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.



  • Content Count

  • Joined

  • Last visited

Community Reputation

223 Excellent

About Ikonnikov

  • Rank
    Star Forming

Profile Information

  • Location
    West Yorkshire
  1. If the reference scope is much larger in aperture than the test scope wouldn't this help to reduce the apparent error (assuming the lager scope was reasonably well collimated)? I seem to remember people on CN using big dobs for this purpose.
  2. Yes, its a reducer/flattener; as far as I know all Takahashi reducers are. You can see the optical diagrams here (under documentation): https://www.telescopes-et-accessoires.fr/fc-76dcu-ota-tube-seul-au-coulant-3175-pare-buee-non-retractable-c2x30343677
  3. I also had a go at a drizzle experiment recently, and came to a similar conclusion: Capturing subs on my E130D with a ASI290MM (giving 1.39 arcsec/pix image scale bin 1x1) with medium SGPro/PHD2 dither between each sub, using PI for (pre)processing. Capture 1: 64x subs bin 1x1 15sec exposure, registered with auto settings and integrated with default linear clipping rejection settings. Average FWHM about 2.8 arcsec by PI FWHM eccentricity script (Moffat 2.5), similar values by dynamic PSF of a selection of stars. Capture 2: (back to back with capture 1, no obvious change in sky conditions) 80x subs bin 2x2 5 sec exposure registered with bicubic spline setting (to avoid horrible dark ringing resulting from auto/lanczos interpolation) and create drizzle files, integrated with linear fit clipping rejection or additionally drizzle integrated with scale set to 2. In short the drizzled 2x2 data never really got close to the original 1x1 image in terms of star FWHM . I kept reducing the dropshrink setting which did lower the FWHM slightly but 0.5 was the lowest drop shrink value I could use without major artifacts (caused by lack of coverage) and this still gave stars with ~30% higher FWHMs than the integration of 1x1 subs. Also the FWHM in the drizzled image (drop shrink 0.5) is barely lower than that of the undrizzled 2x2 integration and using the default drop shrink of 0.9 it was actually higher! I appreciate that at 2x2 binning here the FWHM measurement accuracy might be compromised due to undersampling but I can't say my results fill me with confidence as to drizzle being worth the bother either! Maybe a different selection of pixel interpolation in image registration might help? Paul Integrated masters attached drizzle_integration_BicubicSpline_2x2_DropShrink_0p5.fit integration_bicubic_spline_2x2.fit integration1x1.fit
  4. These images have stars with low snr (especially Ha one) and a lot of hot pixels which are being picked up as stars by the PixInsight script and very significantly lowering the reported FWHM. Manually measuring each of the stars with dynamic PSF gives an average of about 5pixels (2.6arcsec) FWHM for Ha and 3.2 pixels (3.3 arcsec) for OIII, so I'm in agreement that by most measures you're considerably oversampling here. From my understanding, optimal sampling rate for DSO imaging (i.e. to record maximum detail without oversampling) is a continued source of debate on this and other astro forums; Vlaiv has made a detailed argument for a relatively low rate of about 1.6x smaller than the real-world FWHM of your system whereas folks on Cloudy Nights generally seem to go in the other direction with rates 2-3.5 x smaller than seeing with their own technical rationalisation (e.g. see https://www.cloudynights.com/topic/650493-understanding-criteria-for-what-is-proper-sampling-of-imaging-system/?p=9150558 ). Personally I guess I'm of the opinion that for long FL DSO imaging it's better to slightly oversample than to undersample (which can easily result from increasing binning level) as you're never going to recover lost resolution fully from the undersampled subs (even with drizzle which also is a controversial topic here!) but you can (eventually) get the snr up from oversampled subs by taking more of them. Paul
  5. What you don't show/mention is the quality of the in-focus image; if that's good after adjusting with the star test then why worry about the Cheshire? Personally I would use mechanical alignment as a starting point and always fine tune collimation with star testing when possible; with an RC scope you can follow the procedure below for adjusting your primary and secondary with your imaging camera attached to get very accurate collimation. Since it also relies on images of (slightly) de-focused stars you don't need amazing seeing to get a good result. https://www.deepskyinstruments.com/truerc/docs/DSI_Collimation_Procedure_Ver_1.0.pdf Paul
  6. Great first light with your new camera, agree that the OIII filter is very likely the cause of the halos. I wonder if you might be able to get even better signal to noise using longer sub exposures since the KAF16200 sensor has quite high read-noise; certainly I find that 30 mins per sub helps with the Moravian G3 camera and 3nm filters even on an Epsilon at F3.3 (although I've not gone beyond this so far for reasons of pixel rejection, imaging time wasted by spoiled subs etc). Paul
  7. Using various online calculators (e.g. the Wilmslow Astro website) the airy disk size for green light (510nm) at 80mm aperture comes out at 3.2 arcsec and for red (650nm) as 4.1 arcsec i.e much larger than typical guiding error and larger than typical uk seeing. This being the case then wavelength would have more of an effect on the final image fwhm between oiii and Ha filters than you suggest above. I still typically see smaller star sizes with shorter wavelength filters when imaging with a mirrors only 250mm RC scope (where the airy disc size is in the order of 1 arcsecond for green and 1.3 for red) suggesting some effect of wavelength on fwhm even at larger apertures. Paul
  8. As you mention above, shorter wavelengths will intrinsically give higher resolution images (with a smaller airy disk and therefore star size) than longer wavelengths. The theoretical airy disk size for Ha emission wavelength is 31% larger than for for OIII primary emission wavelength so could this not account for the differences in star sizes? I appreciate that bad seeing affects longer wavelengths less but i don't know at what point this would outweigh the intrinsically larger airy disk size. From experience I've consistently seen smaller star fwhm measurements for OIII over Ha with reflectors and refractors. Paul
  9. Hi Richard, this article might be worth looking at as it highlights some of the potential problems with star shapes that can arise in the esprit scopes: http://interferometrie.blogspot.com/2014/08/esprit-tuning-how-we-finetune-esprit80.html I was going to hold off on my own Esprit experiences for now but since it seems relevant to this thread I'll continue... TLDR is, (especially if imaging in colder weather) it can be a delicate balance with these scopes between preserving element centering/collimation and preventing some pinching of the optics. I bought a new Esprit 120ED last autumn and although perfectly collimated I noticed some funny star shapes & (asymmetric) diffraction spikes in images appearing only when the weather is colder (below about 4 or 5 C) suggesting mild pinching by one or more of the element centering screws (e.g. below). p Seeking perfection (foolishly perhaps) I decided to very slightly loosen the centering screws for the front element (WARNING this is a high risk procedure!!!) The good news is that I managed to isolate the screw causing the problem and stop the uneven spikes regardless of temperature. The bad news is that I managed to very slightly de-centre the element which has made the star halo slightly asymmetric and resulted in a slight lateral colour shift. I can get good star shapes now if I combine subs from each side of the meridian flip as the distortions essentially cancel each other out (see below) but this is hardly ideal. Also aligning RGB channels is problematic. I figured I had two options from here, admit defeat and fork out to have the scope sent back to Es Reid or attempt more advanced adjustment techniques. Since there was no guarantee that pinching wouldn't result again in cold weather after the scope was recollimated I figured it would be better (and more fun!) if I could attempt this myself at home as required. At a not inconsiderable expense I've managed to put together a double pass autocollimation setup with a beam-splitter, focuser-insertable artificial star and optical flat as described in the Teleskop Austria article above and the test setup seems to check out on my (well collimated non adjustable) Tak FC76. Next step is to try it out on the Esprit... Paul
  10. I'm guessing the signal to noise for the short exposures required in this study wouldn't be high enough using a smaller aperture system (with much lower limiting magnitude). Camera lenses have been used in other professional setups I've seen e.g. http://mascara.strw.leidenuniv.nl/technical/ and https://www.canonwatch.com/canon-ef-400mm-f2-8l-ii-lenses-used-discover-new-dark-galaxy-dragonfly-telephoto-array/ although in the latter one, each lens costs the same as 3x 11" RASA scopes! ?
  11. An interesting study published in Nature Astronomy using amateur off the shelf gear for some occultation photometry (alas full article requires online purchase or journal subscription but abstract, figures and raw fits data available here https://www.nature.com/articles/s41550-018-0685-8). In essence they’ve done a lot of high-speed photometry using two identical setups situated on opposite sides of a building roof in Okinawa (both observing the same region of sky) and looked for occultations of any stars with the appropriate decrease in magnitude for the appropriate duration (up to around 1 second for ~1km diameter object) recorded simultaneously with both scopes (to rule out non-astronomical interference like birds/bats etc). To record such a fast transit with enough (temporal) resolution they had to use very short exposures (~65ms) which requires a very fast optical system to gather sufficient light. To achieve this, they used a Celestron Rowe Ackerman 11” scope (already F2.2) and made it even faster with a DSLR to M43 Speed Booster (Metabones 0.71x) and acquired images with a ZWO ASI1600 CMOS camera. Crazily they can get a usable signal for stars down to magnitude 13 with these very short exposures. Unfortunately for the budding amateur scientist, what is described in the paper as an ‘extremely low cost’ system still comes to $16,000 per setup, but nonetheless it’s interesting to see what can be done with gear that people do own here on SGL (albeit not usually in duplicate!).
  12. Thanks for your reply Peter. Yes, on balance I think a small roll off is probably the best option for me all things considered. The functionality of a dome is attractive and the price of the basic small pulsar within reach, but I think the added cost to allow full automation and the appearance in my smallish garden make a ROR option my preferred choice. Paul
  13. Hi Folks, I've been thinking for a while about trying to increase my imaging time by either using remote scope hosting or getting my own observatory and am leaning towards the latter currently (if only I had the dosh to do both...). I currently image from a concrete-mounted metal pier in the corner of my back garden (mount/pier covered with a Telegizomo 365 when not in use) so was thinking of putting a simple small obsy over this (no requirements for warm room or doing visual from it). I don't have the time or skill level required to build my own (certainly to the quality I would be happy with!) so I'm looking at commercial options. After some internet searching I came across the 'Astro Booth' http://astrograph.net/epages/www_astrograph_net.sf/en_GB/?ObjectPath=/Shops/www_astrograph_net/Products/AGOABOOTH which seems to meet all my requirements and be reasonably good value compared with similar alternatives, being fairly compact/robust, relatively easy to construct myself (and dismantle if I have to move house) and with a motorised roof and delivery included in the price. I've only come across one account (at least in English) of someone buying one and they seem to like it, although they used it in Spain so might take a bit more of a battering/soaking here in Yorkshire. Would be grateful if anyone else has encountered one or has any thoughts/opinions on it (good or bad) from the website material etc. Cheers Paul
  14. Very nice Barry. This remote imaging thing is looking increasingly tempting!
  15. When I bought the RC second hand it came with this corrector https://www.teleskop-express.de/shop/product_info.php/info/p2646_TS-Optics-PHOTOLINE-Full-Frame-APO-Corrector---Flattener-for-astrophotography.html (if I remember correctly) which is supposed to be very good for RC from reading around and looks the same as the one you're referring to. Unfortunately with my modified backplate/replacement TS focuser I didn't have enough in-focus when using it at the correct spacing distance. Instead I have this now https://www.teleskop-express.de/shop/product_info.php/info/p4006_TS-Optics-2--corrector-for-GSO-Ritchey-Chr-tiens-w-o-focal-reduction.html (and also this https://www.teleskop-express.de/shop/product_info.php/info/p5120_Riccardi-0-75x-APO-Reducer-and-Flattener-with-M63x1-Thread.html which I'm currently trying out with an Apo refractor), both are small enough to insert inside the 3" focuser drawtube so I can achieve focus ok with them, also they're big enough to avoid significant vignetting on the 16200 chip. Obviously this applies to my 10" RC so things might be different for a 12". Definitely handy if you can fit your corrector in the focuser though.
  • Create New...

Important Information

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