exposing over more than one session

[neil groves]

I have read where people have done some exposures on an object, then gone back a second night and taken some more exposures to add to the first ones for stacking, how would you get DSS to stack the second batch since the chances of lining up the scope/camera exactly as you did the first night on the object are probably as remote as the object itself?

Neil.

[neil groves]

But if the stars don't line up with the last session, how does DSS cope with the stacking?

Neil.

[russellhq]

Neil, some of us use plate solving software that's able to point the telescope to exactly the same position as the previous nights session.

There's a thread in the software section that details how to get it up and running with a piece of software called astrotortilla.

[ollypenrice]

Even without plate solving it isn't an issue if you have reasonable agreement between nights. What alignment software like DSS doesn't cope with is a flipped image (rather than just a rotated one.) They won't recognise that.

Remember that DSS is aligning your sub exposures in x, y and rotation anyway. It will have its limits, I don't doubt, but plenty of people just throw the lot it.

I do a lot of combining of images over years, not days, and I combine data from entirely different focal lengths and field curvatures. I'd be happy to claim that this made me clever but it wouldn't be true! It is dead easy in a programme called Registar. (Not Registax and not free!!) My M33 now on the DS Imaging board has data from Tak 85, Tak106, TEC140 and ODK14 scopes and four different cameras. Piece of cake in Registar.

Olly

[RikM]

I use either a mask overly on APT or the crosshair tool in Maxim DL to realign manually.

With APT, on the first night, once you have the object framed the way you want, click on a few bright stars with the mask tool and save the mask overlay. The next imaging night, get in roughly the right place, take an image and load up the mask overlay (it shows as a bunch of circles where you clicked last time), slew the scope a bit, taking short images as you go until the bright stars fall inside the circles again. Ta Da! you are now fairly well aligned to where you were last time.

With Maxim DL I frame the image how I want it on the first night and capture data. The next night, I open one of the previous images and turn on the crosshair display. Then take a new image and again fiddle the mount about until the crosshairs are centred in the same place in both old and new images.

Both manual methods are a bit of a faff compared to plate solving but I don't have the mount computer controlled yet so this the best I can do.

To combine the data, either load it up in separate groups using DSS, with calibration files for each night if necessary. 

[Freddie]

DSS can cope with images before and after a meridian flip so a slight off set will be no problem.

[Nicho]

To stack an image from a different night but the same object .

I would open all of your first nights work and select your best shot by looking at its score and then right clicking that shot and selecting ( Use as Reference Frame )

Add all of your Bias Flats ect

Then in the box where all your light frames and flats Bias are there is a tab at the bottom where there should be Group 1. Click on that then add your light frames from your second night in there .

You will get a few black areas around the shot depending how well you have lined up your shot from the night before but thats the general idear of how to stack over a few nights with DSS

I hope this helps

Nick