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Help! Cannot figure out what went wrong with this veil nebula mosaic.

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So, last night I did a quick test to check my veil nebula mosaic coordinates and camera orientation were right.

It came out like this, after running it through ICE...


According to Telescopius, which I used to create the coordinates, it should look like this:



Now, if I go through Stellarium and use the exact same coordinates, with the camera rotated by 90 degrees (which is what Telescopius says this would be), it works perfectly ie I can do a screen grab of the camera's FOV, copy and paste it into Photoshop, and manually rebuild the mosaic and it looks just like the above graphic.

I have literally spent all afternoon scratching my head to figure out what's wrong here. The coordinates are right, I'm sure. The camera is rotated by 90 degrees. So why are the subs in the mosaic all out? Is it really a camera rotation issue?

I've been through everything several times and that's all I can think has gone wrong here. But I just don't know for sure.

I'll be running it again tonight. I've redone this in Telescopius and rebuilt all the plans, but I don't see anything radically different and I don't expect a radically different result tomorrow.

Any more suggestions?

Thanks, Brendan

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Posted (edited)

OK so I took a different approach - plate solved an image in APT and then used the Show function to see it in Stellarium. This shows that the image was at 64 degrees, not 90 degrees.


So my very first thought, that it's just the camera rotation, was right. But the camera looked to be exactly at 90 degrees. I cannot figure out for the life of me how I managed to be 26 degrees out.

Tonight I'm going to take just one shot, then do the same check: show the result in Stellarium and make sure I've got the right place and the right rotation.


Edited by BrendanC
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Me again.

A sudden thought: when the camera is supposed to be rotated by 90 degrees, that's not at 90 degrees to the scope, is it?

It's at 90 degrees to the horizon. 

So I need to rake into account the angle of the scope - which, at 26 degrees, is about right for the veil nebula - and then rotate the camera by 64 degrees.

I've done mosaics twice before, not had this problem, and I can only assume it was blind luck that they worked out.

If someone could confirm I'm right about this, I'll have learned something! No such thing as failure, just a lesson learned and all that...

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When doing mosaics, it is usually easiest to do it with camera oriented in RA-DEC or DEC-RA (what we would call landscape or portrait).

In your example - you calculated coordinates in Portrait mode, but camera was randomly aligned.

In order to align it properly - there is a simple trick if you don't plate solve. With plate solving - it is simple, plate solving in field will give you camera angle - and you rotate it until you get 0° or 90°.

If you don't have plate solving set up - then aim your scope at bright star, start exposure and slew scope in RA with your handset (or computer if you control it via computer). Star should make a streak on your exposure.

If streak is horizontal - you have landscape orientation. If streak is vertical - you have portrait orientation. If its neither - well, turn your camera a bit and check again until you reach horizontal or vertical - depending on what you need for best framing.

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Posted (edited)

Hey @vlaiv to the rescue again! :)

That's exactly what I'm going to do. Take one shot, plate solve, show in Stellarium, rotate, repeat until it's at 90 degrees. 

I was so certain I had the camera right before but obviously not. A check beforehand is definitely the right way to do this, especially with mosaics. And that's exactly why I did a trial run first. 

The geometry of RA, Dec and a camera beat me I'm afraid... 

Thanks again. 

Edited by BrendanC
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I seem to recall that in SGP if you use the mosaic planner, it platesolves the first panel and tells you how much to rotate the camera. This process is repeated until the camera angle comes within the pre-defined tolerance angle. A powered rotator would be nice but I do have a finite Astro spend budget.

To keep things simple I always use the mosaic planner that is part of the imaging imaging software that is controlling the rig.

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Thanks - I use APT which doesn't have a mosaic feature, sadly.

I sorted it last night though, got to 89.7 degrees which I think is close enough!

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