Rings for atlas?

[leo82]

Hi all,

As I’ve now received my copy of Cambridge double star atlas I want to make a ring to help me learn to star hop, I can’t seem to find much info online, so I turn to you. As I use a red dot finder, my actual finder scope is the standard 25mm skywatcher eyepiece, how many degrees is this circle? If it’s say for example 5 degrees, how do I draw a 5 degree circle? Sorry, maths was never my strong point. 

Many thanks.

[Dave In Vermont]

I'm not quite clear on the task. If your copy of the Cambridge Double Star Atlas is a computerized edition - which I have - you could align Polaris (using the star, Polaris, is best as it doesn't move - much) in your red-dot finder and open the page in your atlas to Polaris. Then overlay a reticle on the star-chart and adjust until the reticle encircles what you are seeing live in the finder.

Here's a website with quite a few very useful downloads (free) of astronomy-software, including a reticle you can fit on top of a map from the Cambrdge DS Atlas and position and size to your desired size and colour. Or any other star-charts you can use - like Stallarium:

https://davesastrotools.weebly.com/

This website and free astro-software programs are by David Lloyd-Jones - a great guy for having done this for all us 'astro-nuts!' I use them all here and there.

Hope this helps,

Dave

 

[leo82]

My copy is the book, here is a picture to help show what I mean. The field of view for the 9x50 finderscope is 5 degrees so the person has made a circle to show what is in the finder onto the map to allow for easier starhopping

[Ricochet]

To estimate the TrueFoV of your telescope eyepiece combination you need to know the ApparentFoV of the eyepiece as well as the focal lengths of the telescope and eyepiece. Assuming you are using the Heritage 130p in your signature then we can estimate with:

AFoV = 50°
FLe = 25mm
FLt = 650mm

TFoV ~ FLe x AFoV / FLt
TFoV ~ 25 x 50 / 650
TFoV ~ 1.92°

You will have to look at the scales on your atlas to work out large a circle will equate to a 1.92° field. However, 1.92° is really quite small for a finder and you may struggle. a 24mm 68° or 32mm 50° eyepiece is as wide as your focuser will allow and will give you 2.5° of sky. I don't know what everyone else with this scope does but I would probably try bolting a 9x50 RACI somewhere on the solid part of the tube to give 5°.

[Dave In Vermont]

Stellarium can be programmed to give you the view you'll see in many common telescopes and eyepiece combinations. And Stellarium is free and will print-out star-charts as desired. From such data, you could make a tool to use with your CDSA.

Have fun up there -

Dave

[symmetal]

On your star atlas, measure the distance with a ruler between two lines of declination along a line of RA. Using your example above the length of the red line corresponds to a 10 degree FOV distance. (30 deg DEC - 20 deg DEC). So the diameter of a 5 degree FOV circle is half of the length of the red line. Just like the  diameter of the 5 degree circle tool in your picture is half the length of the red line (within manufacturing accuracy ). Correspondingly, the 1 degree FOV tool diameter is one tenth the length of the red line.

Hope that's helps with your star hopping.

Alan

[leo82]

Thanks for the replies especially the calculations and visual reference it all seems a lot clearer now

[skfboiler]

For folks that are trying to learn how to star hop, I am that glad you brought this topic up about making rings to match the FOV of the finders scope.  For me, star hopping did not click in my head until I made such a ring.  During the day I can practice the star hops with my star chart of the objects I plan to see that night.  Sure enough the star field inside the ring on my star chart is pretty close to the star field at the finders scope.  Just keep in mind the type of finder that you have, a straight through finder or a right angle, correct image (RACI) finder.  If you have a straight through finder that does not correct, turn your star chart upside down.  Now, the star hop movements with the ring on the chart will match the star hop movements at the finders scope.  The link below really helped me learn how to star hop with my Orion Astroview 6 EQ.

http://www.washedoutastronomy.com/washedoutastronomy.com/content/star-hopping-tutorial-lesson-one-m57/index.html

 

[JOC]

Do the pictures in the atlases assume you are viewing the stars with the naked eye or with some magnification through a finder scope?  I was just wondering about confusing the hops if you saw more or less than the reference stars drawn in the books?

[skfboiler]

If you are using the ring technique,  you need to know the FOV of your finder scope.  Then with what ever atlas you are using, make the ring the diameter that will match the FOV of the finder scope.  The ring will now only be unique to that particular atlas.  I made my ring with my atlas, "Pocket Sky Atlas".  I cannot use this ring with "Cambridge" or any other atlas.