iTelescope data is supplied in both raw and calibrated versions.
These are downloaded from the FTP server;
We do offer calibration files for all telescopes, but we also automatically calibrate images, so it's entirely up to you!
Raw calibration files are available to download from the FTP server and you can also take raw bias and darks for free when the roofs are closed (CMOS cameras do not have this feature available as they do not have shutters), also taking bias or darks when the observatory roofs are open will be charged at the usual imaging rates.
Digital calibration is generally the same for all CCD astrophotographers. Calibration is the time-tested process of characterizing noise (both repeatable and random) in the camera and optics, and minimizing such noise, thus increasing the signal to noise ratio and giving better images.
Bias frames are zero-length exposures. They characterize the read noise of the camera, or the noise introduced during the process of converting the analog signal to digital. Read noise is mostly the same image to image and thus Bias frames are subtracted from every image to remove that "pedestal".
Dark frames characterize noise that is linear with exposure time, i.e. hot pixels that grow brighter with longer exposures. Darks inherently contain bias (since bias is in EVERY image, part of the download), so usually bias is NOT directly subtracted from images. When Darks are subtracted, both the bias and the hot pixels are subtracted properly.
Flat frames characterize dust motes, vignetting, and flaws in the optics. Nothing is saturated -- they are set up to characterize the light distribution across the chip. They are very short exposures, and are brighter toward the middle, darker toward the edges. Flats are divided into (not subtracted from) images, making for much more even illumination and the removal of dust motes. Since flats are very short exposures, they will have bias frames subtracted from them before being divided into the light frames.
During a session on the telescopes Both Calibrated and RAW FITs files are processed on the telescope servers.
Once an exposure is finished it has matching Bias, Darks and Flats applied to it and saved as "Calibrated" and sent to your FTP folder.
This of course removes most, but not all of the noise and optical flaws from an image. It is up to the end users to process to image further to whatever level needed.
We do make efforts to gather fresh calibration data whenever necessary. We monitor these aspects closely.
Our Darks are taken for our 'Standard' times. e.g. 60, 120, 180, 300, 600, 900 seconds - including 1200 and 1800 seconds for some telescopes.
Our Flats are taken with the 'Sky Flats' method and we mostly use plugin apps or our own automated system to gather these during dusk/dawn on the telescopes. Users cannot take Flats themselves. In the near future we will be using illuminated light panels to have more control over our flats conditions and times.
Dark & Bias frames can also be taken by the users themselves via options on the telescope interface.
We suggest this be done during closed dome / bad weather nights so as not to tie up the systems during busy clear periods. Simply bookmark the actual telescope webpage and log in. If the roof is closed then the calibration data you take is free.
All calibration data is performed with a Median/Average combine in MaximDL.