SkySafari | FOV circle on the screen seems much bigger than the actual FOV


    Originally from ticket #26039.

    Hello. I have SkySafari Plus, v4.4.7 installed on my Android phone. I'm using it to control my Evolution scope via WIFI.

    I have correctly entered my scope's aperture/focal length into the 'Equipment' screen as well as my eyepieces. I set the FOV to display the 24mm, 68 degree eyepiece. However, when I then zoom in on an object, the circle on the screen seems much bigger than the actual FOV I see when I look through the scope and I'm not sure why this is. For example: I'm using a focal reducer on my 8" scope so on the "equipment settings" screen, I set the scope aperture for 203.3mm and the FL to 1280mm which equates to about 1.27 degrees (please let me know if this is not correct!) When I look at an object like the Double Cluster through my scope, both clusters BARELY fit into the actual FOV. However, in SkySafari, that FOV is almost doubled and it seems to be showing that I should be seeing much more of the sky in my scope's FOV. So, it seems like SkySafari is overestimating my FOV. Do I have something setup incorrectly? I have attached a screenshot from my phone that shows the FOV.

    Thank you,


    Bill Tschumy replies:


    I can't figure out what is happening without more information. What is the actual FL of the scope? What power focal reducer are you using?

    If you had a 1000mm FL scope and were using a 0.6x focal reducer, you would need to create a "scope" with a 1000 * 0.6 = 600mm FL (In SkySafari 5 you can specify a focal reducer separate from the scope, but that is not yet available for Android).

    Bill Tschumy
    SkySafari Developer


    Ticket update:

    Hi Bill, thanks for the quick reply. OK.

    The scope's aperture is 203.2mm. The FL is 2032 so it's an f/10 scope. But, I'm using a 6.3 focal reducer so it's now operating as an f/6.3 scope.

    In SS, for the "focal length" setting for the scope, I typed 1280 (2032*.63) which should agree with what you said below. For aperture I left it 203.2 (please verify this is right.) But, again, the FOV circle in SS is much larger than what I see with the scope (as shown by that screenshot.)

    Also, just to experiment, I changed the focal length setting to 2032 which would be correct if I didn't use the reducer. Interestingly, that changed the FOV circle to about what I actually see. So, I'm wondering if SS is somehow miscalculating the FOV size and displaying an incorrectly large circle? Another possibility, should I change the aperture size to something different since I'm using the reducer?


    Bill Tschumy replies:

    Those FOV calculations have been in use for a couple of years. I think it is unlikely that there is an error.

    You are correct that the aperture doesn't come into play when determining FOV.

    I assume you entered the eyepiece correctly? You said it was 24mm with a 68º AFOV.

    Is this a SCT? I wonder if the 6.3 focal reducer actually has a different effect when used visually.

    If you would like to send my your [CurrentSettings].skyset file, I can look at it and see if anything is amiss. You generally can find it at:

    /sdcard/SkySafari Plus/Saved Settings

    Bill Tschumy
    SkySafari Developer


    Ticket update:

    Hmm, I checked the FOV circle that SS reports vs. the FOV in Stellarium, using the same numbers.  They agree.  So, I'm wondering if the problem is actually just my perception of the field of view.  Perhaps my eye isn't close enough to the EP to gather the entire field.  If that's the case, I feel pretty silly!  I will more closely examine it tonight.  I did have 2 other minor questions. When using 'manual align' I understand it uses only one star for alignment which is not very accurate.  If you then slew to another star and select 'align' again, will SS add that star to the previous star making the alignment more accurate or does it simply replace the first star?  Also, do you know if Celestron has been working on any kind of firmware fix for the dropped WIFI connections?  I know many, many people have this problem (including me).  I've opened tickets with them but so far, have not gotten anything useful.


    Bill Tschumy replies:

    Glad to hear that Stellarium agrees.

    With non-Celestron Wi_Fi scopes, the Align button just corrects the alignment only in the region of the sky you are in. It basically store an offset from where the model says you are pointing and where you say it is pointing.

    With the Celestron Wi-Fi, I believe the Align button actually updates the model which will help all over the sky. Celestron wrote this code so I'm not 100% sure.

    I don't have any insider info about Celestron's Wi-Fi problems. I know that they have replaced the Wi-Fi adapter in some cases where people have problems. I do agree it can be somewhat flakey. We have much better success with our SkyFi unit.

    Bill Tschumy
    SkySafari Developer