Jump to content

Stargazers Lounge Uses Cookies

Like most websites, SGL uses cookies in order to deliver a secure, personalised service, to provide social media functions and to analyse our traffic. Continued use of SGL indicates your acceptance of our cookie policy.

sgl_imaging_challenge_celestial_motion.thumb.jpg.a9e9349c45f96ed7928eb32f1baf76ed.jpg

Darryl

New Members
  • Content Count

    9
  • Joined

  • Last visited

Community Reputation

15 Good

About Darryl

  • Rank
    Vacuum

Profile Information

  • Gender
    Male
  • Location
    Cornwall, UK
  1. You could try taking evening (or dawn) sky flats first on one side of the pier, then after a flip and then back on the other side of the pier. Differencing the resulting flats would give you an indication of their repeatability. I've had similar problems with flat calibration in the past on the Issac Newton Telescope (in a previous life when I had access to big scopes), we spent a lot of time trying to track down flat field errors and in the end put it down to scattered light upsetting the flats. Flat fields can be a right pain, best to avoid the meridian flip if at all possible. The only other though that I had is whether the star PSF varies significantly as a function of field position. If the photometry apperture is relatively small then there could be a significant difference in the amount of light outside of the apperture depending on where the star is in the field. An 'apperture correction' is often used to overcome this problem, particularly with systems with strong coma or field curvature. Cheers, Darryl
  2. Aperture is generally king for photometry. The larger the scope, the more photons you can catch and the better your signal to noise ratio will be. The only exception to this is if you have very bright targets and your exposure times get below about 15 seconds then scintillation noise can be a problem. As far as scopes go, it doesn't really matter. Essentially you need at least two pixels per stellar FWHM but oversampling much more than that doesn't hugely help and as you stated you just cut down your field of view. In my experience if you have a FOV of 0.25 degrees then you can normally get enough comp stars from a catalogue like UCAC or CMC14. Half a degree is probably optimal. Much larger than this systematics across the field of view can start affecting your precision. Another thing to consider is vignetting. A heavily vignetted system (like an SCT with an F3.3 focal reducer) can offer difficulties in terms of flat fielding. I think that most prolific photometrists tend to either use an SCT (with a focal reducer or binning) or a fast-ish Newt. I'm just building up my kit again for photometry and have a C11. Previously I had a 10" Meade SCT with a QHY6pro CCD and F6.3 focal reducer. I haven't got a CCD for my new set-up yet (need a few more credits first) but i'm planning to get an Atik 414 and a Starizona F6.6 focal reducer. Don't forget you will almost certainly need to guide too, and finding good guide stars in obscure fields is often much harder than finding comparison stars. Cheers, Darryl
  3. I've just had a look at the plate fit for your first post, 2.65 arcsec per pixel is pretty large to get decent photometry. I'm not sure what the seeing was at the time but i'd always aim for at least 2 pixels per FWHM. Less than this and pixel to pixel sensitivity variations/ flat field errors are really problematic especially as moving a filter wheel or any other element in the optical train (or a bit of flex in the system as you change elevation) between taking flats and lights will cause small offsets in the flat. I've done a fair bit of photometry both with amateur and professional scopes so have some experience with chasing down the 'last few percent'. It can be both fun and frustrating all at the same time! It's also worth remembering that a significant fraction of stars do genuinely vary. Variables would have been weeded out if comparing your photometry with a 'standard' such as a Landolt or Henden field, however out in the 'wild' i'd guess that perhaps 5-10% of 'field' stars might either vary intrinsically or due to binarity or spots. This fraction would increase further in 'young' star fields such as in Orion or Taurus. Cheers, Darryl
  4. There are a multitude of variables that can affect photometric measurements. In particular, the colour response of your system compared with the catalogue system and air-mass effects can easily add a few percent uncertainty. Also, the point spread function of your stars is likely to vary across the field of view, meaning that stars near the edge will measure slightly faint compared with those close to the centre for a given measurement aperture diameter. You can combat this by using large measurement apertures but that then risks adding uncertainty from background sources. Systematics casued by imperfect flat-fielding can also play havoc, a problem i've had most significantly when using a 2.5m class professional telescope. An excellent primer on photometry is "A Practical Guide to Lightcurve Photometry and Analysis" by Brian Warner: http://www.springer.com/us/book/9780387333915. Well worth the $20 investment. Cheers, Darryl
  5. Thanks for the suggestions, I will do some more research on those. Cheers, Darryl
  6. Hi All, I've just purchased a secondhand Gemini G41 'field' mount as a step up from my previous EQ6. Mechanically it's good but the controller is defunct. So....I'm looking to replace the aged Pulsar 1 control system with something that will drive it for imaging, hopefully semi-remote (from the house rather than properly remote) and will be robust and reliable in use. Previously I've used EQMOD and Maxim to drive an LX200 and EQ6 but i'm not too familiar with other control systems. So what are my options? Does anyone have any experience of retro-fitting controllers? My initial list of options seem to be... - Gemini Pulsar 2 - Avalon StarGo - AWR Others? Any thoughts gratefully received. Cheers, Darryl
  7. Hi James, Getting image scale right is crucial for doing good photometry. The image scale to choose depends on your local seeing conditions and you should aim for between 2 and 4 pixels to sample the FWHM seeing. As an example, using my old LX200 at home in the UK to do asteroid and exoplanet photometry, I typically had seeing of 3-4 arcseconds. Thus I set my image scale to ~1 arcsec per pixel. In contrast, recent photometry that i've done using the wide field camera on the INT on La Palma required an image scale of 0.33 arcsec/pixel, to capitalise on the often sub-arcsec seeing. It is of course a bit of a trade off as you ideally want a field of view of at least 15 arcmin or so to get a reasonable number of good comparison stars. I'm not sure where you are but a low altitude site (such as all of the UK) will typically exhibit seeing of 3-4 arcsec, I've rarely seen much better. The 2-4 pix per FWHM is important for PSF fitting to get good photometry in crowded fields, but it is also crucial when using aperture photometry too. This is because to do good aperture photometry you need to be able to calculate where the centroid of the star is (so you can accurately place the aperture) and also because spreading the light over a few pixels will reduce the systematic errors that you can suffer from pixel to pixel sensitivity variability (flat fields only fix this up to a point - hence the exoplanet people defocussing their images for photometry). A great reference book for doing good photometry is this: http://www.minorplanetobserver.com/pgbook/PracticalGuide.htm Also Bruce Gary's book is available free here: http://brucegary.net/book_EOA/EOA.pdfand is well worth a read. Cheers, Darryl
  8. Look in the yellow pages for "steel stockholders". Find a local one and give them a call, load of diameters/ wall thicknesses will be available. Darryl
  9. Hi, I am doing my homework prior to purchasing my first cooled CCD. I've done the thinking etc about image scale and cost and had settled on the QHY6 Pro as being the camera to get. I have hust seen though that the Orion Starshoot II deep space monochrome is available for about £250 and seems to use the same chip as the QHY6 Pro. It is also thermoelectrically (presumambly peltier?) cooled and otherwise I cannot see why this should be less than 1/2 the price of the QHY. I was wondering if anyone could shed any light on this? Does anyone have any experience of the starshoot and are there any obvious issues? Cheers in advance, Darryl
×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.