Method for the data retrieving of the extended objects
The extended nebulae will be search for via a visual detection of ALL the H-alpha emitters localised in the field. The Nothern Galactic Plane has therefore been "cut" into mosaics that are overlapped of 0.5 deg in RA and DEC. The parameters used to retrieve these mosaics are the following:
- Search box: 120 arcmin
- Filters: Ha,r
- Sky level: < 600 (in Ha)
- Ellipticity: < 0.3
- Seeing: < 5 arcsec
- Maglim: > 18
- Binning: 15 pixels
Using the data reduction pipeline set by Jonathan Irwin, Albert Zijlstra defined a "recipe" for the mosaics creation. They have been produced in an automatic way given their center coordinates.
Albert's mosaic cookbook:
-
Register for an account at Registration
.
-
At INT WFS Archive: select your pointing, select Ha and r (not i) and select a field size: use 120 arcmin. Click 'sky level' to make sure it is reported in the next frame. Increase the number of select frames to 500, and submit the query
-
You now get a long list of frames (unless you picked the wrong position). Select them all, but you may want tot deselect fields with a high background: they tend to cause problems. At the bottom, select 'no preview' and (this is the confusing bit) now click 'preview selected images'
-
Just as you asked, you get a preview of nothing. Click 'download datasets'. You do NOT want to download the catalogues, by the way.
-
Now you get a screen listing of all your selected images. Make sure to download 'all' images and none of the catalogues (click on 'all' will toggle the selections between all and nothing). Now very carefully:
1. click the 'by filter' button
2. set image download format to mosaic
3. on the subtraction box, click on the 'add' to get all the Ha - r entries.
4. set mosaic scale factor to 15
6. set ' Use simple (non-adaptive) subtraction algorithm'
- Wait for about 2-6 hours, typically.
Steps 1-6 are important: miss one and you are using up HUGE amounts of CASU CPU's and will not get a result. Note that that if you 'back' your way to this page, the selections of these buttons will have often have reset to their defaults (at least for me)
More warnings
GAIA can not load images of more than 1Gb. I have gone over this limit on occasions.
CASU is not unlimited in resources. It is possible to run out of disk space or CPU allocation.
You will get a field which will have bad data in it. If problematic, go back and deselect the fields which cause the problems (normally ones with a high background)
Interesting objects can be followed up with retrieving small fields (typical selection radius of 10') and use the proper subtraction rather than the simple one. Do not try this for large areas though.
Last Improvment
For the extended-emission surveys, when retrieving mosaics, the system by default does a differential background removal. This routine overdoes things a bit, and removes most of the extended emission. It is also the cause of the negative features in maps with sharp features.
Jonathan has now implemented an option not to use this. On the form where
the final mosaic is requested, you will see a box "Disable differential background removal in subtraction". This box should be ticked.
Additional information
- Number of mosaics: 2576
- Storage : Cambridge
- Name: RA_DEC_mosaic.fit