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Imaging over several nights.


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Hi all I’m building a small observatory/shed to try and get better setup for AP. I want to be able to return to objects on different nights to add more data to an image. How do you guys go about realigning an object to image it over several nights and how accurate do you need to be? Thanks,,,,Paul.

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The more accurate you are the less spillage you will have when stacking exposures

The best way is by plate solving, but it's also possible to do it if you have good alignment and your pointing is accurate. What mount and scope / camera combination are you using?

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What is critical is that your camera and scope stay in the same position together. I usually set mine in relation to the rings, or some other non moveable part, so that I can return it to the same angle if needs be. My kit is permanently pier mounted now, but I used to leave it all covered over in the garden on a tripod. As long as you dont remove the camera, it is a lot easier, but if you do remove it, then it has to go back in the same position as before.

Then, once you settle on your target, and framing of it, look for an obvious reference point within the image, a particularly bright star, or a little cluster etc, and carefully note the position of it within the image.

The next time you align, just return that feature to the same position on the camera view and you should be laughing.

Some things to watch for;

Watch out for data taken over two weeks, the moon can go from nothing to blinding and really adversely affect your final stack if some of the subs are not up to the same standard as the moonless night ones. The background sky indicator in DSS is a good measure.

One of the great pleasures for me is turning my kit on after a couple of weeks of cloud, clicking the first alignment star, and having it drop dead centre first time. Very satisfying :o

Hope you get on ok :)

TJ

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Thanks for the great answers guys, I did start to centre the objects as best I could but now I will use the crosshairs in Maxim5 and put them on a specific star. I am using the QSI CCD with the lodestar I may even add a couple of pins to the scope tube then I can let the lodestar rest against the pin(s) and it will always be in the same position.

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If you use EQMOd you can bookmark locations - slew to your target, sort the framing the bookmark the location.

Disclaimer: I aint never tried yet!

The bookmarks within EQASCOM itself only persist for your current EQASCOM session. If you want more permanent bookmarks then use EQTour to save your current location to the "MyTour" list. Of course if the object moves significantly in the sky then you may need to add new alignment points around the new position for subsequent sessions.

Chris.

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I load a frame from the last session and drop Al's reticule Als Reticle (for your PC) - Page 3 - IceInSpace

over a bright star or galaxy core or whatever and then line up to that.

That is exactly what I do and very effective it is too, however, I have a fixed mount so don't have to worry about camera rotation from session to session as I can leave it all set up.

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Having square chips on the Atik 4000s means you only have 45 degrees of rotation before you are heading back again so I like not having much choice! I try to keep the camera orthogonal whenever I can since this is repeatable.

Often I just reframe by eye, looking at what is on the edges and where. Sometimes I write down the x and y location of a star, too, from the capture programme or AstrArt.

What I don't do is take out the camera or turn it. I would rather bring in the scope-camera if necessary but it usually stays outside. Means you have durable flats as well.

Al's reticle looks good. Thanks for that.

Olly

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In Nebulosity I load up an old image and ctrl click on three objects/stars which leaves a cross hair marking on them, I then get into the right area and line things up accordingly. This way I can also see if the camera is rotated since the previous session as well.

Of course that's Nebulosity, no doubt other apps have difference ways of achieving the same thing.

James

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Hi Paul,

I use MaximDL V5 and that makes it a breeze. This is what I do...

As TJ has mentioned - make sure the camera either doesn't move or you can align it rotationally to well within 0.2 of a degree between nights. Even if not this method will still work even if your images will be rotated compared to any prior sets.

1) Center a bright star in the finder scope so it's going to be on or nearly on the imaging chip.

2) Sync to the star in the Observatory whole sky screen.

3) Take a 5s exposure and plate solve it using Pinpoint Astrometry, go to the Observatory 'Telescope tab' then sync to the plate solved location. At this point the scope is sync'd and maxim knows where it's pointing.

4) Open one of your previous images, preferably the first in the sequence (especially if you've used dithered sub exposures) - keep that image open.

5) Plate solve it.

6) Right click on the plate solved image and select 'slew to pinpoint center'. This will slew the scope to your image location.

7) Take an image (5s exposure) and check you are in the precise location. If not then repeat step 3 - that will be because you aren't properly polar aligned.

If you stretch the histogram then plate solve you can see your image in the Observatorys zoom tab (providing you switch that option on).

A couple of things need to be setup the first time for the plate solve to work:

1) Set the right Epoch - 2000.

2) Have the optics set in 'site setup' and set the pixel size so the plate scale can be worked out.

3) Have a corrected version of the Hubble GSC for Pinpoint to use.

I think that's it.

Version 4 of MaximDL was good but Version 5 is a quantum leap in usability. I also use it with field of view indicators in the Observatory zoom to frame targets as often there's something else interesting that could included with a slightly different image center. Yes it is expensive but I consider it a worthwhile investment that saves me heaps of time.

Anyway enough of my ramble...

Hope that helps

Robert

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Some very useful info here... Thanks !!

I've just been trying that, taking more frames of M33 last night. Of course I didn't put the camera on at the same angle. Somehow I assumed DSS would re-rotate.

Besides the rotation, how do you go about adding the new frames to your data from previous nights?

Do you re-stack all frames from all nights? If so, do you just load all flats/darks/bias from all nights?

Or do you make a master TIFF/FITS per night (with the darks/flats/bias of that night only) and then stack the master frames separately?

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In DSS load your data as normal - Lights, Darks, Flats, then looking at the picture you will see a new tab at the bottom of the screen (Group 1) click on this and load the new data into that plus any new calibration frames - so Main Group holds 1st lights plus any common calibration frames, Group 1 holds new data plus any new calibration frames eg new flats if you have re-set up, then proceed as normal.

Once you have loaded Group 1 a new tab will appear - Group 2 etc

post-16950-13387749008_thumb.jpg

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Yep. But dont forget to add any new calibration frames to the new set......The Main Group will hold the calibration frames that are 'common' to all groups. So if your first group was 10 min exposures and your second group was 15 min exposures you would need to put 10 min darks in the main group and 15 min darks with the second group. All the calibration frames that are common to both sets go in the Main Group

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Besides the rotation, how do you go about adding the new frames to your data from previous nights?

Do you re-stack all frames from all nights? If so, do you just load all flats/darks/bias from all nights?

Or do you make a master TIFF/FITS per night (with the darks/flats/bias of that night only) and then stack the master frames separately?

I restack everything all together. You can stack the master tiffs if you so desire, use the entropy stacking method I think for that.

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OK, this is running now. Could take a while to process...

In the first session I took 30 sec exposures and last night 60 sec exposures (unguided, so I get star trails if I go longer...).

I loaded the first session into Main Group and the lights, darks, bias and flats from last night into Group 1. It "complained" that the dark frames don't have the same exposure time. I ignored that for now and see what comes out.

Would it be possible to ignore the Main Group and load the separate sessions in Group 1 and Group 2 and so on? I don't see how darks would be common to all groups when the exposure times are different.

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hm, this has worked quite well. :(

DSS has automagically corrected the orientation difference between the 2 sessions. It has left a bit of a line at the border (see bottom left of the image) but that can be taken care of. DSS has determined the rotation between the 2 sessions to be about 6.6 degrees.

The result of both sessions is definitely better than the 2 separate sessions.

I just need to process the stacked image again in Photoshop. I only have GIMP here at work and that only uses 8 bits per channel.

And I need to figure out a way to remove that ugly gradient.

Imgage info:

session 1

34x 30 sec light frames

10 bias

20 darks

24 stock flats (flats from that night were unusable)

session 2

30x 60sec light frames

20 bias

12 darks

20 flats

Celestron C8N-GT, EOS 1000D

unguided

post-14790-133877490095_thumb.jpg

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