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Using RA/Dec to help plan sessions

Astrometry Beginner Observing Visual
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#1 jlg84

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Posted 18 September 2023 - 04:45 PM

This is a *very* basic question, but I hope you'll indulge my ignorance! I am trying to make better use of my observing and photography time by planning what will be located in prime viewing/photographing locations on any given night. I would have thought there'd be some easy or at least practical way to take a look at an object's RA/Dec and determine when it will rise, when it will set, and when it will be crossing the meridian, but I cannot figure out what the method would be. Would anyone care to illuminate it for me?



#2 rutherfordt

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Posted 18 September 2023 - 04:54 PM

An object crosses your meridian when its right ascension is equal to your sidereal time-- another way of thinking of that is that the sidereal time is the same as the right ascension of objects crossing your meridian.

 

I would think two hours on either side of that time should still put your targets in a pretty good position.


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#3 jlg84

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Posted 18 September 2023 - 05:01 PM

An object crosses your meridian when its right ascension is equal to your sidereal time-- another way of thinking of that is that the sidereal time is the same as the right ascension of objects crossing your meridian.

 

I would think two hours on either side of that time should still put your targets in a pretty good position.

That is EXACTLY the sort of information I was after! That makes perfect sense to me, and is something I can now work with! Many thanks!



#4 DeepSky Di

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Posted 18 September 2023 - 05:07 PM

Astrospheric app now has a circular diagram where you can display objects of interest. Also check out Martin's Astrophotography series - here's September https://youtu.be/-lt...Uk6LlvNZY84eWGL and here's the instructions if needed: https://youtu.be/fvZJd3Q4Bq4?si=Zex6j0-kz5bhLhjE



#5 Tony Flanders

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Posted 19 September 2023 - 04:56 AM

Realistically, you should use a planetarium app that will allow you to input the object's name and will then tell you precisely the information that you want to know. There are many good, free planetarium apps, depending which electronic device you prefer to use. And also some good, cheap ones that are well worth their price.

It's possible to compute all of this from first principles, and it's worth understanding the calculation for your sake. But unless you really enjoy looking things up in trig tables, it's much more fun to let a computer do the work for you.

To expand on the excellent answer given above, sidereal time is equal to civil clock time on the September equinox (that's more or less now!) if you live on the central latitude of your time zone and if your location doesn't observe daylight-saving time. So right now at 1 a.m. -- which would be called midnight if not for daylight-saving time -- the eastern side of the Great Square of Pegasus, which lies almost spang atop the RA 0 line, is at its highest. Four hours earlier, around when the sky gets fully dark, an object at RA 0-4 = 20 (mod 24) is at its highest.

 

Every month the difference between sidereal time and civil time gets two hours longer, until after a full year it gets 12*2 = 24 hours later, is to say, back where it started.

 

So around October 20, an object at RA 2 will be at its highest around 1 a.m. And on November 20, when we will blessedly be off daylight-saving time, an object at RA 4 will be highest around midnight.

 

Since you live at mid-southern latitudes, you can figure on observing far-northern objects one hour plus or minus transit time. For objects on the celestial equator, you can reasonably stretch that to three hours plus or minus. And far-southerly objects are still high in the sky six hours before or after transit time. In the extreme case, Sigma Octantis doesn't change altitude at all.


Edited by Tony Flanders, 19 September 2023 - 05:37 AM.


#6 BlueMoon

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Posted 19 September 2023 - 05:47 AM

 

I would have thought there'd be some easy or at least practical way to take a look at an object's RA/Dec and determine when it will rise, when it will set, and when it will be crossing the meridian, but I cannot figure out what the method would be. Would anyone care to illuminate it for me?

Grab a copy of Stellarium: https://stellarium.org/ (It's free) install it and it should set itself to your location, or close to it, automatically. Click on the Astronomical Calculations icon (or press F10) then click on the WUT (What's Up Tonight) icon. It's a powerful tool to use. You can filter a huge number of different targets for observation using this feature. You can also manipulate the time scale by advancing time for minutes, hours, days by using the time controls in the control bar to see what targets will be up at certain dates and times. I use this function frequently to plan my photography targets for particular nights.

 

Clear skies.


Edited by BlueMoon, 19 September 2023 - 05:49 AM.


#7 MikiBee

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Posted 19 September 2023 - 06:50 AM

Advanced astronomy apps like Sky Safari or Stellarium do these things really well. They are an excellent tool for planning your night whether it is visual or AP.

 

However, just so we don't depend on technology 100% it would be good to know roughly what can be observed in each season. That comes slowly, through hours of observing at different seasons.



#8 rrpallechio

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Posted 19 September 2023 - 01:04 PM

Polar Scope Align Pro (PS Align Pro) has a neat little function that gives the current local sidereal time. AND, you can change the local time and it will calculate the new LST. So, for example, right now as I type it says the local time is 11:02 and the LST is 09:44. Then, if I manually set my local time to 22:02, it shows that LST will be 20:46.

 

THEN, I can go to my star charts and turn to the pages that show RA around 22 and see what is up.

 

It's probably a lot easier to use one of the apps mentioned above :)



#9 ecuador

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Posted 23 September 2023 - 06:23 AM

Polar Scope Align Pro (PS Align Pro) has a neat little function that gives the current local sidereal time. AND, you can change the local time and it will calculate the new LST. So, for example, right now as I type it says the local time is 11:02 and the LST is 09:44. Then, if I manually set my local time to 22:02, it shows that LST will be 20:46.

 

THEN, I can go to my star charts and turn to the pages that show RA around 22 and see what is up.

 

It's probably a lot easier to use one of the apps mentioned above smile.gif

I am the developer, and I can share the way I plan my sessions with the app, since I was the main target user for it, and it is especially useful for people with a limited view of the sky - I have a building on one side, trees etc.

So, as you correctly said, you can set the date/time forward, I set it to roughly the time I want to start my session. I then open the DSO DB tool, and select from the filters the type of objects I want etc (there are many types of filters to combine, like minimum altitude, magnitude etc).

 

Simulator Screenshot - iPhone 8 Plus - 2023-09-23 at 12.11.31.png

 

If you have an unlimited view of the sky and you have at least chosen to limit by a minimum altitude, you can just "Show Data" at this point to get a list of targets tailored to your requirements. But for my limited backyard I then click on sorting configure and  by distance from: -> "select by pointing". WIth this tool you hold the phone like a remote control pointing towards the general area of the night sky you have access to. Holding down the Select Point button and moving gives you a live preview of the nearest objects that match your filters on the time/date you had selected for your session:

 

Simulator Screenshot - iPhone 8 Plus - 2023-09-23 at 12.11.20.png

 

In the end you hit apply, show data, and you get the list with information on each target:

 

Simulator Screenshot - iPhone 8 Plus - 2023-09-23 at 12.25.02.png

 

Hope this helps someone, as the niche features of the app are not known to all.


Edited by ecuador, 23 September 2023 - 06:25 AM.


#10 gnowellsct

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Posted 24 September 2023 - 07:13 PM

When a constellation culminates that means it reaches its highest point on the meridian. Lugibuhl and Skiff in their book give the culminating date for every constellation.

You can also use an old fashioned planisphere to see which constellation is crossing the meridian at the date and time you set.

It is absolutely true that modern folks use software to do this and I use software too, but actually for something as simple as planning around which constellations I want to view in I use old fashioned tools.

Greg N

#11 555aaa

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Posted 24 September 2023 - 07:28 PM

The spring (vernal) equinox is when the sun is at right ascension of 0 hours. That means that at midnight, an object with a RA of 12 hrs will be overhead at midnight. That’s generally Mar 21. On June 21 an object at RA of 18 hrs will be overhead at midnight, and Sept 21 it will be 0 hrs. Dec 21 will be 6 hrs. This is true everywhere in the world. So you just have to remember that the sun is at 0 hrs in spring and from that you can figure out the rest. Over time you’ll get a feel for what RA and Dec is visible at your location in the evening when you observe.

#12 gmiller123456

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Posted 25 September 2023 - 05:02 PM

The equations for rise, set, and transit aren't exactly complicated, but developing an intuitive algorithm you can do in your head is going to require some pretty amazing mental math skills.  I find it better to work in terms of constellations.  Since there's not that many in a good position on a given month, it's a small enough amount of data to remember, and it changes slow enough that you don't have to sit down and memorize it every night.  Over the years, you'll eventually develop an intuition as to which ones will be visible, blocked by trees, or in a light polluted part of the sky.  If someone gives you RA/Dec coordinates, look up the position in some planetarium software and note where it is relative to constellations you know.



#13 rrpallechio

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Posted 25 September 2023 - 06:30 PM

I am the developer, and I can share the way I plan my sessions with the app, since I was the main target user for it, and it is especially useful for people with a limited view of the sky - I have a building on one side, trees etc.

So, as you correctly said, you can set the date/time forward, I set it to roughly the time I want to start my session. I then open the DSO DB tool, and select from the filters the type of objects I want etc (there are many types of filters to combine, like minimum altitude, magnitude etc).

 

attachicon.gif Simulator Screenshot - iPhone 8 Plus - 2023-09-23 at 12.11.31.png

 

If you have an unlimited view of the sky and you have at least chosen to limit by a minimum altitude, you can just "Show Data" at this point to get a list of targets tailored to your requirements. But for my limited backyard I then click on sorting configure and  by distance from: -> "select by pointing". WIth this tool you hold the phone like a remote control pointing towards the general area of the night sky you have access to. Holding down the Select Point button and moving gives you a live preview of the nearest objects that match your filters on the time/date you had selected for your session:

 

attachicon.gif Simulator Screenshot - iPhone 8 Plus - 2023-09-23 at 12.11.20.png

 

In the end you hit apply, show data, and you get the list with information on each target:

 

attachicon.gif Simulator Screenshot - iPhone 8 Plus - 2023-09-23 at 12.25.02.png

 

Hope this helps someone, as the niche features of the app are not known to all.

I had no idea! Thank you for the lesson!



#14 Tony Flanders

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Posted 26 September 2023 - 04:23 AM

When a constellation culminates that means it reaches its highest point on the meridian. Lugibuhl and Skiff in their book give the culminating date for every constellation.


That runs into a problem in the case of Hydra, which covers 7 hours of right ascension. But for the most part thinking in terms of constellations works fine.

It doesn't take all that long before you get a good gut sense of when any given object will be visible at your favorite observing site.

#15 ecuador

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Posted 26 September 2023 - 07:13 AM

I had no idea! Thank you for the lesson!

Anytime. I develop it for fun (it's more reliable fun than the skies where I am currently living) and for my own needs, but I do wish more people were aware of some nifty little features I have built into the app. I guess it's my fault for not having the time and the competence to do nice instructional videos etc ;)




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