DECam Frequently Asked Questions
Updated on June 29, 2021, 3:07 pm
> I'd like to use my own filters or Mosaic filters with DECam, even if that means obtaining images without the entire field of view. Is this possible? What would the reduced field be?
The location of the DECam filter changer is between the lens C4 and C3, whereas the Mosaic2 filter wheel was located close to the focal plane, just in front of the dewar window. Therefore the telescope beam is much larger where it passes through the filters as it was when Mosaic2 was on the Blanco. Using smaller filters would cause vignetting, reducing the star's brightness at the periphery compared to the image center and making the images impossible to calibrate. One would need at least a 40 cm diameter filter to get any field unvignetted. The Mosaic2 filters are 14.6 cm in diameter, so these are not suitable to use with DECam.
> I'd like to go as deep as possible using DECam. Do I have the option of not using a filter?
Yes, but don't.
The DECam corrector is not achromatic over its full wavelength range, although approximately so from 500 nm longwards. Plus the 13mm of fused silica of the filter substrate is essential to obtain good images. Plus you will include lots of sky background, including the 557.7 nm sky line that is between the g and r filters. Plus you will get fringes from the 1000-1050 nm region. Plus the guide stars won't look good. Plus unless you are near the zenith images will be be degraded by differential fraction. Hence there is no advantage in trying to go deep by using no filter in comparison to using one of the broadband filters. If you would like to go as deep as possible, we would recommend using the VR filter, although for a very blue or very red target another filter might be more appropriate. This filter goes ~0.4 mag deeper than the r filter in equal time.
> What are the Blanco horizon limits?
> I have this position I want to observe - RA, DEC, but I want the object to appear center of N4 (or S4) , what is the offset?
N4 and S4 are both the central CCDs of DECam focal plane array. If you do not center your object in one of them, your object is going to fall in the CCD gap between N4 and S4. For this reason, we strongly recommend to center your object either in N4 or in S4. To do so, you should add +4.9 arcmin to the declination of your target if you want to have it centered in N4 (or -4.9 arcmin if you prefer to have it centered in S4). More info about the characteristic of DECam focal plane here.