Geographic longitude

Using SharpCap's Polar Alignment feature
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oopfan
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Geographic longitude

Post by oopfan »

I am, in fact, a bad person who uses 2.9.3055. I promise I will invest in SharpCap Pro today! But last night was an opportunity to get the scope out after suffering through a long stretch of terrible weather.

I, like many of you, suffer from up-down-left-right confusion. Last night when SC told me to go up, I really needed to go right.

So I tried to think about how the algorithm sees the world. If I am not mistaken, I think it only needs to know your geographic longitude, and that your mount is level.

To my knowledge there is no settings dialog that allows me to enter my longitude. (I could be wrong.) So the only other possibility is that SC estimates my longitude by the information from my computer's time. For me I am west of the prime meridian by 5 hours during standard time, and 4 hours during daylight savings time (which is the current condition.) In the past I have written code to print out the time. My program formats the computer time to include the number of hours from UTC. For me it says "-04".

So my question is: Does SC know the difference between daylight savings time and standard time for my location when it estimates my longitude? If not then it thinks that I am somewhere out in the Atlantic Ocean. That might explain the confusion it is having with up-down-left-right.

Thank you.
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admin
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Re: Geographic longitude

Post by admin »

So you are perfectly correct, SharpCap uses your computer time zone to estimate your longitude - 1 hour behind GMT - 15 degrees west longitude.

It should take account of daylight saving time correctly, but even if it doesn't, the error of a 1 hour miscalculation should be relatively small - the direction would be off by 15 degrees, certainly not enough to turn up into right. In SharpCap 3 the longitude is taken from your ASCOM mount if you happen to have connected one via SharpCap, which should obviously be much more accurate.

I'm very confused by the direction problem - I have tested several times and looked through images taken by those aligning in other time zones and not found a problem yet.

One possible confusion is what is meant by up or right - SharpCap's view is that to move the scope right means that if you are standing looking past the scope to the pole you need to move the far end of the scope (the objective) to the right. Of course that doesn't explain up and right getting muddled...

I would be very interested in collecting more data to try to sort this problem once and for all - what would be useful would be

* A saved frame (use the sharpcap snapshot button) in the first orientation
* A saved frame in the second orientation after rotation
* A screen capture at the start of the adjustment phase showing the arrow and the direction text
* A description of how you actually had to move the mount to correct the alignment.

Ideally the snapshots should be taken with SharpCap3.0 as the older versions included the overlay graphics in the saved image which makes it hard for me to re-run the calculation if that turns out to be necessary.

cheers,

Robin
woldsweather
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Re: Geographic longitude

Post by woldsweather »

Is it documented anywhere where to say go up you tighten front or back screw and to go left you tighten left or right screw? I need to write thi downb!
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oopfan
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Re: Geographic longitude

Post by oopfan »

Like a lot of things, much can be accomplished in the light of day.

In the Northern Hemisphere (since that is my experience), SharpCap's "Up" is saying "twist the altitude adjustment knob in a direction that results in the polar axis moving towards the zenith," and "Down" is "twist the knob to move the axis towards the horizon." Now whether that is clockwise or anti-clockwise depends on the specifics of your mount.

Brian
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Re: Geographic longitude

Post by admin »

Yep,

the up/down/left/right indicate the direction that you should make the 'sky' end of the telescope move if you are standing looking past the telescope towards the NCP (or the SCP for southern hemisphere observers).

As Brian says, the specifics of which way to turn the knobs will vary from mount to mount - experiment in daytime and note it down:)

cheers,

Robin
SteveInNZ
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Re: Geographic longitude

Post by SteveInNZ »

The first thing you need to check is that your camera is oriented correctly. You can put it into the drawtube in any orientation so up/down left/right makes no sense if the camera is at 45 degrees. Check by putting the telescope in the home position and moving in RA only. The star movement should be horizontal.

What I do, prior to starting the polar align process, is slew the telescope in RA about 90 degrees. Start the process and when it tells you to rotate, I tell the mount to go to the home position. This does several things.
As it's the first move of the night, I use it to check that everything is working correctly and no cables are caught up, etc.
When you get to the adjustment step, the telescope is in the correct orientation so that left/right and up/down match Az and Alt respectively.
Lastly, I can do a plate solve (without slew & sync) to confirm that the OTA is pointing at the pole. If not, I can investigate why not and/or adjust to be starting from the correct position.

Steve.
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oopfan
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Re: Geographic longitude

Post by oopfan »

Steve,

Really? I can rotate my camera any which way but it doesn't matter. My understanding is that the step to rotate 90 degrees enables SharpCap to know which is up/down/left/right relative to your horizon. I could be wrong, I guess. I've been using SharpCap for three years with different scopes, and I've found it to work reliably the way I described.

I understand that my statement stands in contrast to my original post, but that was an old SharpCap version and I was new to computerized astronomy. There is a distinct possibility that I was observing something that was factually false.

Brian
SteveInNZ
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Re: Geographic longitude

Post by SteveInNZ »

Yes, SharpCap works fine, regardless of the orientation of the camera.
But if you line up the camera with the Alt and Az axis, what you see on the screen matches what knob you turn on the base.
So if I want to move the telescope left, I tighten the left or right knob (depending on your mount) rather than a bit of Alt, then a bit Az, then a bit more Alt, etc.

Steve
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oopfan
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Re: Geographic longitude

Post by oopfan »

Steve,

Ah, yes, I see what you are saying. The difference for me is that I only concentrate on the text display: Up degrees/Down degrees. Everything else is just sugar.

Thanks,
Brian
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