Grouping individual targets eg Leo Triplets

[astrobrandx]

Is there a way to have a group label for targets in a target list which may be imaged together.  For example I have added M65, M66 and NGC 3628 separately in a target list but together they are commonly known as Leo Triple or Triplets and are often imaged together with a suitable set up.  Another example would be M81 and M82 which are often imaged together.

[theskyhound]

Is there a way to have a group label for targets in a target list which may be imaged together.  For example I have added M65, M66 and NGC 3628 separately in a target list but together they are commonly known as Leo Triple or Triplets and are often imaged together with a suitable set up.  Another example would be M81 and M82 which are often imaged together.

Hello. There are many possible ways to answer your question, including that most prominent galaxy groups/clusters do have catalog designations. Examples range from the Virgo cluster to Seyfert's Sextet. The Leo Triplet hasn't been cataloged as a galaxy cluster because its just a grouping of three bright galaxies and the astronomers who compiled the galaxy group catalogs didn't find them useful to include. You could make your own Leo Triplet database entry using the Supplementary Deep Sky feature, accessed via the Data menu. But that just gets you a position in the sky with a label. 

Here's the thing: you can't do exposure calculations on a position in the sky. So better to choose one of the galaxies as your target and expose for that one. When you create a project, drag the FOV on the Composition tab to include all three galaxies.

[astrobrandx]

Is there a way to have a group label for targets in a target list which may be imaged together.  For example I have added M65, M66 and NGC 3628 separately in a target list but together they are commonly known as Leo Triple or Triplets and are often imaged together with a suitable set up.  Another example would be M81 and M82 which are often imaged together.

Hello. There are many possible ways to answer your question, including that most prominent galaxy groups/clusters do have catalog designations. Examples range from the Virgo cluster to Seyfert's Sextet. The Leo Triplet hasn't been cataloged as a galaxy cluster because its just a grouping of three bright galaxies and the astronomers who compiled the galaxy group catalogs didn't find them useful to include. You could make your own Leo Triplet database entry using the Supplementary Deep Sky feature, accessed via the Data menu. But that just gets you a position in the sky with a label. 

Here's the thing: you can't do exposure calculations on a position in the sky. So better to choose one of the galaxies as your target and expose for that one. When you create a project, drag the FOV on the Composition tab to include all three galaxies.

Thanks Greg - very helpful.  So in the case of something like Horsehead Nebula and Flame Nebula within the same field of view how would ST4 treat that? Two very different target types being imaged together? What would be your recommendation when creating an observing project, sub exposures etc? Cheers, Les

[theskyhound]

Is there a way to have a group label for targets in a target list which may be imaged together.  For example I have added M65, M66 and NGC 3628 separately in a target list but together they are commonly known as Leo Triple or Triplets and are often imaged together with a suitable set up.  Another example would be M81 and M82 which are often imaged together.

Hello. There are many possible ways to answer your question, including that most prominent galaxy groups/clusters do have catalog designations. Examples range from the Virgo cluster to Seyfert's Sextet. The Leo Triplet hasn't been cataloged as a galaxy cluster because its just a grouping of three bright galaxies and the astronomers who compiled the galaxy group catalogs didn't find them useful to include. You could make your own Leo Triplet database entry using the Supplementary Deep Sky feature, accessed via the Data menu. But that just gets you a position in the sky with a label. 

Here's the thing: you can't do exposure calculations on a position in the sky. So better to choose one of the galaxies as your target and expose for that one. When you create a project, drag the FOV on the Composition tab to include all three galaxies.

Thanks Greg - very helpful.  So in the case of something like Horsehead Nebula and Flame Nebula within the same field of view how would ST4 treat that? Two very different target types being imaged together? What would be your recommendation when creating an observing project, sub exposures etc? Cheers, Les

To plan ahead for a complex image, you want to think about the final result. What do you want the stars to look like? Is it possible that the bright part of the Flame Nebula might actually saturate the detector? Maybe your goal is high SNR in the faint detail in the outer regions of the nebula. In that case, focus on the quality you want for that signal. 

The overall goal is to avoid spending a lot of time making test images under a clear dark sky.

What things do we want to determine going in:

Filter selection: narrow band? RGB? Mono?
Sub exposure time
Binning
Composition
Total Exposure time to obtain the quality you want

There are two tools in particular you might find useful for selecting filters and considering total exposure time: the Smart Project Builder, and the Advanced Exposure Calculator. You can't expose for the whole scene at once, but what you can do is break it into parts. Start with the Smart Project Builder. Try IC 434 as the target (remember the Horsehead is technically a dark nebula, so use the IC number). Then try the Flame. There are also some reflection nebulae in there, so it can't hurt to have a look at NGC 2023. This tool can allow you get a good idea of how many hours of exposing you would need to get the quality of image you want. Based on that, it can help you decide what filters you want to use. Broadband filters will usually take a lot less total exposure time, for example.

The Advanced Exposure Calculator can help you with the same things; but for one part of each object at a time. Its particularity good at stars, and determining the exposure time that will cause them to saturate. Imaging systems respond to saturation very differently, so knowing what you can tolerate is key. You can enter the brighter stars in the field, and try different exposure times. Sometimes this will tell you that you want to keep the sub exposure time below a certain time. 

The rule of thumb on sub exposure time is this: longer is always better, so you want to expose as long as is practical. Some people are confused by this, because "practical" can mean just about anything. Something will limit your exposure time, but its not going to be light pollution, For example, in practice there are limits to how long we can exposure due to mount alignment or drift, airplane lights, satellites, too much signal (the sky itself can be too bright in some rare instances), and how the bright stars will look, particularly in multiple filters. If a star saturates in one filter but not another, it can make it very difficult to process into a pleasing color composite.

Anyhow, these are some ideas for how to think about a complex scene like this. Break into smaller parts, and explore each one, then try to make decisions that will best lead to the result you want in the amount of time you have available. Experiment with things like sub exposure time and binning.