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Exposure Calculator for RGB Stars for Dual Band Image

I use Optolong L Extreme dual band filter which works great for the nebulas, but would like to start collecting full color stars using my lum filter for my image.   I then plan to add the full color stars into my images.

I often use the exposure calculator when planning a session for a nebula which works great.  Is there a way to have the calculator while pointing at the same object calculate just on the stars?     I see I can change expose for target object to stars but I am not sure if that takes into account the brighter starts in the FOV as I do not want to saturate them.

Or am I going about this all wrong?

Thank you for any advice

Hi Shawn,

If you select 'Star' as the Target Object in the Exposure Calculator, I believe that ST4i will indicate which stars may saturate, bloom, etc. given your selected exposure time, filter, etc. See the bottom right section of the EC dialog for this information.

Is this what you're looking for? If not, could you be more specific about the target nebula & star brightness? There's not a lot to go on here  Wink.

Hope this helps,

Phil S.

Phil is correct that if you switch to Expose for: Star, it will display the Faintest Star, Stellar Bloom magnitude, and the Brightest practical star. For example:

Faintest star: 23.2V
Stellar bloom magnitude: 9.2V
Brightest practical star (linear/not clipped): 14.9V

These calculations are independent of the spectral type and magnitude you enter for the star under Expose For. But they do depend on the sub exposure time being used for the exposures. Shorter sub exposure times will allow for brighter stars without clipping/blooming. What the above says is that stars brighter than about 9.2 magnitude will begin to bloom, and the brightest practical star is magnitude 14.9. This is given the sub exposure time being used to compute the results.

Note that when you use the Scheduler to schedule your project, you can right-click on the schedule one the right and click Report to generate a report for the specific observations of a project. That report also tells you the magnitude at which stars bloom and when they become non-linear (practical limit). This will be for the exact time and conditions on the Scheduler.

BTW there is an upcoming version 4.1 that will have expanded information available when selecting Exposure Goals for an Imaging Project that will help with this kind of decision.
Clear skies,
Head Dude at Skyhound
Thank you both, that helps,  I was not sure what to pick or do with "spectral type and magnitude".

So after setting "Expose for" to Stars and entering in my settings and tweaking the exposure time to about 15 s I came up with

Faintest star: 15.4V
Stellar bloom magnitude: 9.4V
Brightest practical star (linear/not clipped): 9.8V

Other than going to the interactive atlas and clicking on the brighter stars, is there better way to know how many stars in a given FOV will be impacted by blooming and clipping so I can compare the impact of different exposure times?

Hi Shawn,

Here's one way to find the magnitudes of the stars in your FOV. Open the Interactive Atlas & set the View options to label the star magnitudes in the desired range. I tried the open cluster M 38 as an example because you didn't specify a target object. There are many stars in your range of interest.

You can label all of the stars as shown here:     

If that's overkill, try selecting the 'Very Dense' option as shown here:     

There's probably a setting that will work better for your actual target object. You mentioned a nebula, but I figured stars are stars for this purpose. 

Hope this helps, if not ask some more questions  Wink

Phil S.

Thank you, that worked great and allows me to know the magnitude of the stars around the Bubble Nebula.   Way better than double clicking individual stars to get the info like I was doing.

Yea, that would get old quick & then trying to remember the magnitudes after you'd clicked on them. This is much more efficient. Tightening the zoom setting will alllow you to get magnitudes of fainter stars too. Just be sure that you've ckecked their catalog designations or they might not get marked.

You could also hover the cursor over each star & look in the bottom left of the IA to check the object's brightness. No need to click really.

Phil S.

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