Norris Adams

Hi Harry - Congrats on the pics...I'm intrigued how you managed to acquire these image without tracking? I understand the need for the short sub-frame time and I guess you rely on SharpCap to manage the rotation of the FoV, but how do you keep the target centred without a motorised mount? Is this achieved entirely manually - or am I missing something? How easy is this to do? I recently started using Dark frames, so I shoot a dozen or so before each session. I normally opt for around 10s exposures so it only takes two or three minutes. They make a big difference to the shape of histogram and the output images (even before processing in GIMP) seem cleaner and more natural. Flats (which SharpCap also allows) seem to be more of a faff because of the need to keep the whole optical chain fixed and unchanged throughout the entire imaging session. Best regards Norris Skywatcher P150i / ASI 183MC

Many thanks Robin, for taking the time to reply. The paper you supplied is certainly very helpful in throwing some more light on this question, even if it seems to show that the jury is still out on exactly why it works as it does. (I haven't reached the end yet)! NB - I looked at some of your spectroscopy links too. Fascinating. (Yet another astronomical interest is the last thing I need right now!!!) Much obliged for all your help!

I am working my way through a Masters textbook on Galaxy Formation. I am an engineer (Bachelor level) , not a physicist, but I have reached page 31 without (completely) losing the thread! My problem is getting my head around the Sersic profile. This equation relates the surface brightness at any radial distance of a given galaxy to the so-called Effective (or Half Light) Radius at a given wavelength. The shape of the curve (i.e. the Sersic profile) defining the drop off in luminosity as you move away from the centre is described by this equation (for varying values of integer n). I just wondered where this equation came from? Is it empirically derived or based on some physical law? Alternatively, is it just a mathematical device to help draw a suite of brightness curves? If so, why does it work in practice? Why should galaxies be constrained by this law to have such fixed and predicable light curves (from centre to rim)? I am hoping there are some clever astronomers or astrophysicists who may be able to help! Many thanks!! Sersic Profiles.docx

Thanks both, I had exactly the same question! A couple of nights ago I took the attached shot of T Cep. On later inspection, I couldn't understand why T Cep was so bright when the star to the right (HD201731) was nominally a very similar magnitude. I discovered that it was a variable star, but even at its brightest, it couldn't account for the apparent difference I was seeing. As you can see, the comparison is striking - so much so that I estimated it to be 3 or 4 magnitudes brighter than it should be! I knew that had read somewhere about CCD cameras having a 'red sensitivity' (maybe on the CBAT site for supernova reporting), but I wasn't sure if the same was true of CMOS. I certainly looks like it is! Equipment: 6 inch Skywatcher GoTo coupled to an ASI 183MC camera.