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

968 Excellent

About IanL

  • Rank
    Sub Dwarf

Contact Methods

  • Website URL

Profile Information

  • Gender
  • Interests
    Developer of the Imaging Toolbox.
  • Location
    Near Colchester, Essex, UK
  1. Reflection of an external light source that changes as the scope tracks?
  2. If you're looking to compare noise, e.g. using patches of the background sky, use MAD (Median Absolute Deviation). It is a much more robust measure than StdDev. For example removing outliers (e.g. hot pixels) will have a significant effect on StdDev whereas it won't on MAD and thus allows you to compare the underlying noise distribution of the before and after images.
  3. The manual is here: https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.baader-planetarium.com/en/downloads/dl/file/id/175/product/2856/instruction_manual_for_all_baader_diamond_steeltrack_bds.pdf&ved=2ahUKEwjcipWQhYbqAhVKTcAKHZDcAEoQFjAAegQIAxAC&usg=AOvVaw1GJ5_6SdkVmNgNQpbo9EfL Read page 10. The pressure screw is set for 6Kg (imaging) and may need slackening slightly for visual. It is the single hex head in the centre of the drive base. Don't mess with the other screws.
  4. I don't know about the ASIAir but I assume it uses the same process. Basically you take an image, rotate the mount 30 (polemaster) or 90 (sharpcap) degrees in RA and take a second image. The software identifies the corresponding stars in both images and uses geometry to work out where the centre of roation is (i.e. where the RA axis is pointing on the sky). Using plate solving it works out where the NCP is in the image and directs you to adjust the mount until the NCP and centre of rotation match.
  5. Again, yes a polarscope can be misaligned and will cause issues. A Polemaster does NOT depend on being well aligned though. The software determines the centre of rotation in the series of images, so any misalignment is automatically taken to into account. (Same for Sharpcap). That is why they are superior at alignment. Yes you are correct that if you have a perfect polar alignment, an accurate home position and zero cone error the main scope should be centered on the NCP. In fact the easiest way to check your cone error is to take an image of the NCP (once polar aligned and homed). Plate solve it and you can see how much cone error you have. You can get to (or close to) zero guiding with a high-end mount, e.g. a direct drive mount with no gears. For normal mounts you still need to guide as the gear train will have backlash and periodic error in the in order of arcminutes, and no amount of polar alignment will overcome that for exposures more than a few tens of seconds.
  6. You're confusing three different concepts: 1. Polar alignment refers to the alignment of the RA axis of the mount so that it is parallel to the Earth's axis of rotation. Neither the mount's home position nor the scope's cone error have any bearing on this. - You can polar align using the mount's polar scope. This is subject to a number of cumulative sources of error; how well the polar scope and reticule are aligned to the mount's RA axis, and your ability to precisely align Polaris on the reticule over several steps to find the appropriate hour angle. Usually this can be done well enough for visual, but unlikely you'll get within more than a few arcminutes of the pole unless very experiences. - You can't really use the main scope for this job, as it is subject to cone error, and you do not need to centre on Polaris, but position Polaris in the field of view such that the North Celestial Pole is centred (assuming no cone error). - Much easier is to use a Polemaster or something like Sharpcap. These don't rely on any precise alignment of anything. They take an image of the sky through the camera (dedicated or through your main scope respectively). You then rotate the mount approximately 90 degrees in RA and a second image is taken. By plate solving both images, the software can determine the centre of rotation in the image, and therefore exactly where the mount's RA axis is pointing (not the camera or scope, the axis). You then adjust the mount until the axis is pointing at the correct position in the sky, very precisely determined by repeated imaging and plate solving. You can achieve a few arc seconds of accuracy very easily, the limit being the mechanical stability of the mount and the coarseness of the alt-az adjuster threads. 2. The home position is only important for mounts that don't have absolute encoders (such as many Skywatcher mounts). The mount controller (handset, EQMod, whatever) only knows where the mount is pointing by counting the number of steps the RA and Dec stepper motors have been told to move. This is unlike an absolute encoder which will usually start from a defined index position (found automatically using a sensor), with the encoders counting the rotations of a given gear axis electronically and reporting back to the controller the encoder position, which the controller converts in to RA and Dec. The problem with stepper counter control is that you have to know where you are starting from. There is no absolute position as such, so the controller can only determine that a given axis has moved X steps (and thus degrees) clockwise or anticlockwise from wherever the mount was pointing when it was switched on. Thus the need for a home position, so that the mount can assume a given start location and go from there. If your home position is off a bit, you'll be correspondingly far off after your first slew to a target. You can of course correct this by moving the mount on to target and syncing the handset or EQMod which updates its pointing model to eliminate this error slightly. 3. Cone error also induces slews to be off target since the handset will of course assume that the scope is pointing at the Celestial Pole (not Polaris) when in the home position. Again the pointing model can and will compensate for this as you do more corrections and syncs, but it is desirable to get a good home position and tune out any cone error mechanically to make life less difficult for the visual observer. For an imager, just use plate solving once you are polar aligned; there are numerous free plate solvers and most capture software supports one or more of them, and it literally saves hours when the software finds the target unaided. The challenge with having a set of home position marks made by the manufacturer is perhaps that you can rotate the dovetail clamp by 90 degrees to accommodate a side-by-side bar? I don't see any reason why they couldn't put fixed index marks at 90 degree intervals around both halves of the RA and Dec Axes to be honest, it would save 10 minutes with stickers and fine-tipped pens, but I suppose there would be additional work to adjust everything so they were properly aligned and represented the actual home position.
  7. This month's public lecture will be given by Ian Lauwerys on the subject of radar meteor detecting. We will look at the practicalities of building your own back-garden meteor detector, delve in to the murky world of top-secret spy installations and cover some of the science behind meteor detecting. The talk will be presented live via our YouTube channel (details below) and will also be available for replay later. The downside if that you'll have to supply your own tea and biscuits, but on the plus side we'll be able to give a demonstration of a meteor detector in action. The stream will be available here at 8:00PM (UK time) on Wednesday 15th April 2020:
  8. I've finally managed to get round to building a remote control flat panel (suitable for use in SGPro and any other software that supports Alnitak Panels). Total cost was less than £25 - I used one of those LED tracing panels that you can find all over Amazon and eBay. Surprisingly the first one I bought turned out to be a real winner - A4 sized, USB power, 60 LEDs and dimmable for £15. The key difference seems to be that many of these tracing panels have a grid of LEDs close behind a diffuser panel which creates dark and light spots, or the are edge-illuminated from one side only. The one I got has two strips of 30 white LEDs along the long edges - internally it has a reflecting layer, a clear acrylic sheet which the LEDs shine in to and then a diffusing layer sandwiched on top. The light seems very uniform and I didn't need to add any futher diffusing or similar elements to make it usable. I hacked out the original controller and hooked everything up to a 5V Arduino Nano clone (£5) and a MOSFET (£2) and used some Alnitak emulation code written by one of the SGPro developers. It is now mounted on my observatory wall and I just have to park the scope and SGPro can take care of the flats for me for a fraction of the cost of the cheapest automated flat panel. At full brightness I can do flats for LRGB filters in a couple of hundredths of a second, and about half a second for narrowband filters. This is ideal for my ASI 1600MM-C (not Pro), since longer exposures use a readout mode that creates gradients in your flats. Used it this week and results are as good as using my DIY manual diffuser and cloudy sky method, with less risk of floating away in a garden under 2 inches of water! Anyway, full write up with shopping list, photos and diagrams is available here: https://www.blackwaterskies.co.uk/2020/03/cheap-diy-remote-controlled-flat-panel/
  9. It will be a lot smaller than you are probably expecting. To give you a rough idea, below are are couple of images of Venus and the Moon that I took a bit over a week ago - both with the same scope and camera, so effectively the same "magnification". Venus should look like a tiny 'half Moon' at the moment when you have it in focus, but as you can see it is way smaller than the actual Moon. It will become more like a crescent Moon over the coming weeks.
  10. Don't know to be honest. All my ISS detections have been main beam but obviously much bigger target
  11. Good luck finding a corresponding motor pulley. I could not find anything with the same belt profile, correct number of teeth and pre-bored to an appropriate diameter for available motor shafts. In the end I put my own pulley in place of the other knob. Very frustrating and believe me I really looked hard. Almost like Baader wanted you to use their own unit!
  12. Caught some Starlink echoes on the GRAVES meteor detector from here in Essex. They were coming across at approximately 10 second intervals for several minutes. Two were strong enough to trigger the meteor capture routine and a couple more coincided with a real meteor, but manually captured a bunch more that were below the threshold.
  13. LBN552 appears as a small orange patch to the lower left of this image, with dark dust and reflection nebulae from the borders of Cepheus, Ursa Minor and Draco. This was a bit of a rescue job as had a bunch of problems during initial acquisition - mount lost power at one point, then auto-focus failed during the second half of the night (still dialling it in with the new scope). Polar alignment is way off as I think the pier has shifted a bit since I last used it, so lots of field rotation caused the outer 10% to be unusable once stacked. I need to make a better flats box too so this is just a crop from the centre of the image complete with dust bunnies. needs four or five times the amount of data really so the overall effect is a bit waxy at the moment, but still it is a pleasing start. Full version here: https://www.blackwaterskies.co.uk/2020/01/lbn552/ Acquisition: William Optics GT81, WO Flat 6AIII 0.8x reducer, ZWO ASI1600MM-Cool, Atik EFW2, Astronomik LRGB 1.25″ Mount/Guiding: Orion ST80, QHY 5, PHD2, Sky-Watcher NEQ6, EQMod, Sequence Generator Pro Processing: PixInsight 1.8.8 Dates: Dec. 18th 2020 Lights: L 60 x 120s, R 30 x 120s, G 30 x 120s, B 30 x 120s, Unity Gain, -15C Bias: No Darks: 100 Flats: No
  14. Didn't get any response on this, maybe being too specific Anyway for reference if anyone else has the same issue and having had time to do some trial and error I can confirm that the reducer needs to be set to at least the 9.1mn mark on the scale for a GT81, the 7.1mm setting in the second diagram is definitely too close. The extra 2mm outwards adjustment goes from visibly egg-shaped stars in the corner to visually acceptable ones. If anything I might try going a bit further out as measurements using FWHMEccentricity script in PixInsight suggest the stars are still somewhat stretched in the corners. Results with the original setting (7.1mm + 0.33mm to account for 1mm thick filter glass): Results with the 9.43mm setting (9.1mm + 0.33mm to account for 1mm filter glass): The stars are visibly rounder in the corners, measurements of eccentricity suggest a bit more spacing needed as ideally would want to be below 0.45 across the entire field. Measuring using a digital caliper, assuming a 1mm thick filter, you're aiming for approximately 90mm between the rear face of the fixed part of the reducer and the front face of the camera body (assuming standard ASI 6.5mm sensor setback), there are a few models with a different value so do check. You'd need to add a further 0.33mm for each mm of filter thickness or reduce by 0.33mm if using an OSC with no filter :
  15. I can confirm the Revelations do have the Lazy Susan bearing by default. One issue is making sure the base is level - if it is the Az action is smooth and can be a problem with the wind moving it like a sail. If not level then it can stick quite badly. With mine the Alt motion is rather too free (probably needs new hold-down springs as it is rather ancient). I use some magnetic welding weights to adjust the front to back balance depending on where I am point it.
  • Create New...

Important Information

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