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Help with rotation of images and stacking in Pixinsight

09 Apr, 2018 07:10
Hello everyone,

After a long period of waiting I finally got the chance to acquire some images of NGC 2903.
I had two evenings spend outside now. But I was confused because the EQ5 drove to the object different in the second night than in the first.

So my result is a rotated , mirrored picture to the ones from the night before.

I still tried to capture the same amount of images like in the night before and I almost got the same field of stars, though it's difficult to compare mirrored images to one another.

My question is now , how did this happen ? or why ?

Can Pixinsight work with these two sets ? or do I have to rotate and mirror them by hand ?

Thank you guys for your help and for sharing your knowledge.

Regards Dennis
09 Apr, 2018 15:19
PixInsight will normally do all this automatically by aligning the images before stacking. Are the images rotated and mirrored or just rotated?
09 Apr, 2018 18:17
It's not only rotated , also mirrored,
but camera position at the Teleskope wasn't changed though.

okay I'll try if it works , maybe I won't have a problem but I wanted to make sure if I'll get problems or not .

thank you Michael
10 Apr, 2018 06:18
If you use Triangle Similarity as the descriptor type for star matching, StarAlignment will work with mirrored images.  The BatchPreprocessing script uses Triangle Similarity by default so it should work with your data.  If you're running StarAlignment manually then make sure you change the descriptor type.  The default value of Pentagons will not handle mirroring - see the mouse over text for details.

10 Apr, 2018 07:06
Hello Rick , thank you for your help, I usually use the manually registration and alignment ,

I'll definitely try what you suggested.

So can someone tell why on one day the EQ drives "different" to one object than the night before ?

It is constellation Leo , it is in the S right now , by about 00:00 it is S-SW direction.

Mast also have something to do with the time i went outside , i was about 1,5 hours later on the second  night.

Regards Dennis
10 Apr, 2018 07:19
Hi Dennis,

On a standard equatorial mount when the object moves through the Zenith (also called 'transit' ) you usually you have to 'flip' your scope in both axis as otherwise something will hit the pier soon. You probably imaged the target before then after the transit on those nights. If you have a planetarium app it will usually tell you when the transit occurs for a given object.
Edited 10 Apr, 2018 07:19
10 Apr, 2018 13:27
Yeah that might be the reason.
I wondered when this happenes. I only use Stellarium, I'll check if i find something about it to avoid this problem again.

Thank you for your help.

Regards Dennis
10 Apr, 2018 21:51
But note that a meridian flip will only rotate images by 180 degrees.  It won't mirror them.  I quite like imaging both sides of the meridian.  It's like a huge dither!

11 Apr, 2018 01:08
Speaking of stacking images rotated do any of you know if there is a function in Deep Sky Stacker that will stack rotated images?
11 Apr, 2018 16:12
Deep Sky Stacker automatically rotates images.  Nothing to set.

11 Apr, 2018 19:36
Excellent thanks Ron.
11 Apr, 2018 19:51
Hey Rick !
so the meridian flip is the one that rotates the axis ? Or is that still something different now ? I'm not so firm with all the terminology , I only do smile

I capture with backyard eos.
and there is a button for the preview of already taken images that rotates by 180°.
i tried that , and it still was mirrored somehow.
what does that mean now ?

is it due to the meridian flip or is it anything else ?

Regards Dennis
12 Apr, 2018 04:28
Hi Dennis,

A meridian flip is what happens when the object transits and the scope and the counterweights switch sides.  This will cause a rotation of 180 degrees in the captured image.  It won't cause the image to be mirrored.

Mirroring would normally only be caused by a change in the imaging train or by differences in how software packages interpret the coordinate origin of the image in a file.  This is especially an issue with FITS format files where the standard isn't specific and some software assumes the origin is the upper left corner and some assumes the bottom left.

I'm not familiar with Backyard EOS and don't have a good theory about what is happening in your specific case.

31 Jul, 2018 07:30
In my experiences with several mounts, they usually advise via the hand pad what you want to do when your OTA reaches the Meridian.  My mount allows several options:  stop, continue through the Meridian/Zenith by so many degrees, depending on how far your mount can go beyond it, you specify how many degrees (within the maximum) and lastly, perform the "dreaded" Meridian Flip.  I've tried a number of times to perform the Meridian Flip and also experienced them when I left the mount unattended and came back outside to find the mount conducted the flip on its own, obviously I didn't get to the mount in time to stop it.  My success rate for a perfectly performed Meridian Flip is like batting zero, so it must be something else I'm doing wrong I reckon or I have a mechanical or software issue.  In view of  these variables,  I avoid the Meridian Flip like the plague and consequently the session is interrupted because I've tried stopping the mount prior to the Meridian, wait ten to fifteen minutes then re-align and reframe the OTA and try to get the target back in the same position in the FOV.   This effort is equally frustrating and most of that can be blamed on my lack of experience in these matters, it doesn't help I've had ongoing issues with the mount's tracking and guiding, so much so it's been back to the factory twice, both resulting in zero improvements.   The one missing component to their repair work is they do not perform live tests on the mount afterward, so how are they to know if the mount is performing as it should? The minimum they will do is hook it up to a graph measuring performance at the encoder.  The graph may look in spec, but real-world performance is another matter.

It would be a joy if my mount seamlessly performed the Meridian Flip and continued the imaging session, but even if it did, my atmospheric conditions and location of targets in the southern sky negates any imaging session anyway.  Targets in the southern sky are directly over a massive light dome, where stars are non-existent up to 60 degrees above the horizon.  Further complicating things, most popular targets are low to the horizon as well, where most are in a zone about 25 degrees above, right in the meat of the light pollution.  Since I use a DSLR for imaging and even when imaging away from the southern sky, say to the north or east, I'm limited to around 280-300 seconds max before the histogram blows out.  Lastly, I'm hampered by a neighbor's rooftop, the only real chunk of sky I have is limited to the east to northeast skies and that is shrinking with close by neighbor's fast-growing trees.  When I moved to this current house 12 years ago, I had unlimited views of the most important parts of the nighttime sky, unfortunately, I didn't get involved in astronomy until five years ago, just when the trees were starting to encroach and I then decided to try imaging a couple years later.  Looking back, I sometimes wished I never got started into imaging, with the technical and environmental problems compounding things, it has been an exercise in frustration.  There were times when I seriously considered getting out of the hobby, but the one thing that keeps me hanging on is the desire to move to a better location.  I finally have my wife thinking this (for other reasons of course), but she doesn't want to go as far out as I would like to, so whether it's remaining in suburbia or otherwise, as long as the yard is big enough where neighbor's trees don't block the view.  But, if we stay in the burbs, the light pollution will still be present, so I might be forced to take on narrowband imaging.  Oh boy, I'm I lucky or what?

I do have a back up mount ready to go, but I made a commitment to get the problem mount sorted first before starting up my new to me AP 900GTO.  Unbelievably, it is a 2011 mount that was never used by the original owner, life matters at the time prevented him from pursuing the hobby, so he hung on to it for over six years before deciding to finally sell.  I was lucky enough to find it on Astromart, absolutely mint and with some nice upgrades atop a G11 tripod.  Nicest guy in the world to deal with and even though tempted, the only thing I have done so far is change out the keypad battery and upgrade the chip.  I'm a little concerned about that because I accidentally installed the V-chip in backward, but initial testing shows everything in working order, testimony to the durability of the electronics (I hope).  However, after the initial logging on and coordinating home location and so forth, there is a very slight flickering of the keypad screen readout, I don't know if this is something to be concerned about or not, but one day I'll know.  Maybe this time I'll catch a break but knowing my previous luck, I'm not so sure.
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