Using Push-to direction for finding targets with a Dobsonian (no tracking)

[MattTexas23]

Dear all,

Coming from Astrophotography, I have been a long fan of the Sharpcap Push-to feature. It extended the life of my startracker with months! In public outreach I am using an 8 inch Dobsonian with Dobsonian box mount (Alt, Az) no tracking, no actuation, no encoders; a classical visual setup. Colleagues use a Pi-Finder to quickly find targets in the (light polluted) night sky.

My plan is to put a guidescope (WO 130 uniguide +ASI120MM) on the dobsonian (replacing the optical finder scope), connect the guide cam to a laptop running Sharpcap. I can then use the Push-to feature to walk towards my next target: manually, aided by Sharpcap. This would work well if my guidescope was exactly aligned with the center of the dob - and that's a major hurdle, especially if you (re)assemble your system for an outreach event.

I heard from PiFinder owners they can put the dob on a bright star (Sirius), tell the PiFinder that's the target they see, and then the PiFinder corrects its "center" as if it was on the bright star. Even if the camera is pointing quite some distance away, it would "shift" to what the dob sees.

Is this routine possible with Sharpcap today?

Other items I was thinking about;
- Let Sharpcap know your location (in platesolve settings), focal length of guide scope.
- not sure if/how I should run calibration wo an "active" mount yet (what is up/down, left/right)

interested in hearing your thoughts,

with kind regards,

Matt

[admin]

Hi Matt,

there sadly isn't a solution for the offset finder issue for push to at the moment - there is one if you are using a GOTO telescope where you can use the (fairly new) 'Offset Plate Solve and Resync Target Position' tool. That tool lets you put a cross on the screen to mark where your main scope is pointing and makes platesolve/resync/regoto operations try to put the target on that cross rather than in the center. I will look into making that tool work for the PushTo mode (it seems like it might be quite simple, but I will have to check).

If you set your location in the polar alignment settigns then the push to routine will use that location if there is no other location source available - that will then get you up/down/left/right guidance as well as +/- RA and Dec. You will only need to be accurate to a degree or so for that to work. If you are using SharpCap's built in plate solving tool then there is very little performance gain from setting the focal length (it's fast anyway), so no real need to do that.

Finally, not quite sure which calibration you are talking about wrt up/down/left/right, sorry.

cheers,

Robin

[MattTexas23]

Thanks Robin,

Yes I think it would be a game changer - with just a small feature change, from the GOTO Sync feature available already. I would recommend it . Imagine all the dobs that can be moved manually with this target help. A messier marathon would be a breeze.

On the calibration - I was referring to the later sections in the manual on Push to. But I understand that this is solved already basically after the plate solve/using the correct location.

thanks so much for your fast response and all the good stuff you are bringing to us!

with kind regards,

Matt

[admin]

Hi Matt,

I had a look at the idea of enabling this for 'PushTo' last night and it turned out to be pretty simple, so you can look forward to that in next week's update

cheers,

Robin

[MattTexas23]

Waaaauw truly fantastic Robin! Many many thanks!

[MattTexas23]

Tried today - could not really find my way. If I want to "sync" the dob (with target in view) with the Push to camera, what do I do?
I tried the other item in the menu : Object syn with platesolve/push to" it shows a red cross that I can move in the FOV of the Push to camera, but not beyond it (which is a likely scenario since the widefield Push to camera is still not aligned with the high focal length dob). I must be missing something... Captured my experience in this unlisted video below : https://youtu.be/d_d1dXJYLcI

[admin]

Hi,

OK, so the idea is that you do the following...

* Find some target in the main scope that is recognizable

* Using the guide scope/camera, identify the same target in view and move the red cross onto that target (the assumption here is that the alignment is good enough that the true target of the main scope is in view of the guide scope)

* Now use PushTo while the guide scope camera image is being shown - follow the directions and it should help you put the target onto the red cross position, rather than the center of the screen

* Now switch back to the camera on the main scope - the target should be in view because of the alignment set up at step 2 above.

Hope that is workable for you - I'm afraid I don't see how this could be handled usefully if the main scope target is out of the field of view of the guide scope.

cheers,

Robin

[Gfischer]

Hi robin,
A while ago we talked about using guide scope as an electronic finder. Your update is a better solution than just locking the crosshair position after aligning scope with finder. Without your solution it would still be necessary after plate solve to push scope to put object on crosshairs. Assuming object is visible in finder view. Not a dim ngc galaxy. I’ll have to update again. Great work looking forward to checking it out.

[MattTexas23]

Hi Robin,

Thanks so much for the clarification - it worked exactly as described. The red cross moves the "perceived" center for Push to, to the where I put the cross.

It indeed needs to be within the FOV of the guide camera - since these have usually small chip size it requires still a bit of alignment. I did have a careful look at the synta mount of the dob, it was very off, so with a small tweak I could easily align the FOV of my guide cam (fl=130mm, ASI715MC) with the dob (D=8inch fl=1200mm). I used a far away light pole for that during the day, it worked but that was not the thing you and I were trying to solve

I had expected you could do a platesolve from your guide camera, find the center of what your guide camera sees (Ra, Dec, rotation) And in "Push to" and you have a button saying "I see my target" - after that Push to is calibrated with the Ra, Dec that your guide cam sees. Not sure if that works that way, but it would not matter then if the target is in the FOV of the guide cam.

What I struggled most with however, was getting a consistent plate solve. Maybe it was the 90% full moon. I am in Bortle 6. We had good seeing and clear skies and I found 100 stars consistently. I used the Sharpcap solver which is usually the fastest, I put in the right location but left the focal length not defined. I would get a good platesolve at one point, but then a few degrees off it would fall back to the all sky solve again and then drop out with a fail. I increased exposure time and gain, but it always found 100 stars. I think that is probably the biggest slow down of the idea honestly. Its a small ZWO guidescope, maybe the aperture is pushing the limits a bit.

I will keep on experimenting, hope to shape it up a bit further,

many thanks, clear skies!

Matt

[admin]

Hi Matt,

glad to hear we are on the same wavelength now

On the issue of intermittent plate solving - it's a bit odd, as I would expect you to have an FOV of over 1 degree in both directions with a standard guide cam/200mm guide scope combination, and that usually works well.

One thing to note is that the PushTo code will default back to an all sky attempt to solve if it fails to solve three frames in a row, currently regardless of the reason for the failure. That can happen if you push consistently for a while - the frames may have trailing stars and not solve because of that, and that can still trigger a global solve. I keep meaning to do something about that.

Is the solve also failing when it drops back to the all sky search too? If that's the case then grab a few sample frames (just press 'snapshot' ) that cause this and send them my way so I can work out why.

cheers,

Robin