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SKySafari 6 Plus pointing accuracy with digital setting circles / push to Dobsonian telescopes

Greetings,

With SkySafari 6 Plus (with iPad 4th generation & iPhone SE) Basic Encoder System (2048 PPR AMT102 encoders) + Alt-Az Push-To

After star aligning using two stars, the target object can only get as close as 0.5 degrees to compared to eyepiece center. This behavior is less evident for targets close to each other. For example I align the scope using Arcturus & Polaris and then got to Mars and as I align it on iPad screen dead center, it is roughly 0.5 degrees off-center on eyepiece. The Dobsonian telescope I'm using has a mirror 6" f/7 and my eyepiece's focal length is 24 mm.

Also I have some other questions which I couldn't find answers in online documentation:

- How critical is selecting the exact coordinates of the observation site in Location? Would a few miles / kilometers in approximation make a difference in pointing accuracy? I'm using my AMT-102 encoders in their maximum resolution, 2048 PPR, ALL 4 DIP switches are OFF

- What is the function of Push To menu item in Scope menu?

- What is the proper way of aligning a Alt-Azimuth push-to Dobsonian telescope using 2 stars so that it may stay consistent with the SkySafari's navigational window throughout the observing session. I've assumed just start with pointing the telescope to the North and make OTA parallel to the ground and connect the scope to SkySafari using WiFi. For precise alignment is a Telrad OK, or should I use a high power eyepiece?

Regards,

 

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6 comments

  • 0
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    Bill Tschumy

    Basar,

    Is the accuracy of push-to much better when using the DSC encoders without SkySafari?  If you are just doing a two star alignment, this depends on your mount axes being orthogonal and there being no flex in your mount.  If you do a 3 or 4 star alignment, these mount error are handles better.

    A few miles accuracy in your location is perfectly fine.  Unless you are tracking satellites, an accuracy of 50-100 miles is probably fine.

    If you select an object and tap "Push-To", we display an arrow showing what direction to push the scope in.

    You should be able to just turn on the encoder system and connect via SkySafari.  Do your alignment on several stars in different parts of the sky.  You definitely need to use a medium to high power eyepiece to center the star.  A Telrad is not going to be accurate enough for a good alignment model.

     

  • 0
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    Basar Titiz

    Hi Bill,

    Thanks a lot for you response & comments.

    I had assumed SkySafari would automatically correct the errors originating from the mount but maybe this was only to a degree.

    Since I'm using a medium magnification (118 x ) for centering the stars, I may have problems in mount orthogonality.

    I will make some indoor tests with my current setup after enabling the visibility of the Meridian line in setup and try to follow this line while moving the scope from horizon to Zenith. If I can stay on the Meridian, then I will know my mount is orthogonal on this axis. I will repeat the same test with azimuth axis,  turning the scope along horizon, OTA being parallel to the ground and check if I can stay on the horizon line on SkySafari screen with some magnification.  I will also try to verify if my altitude encoder is located at the center of rotation or not. I've had eyeballed for the location of this encoder but again I may have some differences that can cause pointing errors.

  • 0
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    Bill Tschumy

    With a two star alignment, there is no way to correct for mount errors. If you do a 3 or 4 star alignment, then it can correct some of the error.

  • 0
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    Basar Titiz

    My problem with aligning the scope with more than two stars is, probably related with mount orthogonality + optical axis alignment  / etc. problems.

    After aligning the first two stars, whenever I point the scope to the third one, it doesn't fall into 0.5° circle of SkySafari's Telrad screen. Mostly I end up within 4° circle. So SkySafari says "Your alignment target is too far from the telescope's actual position.  Please make sure you selected the right alignment target". Then only solution is restarting the alignment. 

    My method above for checking the orthogonality of the telescope mount showed no errors at all but I still  believe, misalignment between optical axis and alt / az  axis  may cause these pointing errors.  I need a better method to find out that telescope mount is good enough to keep the objects within 0.5° circle.

    I believe, this problem needs a kind of study  group or something like this and possibly a systematic checklist which SS users can go over and make sure that everything is alright.

  • 0
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    dksolar3294

    I have an Explore Scientific push-to 12" DOB with an Astro Devices Nexus-2 & magnetic angle encoder.  This device links to an iPad running SkySafari6Pro.  I first do the initial 2 star alignment.  When then moving the scope to a desired object, the SS6 cross-hair is invariably noticeably slightly off target - enough that it is usually not in a wide eyepiece.  If I then tap on the object (in SS6) & hit Align, I get "Command Failure: Object too far from actual position".   I've never been able to do a 3 or 4 star alignment without some error.  All very frustrating.  I suspect the magnetic encoder is a factor to be considered.  SkySafari needs a Syn command to shift the cross-hair to the scope position.   With that capability, star hopping would be greatly enhanced.  A more robust alignment algorithm which uses 4+ stars would be equally useful.  

    PS:  This was re-posted separately to SkySafari | Feature Request. 

  • 0
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    Keiron Smith

    Please create a NEW post to discuss your issue, as while the issue described is similar your equipment setup is different.

    Thanks!

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