less bright but don't seem to contain more of the faintest dark features.

I can't say how you'd add the short subs from a single panel in Pixinsight but in Photoshop it would be routine after the short set had been cropped and padded to fit the main image in Registar.

Olly

[coatesg]

I'd run at the same exposure length across the whole galaxy, exposing to ensure sky background >> read noise as normal. When you're looking at the pane(s) with the core, if the core is saturated, then you could always take a quick series of shorter (5-10x shorter) exposures and then do an HDR Composition to merge the shorter exposure in place of the saturated regions. It wouldn't proabbly take long to take those extra exposures if they are needed.