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Deep sky magnitude limit

This is a follow-up to your previous request #24741 "deep sky magnitude limit request"

I thought I'd try once more to see if there is any chance of changing the way the magnitude limit of deep sky objects changes as you zoom in and out. I may be the only one that is bothered by this, but it is a major annoyance in an otherwise excellent piece of software.

I use Sky Safari on an iPad Air with a 9.5" diagonal screen. I do only visual astronomy with a manual 20" Dobson scope. The issue is that I can set the field of view for one of my eyepieces, and then set the magnitude limit on DSO's to some value. This gives me an idea of what I can see at the eyepiece.

Then, if I zoom out or in, the magnitude changes dramatically, and I have to re-set it to the desired value.

I collected some data on this and it is shown in the attached image. The X-axis is the angular size of my screen (the vertical dimension, in landscape mode). The Y-axis is DSO magnitude. I start by setting the limit to +15 and the angular size to 0.25 degree. The dark blue curve shows how the magnitude limit changes as I zoom in and out. The change is exponential.

The pink line shows that the software is capable of retaining the +15 setting all the way out to a 10 degree field of view, and it only drops to +14 at 30 degrees.

The blue rectangle shows the range of field of view for my various eyepieces. I change eyepieces to get the best framing and exit pupil size for specific targets, yet I want the same magnitude limit. I also need to zoom out much further to look for other objects in the region.

Since magnitude varies on a logarithmic scale, it seems excessive to have that value change exponentially with the inverse of the field of view.

Thanks for your consideration. I still recommend Sky Safari to all my astronomy friends.


  • 0
    Bill Tschumy

    Mr Bill,

    Thanks for your analysis.  We have planned to revisit this for SkySafari 6.  I don't think anything is going to be changing for SS5.  I agree the exponential change at small FOV seems too extreme.  Tim made this algorithm a long time ago and it has been in use for probably 20 years.  He may have some reason why he is doing it this way.

  • 2
    Bill Tschumy

    Mr. Bill,

    Let me tell you what we are thinking of doing for SS6.  I was going to implement a setting next to the magnitude limit setting for Stars and DSOs.  The setting would be something like: "Faintest magnitude displayed".  You would set this to the faintest magnitude you think your scope is capable of showing.  We would never display anything fainter than that.  So if you set this to "15" for stars, you will never see anything fainter than that no matter how much you zoomed in.  It basically puts a hard limit at small fields of view.

    Would this solvate majority of your problems?

  • 0

    Please add the "Faintest magnitude displayed" feature. I think this would solve half of the problem.

    This would work when zooming in, however, when zooming out,  wouldn't it just follow the exponential curve back down?


  • 0
    Bill Tschumy

    This feature is planned.  Thanks for the feedback.

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