Cannot Rotate OTA After PA Initial Step

Using SharpCap's Polar Alignment feature
GaryS
Posts: 175
Joined: Sun Mar 31, 2019 5:18 pm
Location: Boston, USA/Jackson Hole, USA

Cannot Rotate OTA After PA Initial Step

#1

Post by GaryS »

Hi
Give the presence of obstructions ( trees and Porch roof), if I rotate a full 90 counterclockwise after the first step, I’m pointing straight into a tree. If I rotate clockwise a full 90, I’m pointing into our front porch roof. I might be able to rotate 30-40 degrees in either direction and still have a few stars but that’s all.

Given all the other tricky issues with SC PA that I’m reading here, is my situation unlikely to be workable for doing PA with SC?
Thank you for your help..
Gary
User avatar
oopfan
Posts: 1321
Joined: Sat Jul 08, 2017 2:37 pm
Location: New York
Contact:

Re: Cannot Rotate OTA After PA Initial Step

#2

Post by oopfan »

Hi Gary,

My multi-year experience with SC PA suggests that rotating exactly 90 degrees is unimportant. I've never had it complain that I rotated too little or too far. Try rotating only 45 degrees. It might work. Also, I've found that PA is quite tolerant of partial obstructions like trees, depending on the density of foliage. However, there is something I recommend: choose your rotation endpoints carefully. You will spend 90% of your time at the second endpoint while you make adjustments to altitude and azimuth. You don't want one successful plate solve for every ten failures. This normally means that the second endpoint should be clear of obstructions. However, the initial endpoint can be partially obstructed since all you need is one successful plate solve to proceed to the second endpoint.

Brian
GaryS
Posts: 175
Joined: Sun Mar 31, 2019 5:18 pm
Location: Boston, USA/Jackson Hole, USA

Re: Cannot Rotate OTA After PA Initial Step

#3

Post by GaryS »

Thank you Brian.

By initial endpoint do you mean the first rough aim at CNP or the first rotation?....just checking.

Gary
User avatar
admin
Site Admin
Posts: 13177
Joined: Sat Feb 11, 2017 3:52 pm
Location: Vale of the White Horse, UK
Contact:

Re: Cannot Rotate OTA After PA Initial Step

#4

Post by admin »

Hi Gary,

I think Brian means that if you have partial obscuration in one position (say due to tree branches), then start in that position and move back to the position that has the clearest view - the reason being that you only need one image to work (plate solve) in the initial position, but you need reliable plate solving of every image in the final position to allow for adjustment.

Anyway, there's no actual reason to start in the upright position and then rotate to the horizontal, it just makes for nice clear instructions. In your situation you could try starting 30 degrees one side of upright and rotating to 30 degrees the other side to make best use of the available view of the sky.

cheers,

Robin
User avatar
oopfan
Posts: 1321
Joined: Sat Jul 08, 2017 2:37 pm
Location: New York
Contact:

Re: Cannot Rotate OTA After PA Initial Step

#5

Post by oopfan »

Thanks for the translation, Robin. That is exactly what I meant to say.

Brian
GaryS
Posts: 175
Joined: Sun Mar 31, 2019 5:18 pm
Location: Boston, USA/Jackson Hole, USA

Re: Cannot Rotate OTA After PA Initial Step

#6

Post by GaryS »

Thank you Both,
Gary
GaryS
Posts: 175
Joined: Sun Mar 31, 2019 5:18 pm
Location: Boston, USA/Jackson Hole, USA

Re: Cannot Rotate OTA After PA Initial Step

#7

Post by GaryS »

Hi Again, hope you’re still getting alerts on this topic.

I thought a photo would better convey my situation. I’ve been using an AZ Mount because I felt I could not polar align with this view to Polaris which is the small dot in the image. A ‘friend’ at iOptron felt that, based on this image, I could polar align using ipolar at least. So, I went ahead and purchased a CEM70 mount and am about to do its first test run. But, being an ardent SC fan, I am hoping to use SC instead of ipolar.

Could you forgive please this follow up question, take a look at the image and offer your final observations on whether SC can handle this small a FOV around Polaris....many thanks.
Gary

ps: I know that ultimately I just have to try it but knowing the situation I felt you might have specific suggestions on where to start. Being brand new to EQ mounts, I’m not even clear how I aim at a particular point to start off.
Attachments
0B23ECC9-ABB6-418F-AF1E-2683FF5EA6BB.jpeg
0B23ECC9-ABB6-418F-AF1E-2683FF5EA6BB.jpeg (973.61 KiB) Viewed 2533 times
User avatar
oopfan
Posts: 1321
Joined: Sat Jul 08, 2017 2:37 pm
Location: New York
Contact:

Re: Cannot Rotate OTA After PA Initial Step

#8

Post by oopfan »

Hi Gary,

Those are degrees? You've got more than enough room.

I've got a GEM. I've got less than 5 degrees clearance to the right of Polaris where it gets tangled up in a tree. I've got a lot more clearance above Polaris and to the left. For polar alignment I begin at a point to the left of Polaris, and end above it. A month ago, I tried a new location. Polaris sat between two trees. Every rotation angle included tree limbs (dead of Winter). I was able to complete polar alignment but the plate solve success rate was not that high. Thankfully I was capturing lunar that night, so PA wasn't super critical.

Brian
GaryS
Posts: 175
Joined: Sun Mar 31, 2019 5:18 pm
Location: Boston, USA/Jackson Hole, USA

Re: Cannot Rotate OTA After PA Initial Step

#9

Post by GaryS »

Thank you Brian. I’m hoping to give PA a first try.
We have clear skies today in Boston but tonight?.....only the wind knows what we’ll have here.
Cheers
Gary
User avatar
admin
Site Admin
Posts: 13177
Joined: Sat Feb 11, 2017 3:52 pm
Location: Vale of the White Horse, UK
Contact:

Re: Cannot Rotate OTA After PA Initial Step

#10

Post by admin »

Hi,

yes, provding that the guide scope maintains a view of polaris through a rotation of the mount through at least 30 degrees (ideally more, but 30 should do) then you should be OK. The actual field of view of sky that your guide scope will look at will be about 1.25 by 1 degree (if you have a typical guide camera on a 200mm focal length guide scope).

The issue is going to be that as you rotate the telescope in the RA axis the guide scope drops to a lower height which might then mean that it is so low that the porch roof blocks the view of polaris.

cheers,

Robin
Post Reply