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drjolo

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drjolo last won the day on July 23 2018

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About drjolo

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    Nieborowice, Poland
  1. I would like to share with you my measurements of iEXOS-100 mount stepper motor tracking rate recorded in short time scale. The reason we did such measurement was permanent problem with elongated stars, even at short 1-3s exposures during focusing. Together with friend of mine we took off RA stepper motor cover and noticed that stepper motor movement during tracking is not linear. I put Allen key to the RA worm shaft to better record this effect: It did not look good, so I decided to actual measure the period of that wobble. I measured that during 24s of movie axis wobbles 18 times, so the period is 1.3s iEXOS-100 has 1:144 worm gear ratio and 1:4.5 stepper gear ratio. It gives stepper revolution time equals 133s. That means that this wobble corresponds to 1/100 of stepper revolution. Assuming it is 200 steps per revolution motor, then it corresponds to 2 stepper motor steps. That non linearity may have two sources. Either chosen stepper motor model is non linear itself when microstepped. Or microstepping controller is not able to control the motor linearly. We contacted with manufacturer and we got the response it is known problem and it is expected behavior, and it does not affect the images captured with this mount. So I decided to measure amplitude of that effect. I exported 18 frames of the movie to Photoshop and measured at 400% zoom level the distance that Allen key lever travels over one 1.3s period. Then after some calculations it gave me the following results. Orange line is perfect tracking rate 15"/s. Blue line is actual measured axis movement. Gray line is an error. Horizontal axis is time in seconds. Vertical axis is RA main axis movement in arc seconds. So the maximum error is about 5.5 arc seconds. It may cause elongated stars even at few seconds exposures, when you have imaging setup with 300-400mm FL telescope and modern small pixel CMOS camera. At higher declination this effect will be of course smaller. And this is only one of the error sources in the mount. It will sum up with worm periodic error and any worm gear inaccuracy. According to manufacturer that is expected behavior and do not affect imaging results. However I would expect better behavior in the first step of the transmission in the astroimaging mount. 5.5" error does not look large, but it has 1.3s period and cannot be compensated with guiding or periodic error compensation. Then it sums up with all other errors and I would not risk statement, that it does not affect the images captured with this mount.
  2. Thanks! You don't need a direct sunlight to take Sun spectrum - it is enough to point the spectroscope inlet to the cloudy sky. I took my first spectra ever this way It was captured with IMX224 camera
  3. I have been using LowSpec already some time with 600 lpmm grating (from Edmund Optics). Recently I have purchased 1800 lpmm holographic grating from Thorlabs to be able to analyze deeper Ha lines. This grating has pretty large dispersion, so I needed to saw a little (about 1mm) of the grating holder to be able to reach the very red end of the spectrum. Other than that all works fine, and here are first results compared to the ones captured with the same LowSpec device an 600 lpmm grating. Sun Mg triplet at with 600 and 1800 grating and different slits Sun Na doublet with 600 and 1800 grating and different slits Aldebaran spectra around H alpha line, 30um slit 17 Tauri spectra around H alpha line, 30um slit Other equipment used: Meade ACF 10" f/10, Evostar ED72 + IMX224 as electronic viewfinder, ASI290MM as guide camera, EQ6 mount. Suburban sky.
  4. Both Nixie tube converters arrived, both work, however only one meets our requirements. Output voltage in the first device can be set between 80 and 380V. I tested it with two Relco SC480 starters and they start to glow at 220-240 VDC, and optimal brightness is at 260-270VDC. There is 27 kohm resistor in series with the starter bulb (it can be two 47 kohm resistors in paralell). So this is the first one device: This converter is okay Second one is also regulated, but this regulation can be done in a range 100-230VDC, and that is too low. At 240VDC one of my starters just starts to glow, and second one does not even start. So when you choose Nixie tube converter, select the one that gives at least 270-280 VDC output. This one gives too low output voltage Remember to put resistor in series with the starter. Otherwise bimetal will be short in a few seconds, and the converter may be damaged. I left first one turned on for about 15 minutes and all was fine.
  5. Yes, exactly - it is set to f/2.8 and almost at infinity (exact infinity would be when focus is set to vertical part of this L. "Fast" lens means less f number. f/2 is faster than f/4, because at f/2 you need less time to exposure the same scene. Stopping down means to set higher f number, so the actual aperture will be lower.
  6. I think there are two things worth to mention about this lens. One is aperture: it may happen that you will get little bit better image quality when you stop it down a little. But then bright stars will get spikes that comes from aperture blades. Some users (including me) attach "aperture stop ring" in the front of this lens to stop it a little and keep circular aperture shape. Second one is mounting to camera. This connection needs to be stiff, because that is fast optics and do not forgive tilt or bending. And also backfocus distance needs to be kept, so you focus on the stars as close to infinity mark at the focusing ring as possible. This lens has internal focusing, so it is designed to have specific lens configuration at specific object distance. In my setup the ring is set about 5mm left to that mark when I focus at stars. And then I have pinpoint stars in the whole frame, but I have small chip - QHY163M. And SY135 is full frame lens. Then I think you should be very happy about this lens Here are my thoughts after (now) three years of having it https://astrojolo.com/gears/thirty-months-with-samyang-135-f-2/
  7. I had TS130 f/7 for about two years. I used it mainly for imaging ( https://astrojolo.com/tag/ts-apo130/ ) but also had few observing sessions. I remember once we compared views from APO130 and SCT 8". APO provided excellent and contrasty views, very pleasant. But when we moved scopes to Jupiter, then SCT 8" showed noticeably more detail. And APO130 is heavy beast, significantly heavier than SCT 8". I think it is more comparable to SCT 10", but still longer. 2/3 of its mass is located in the front cell, so apparently there is lot of glass It requires two hands to carry, unless you will equip it with some comfortable handle. As I remember with rings and dovetail it was already over 10kg. Here is mounted onto EQ6 and with SW 80/400 as guidescope:
  8. After careful preparation I made another attempt to radial velocity measurements with LowSpec spectroscope. Currently it works at low resolution (600 lpm grating, R~1400) and I expect accuracy at the level of 1/10 R. With 600 lpmm grating it maps to about 20 km/s. So I picked some fast star for the first test - BD+37 1458. And slow HD43094 for comparison. Here are both candidates imaged with EV (Evostar 72, IMX224, 30x15s): Here are both star spectra already with Relco SC480 (light grey plot). Both stars exposure time was 3x180s. It can already be noticed, that Ha lines of both stars are shifted. But rightmost absorption is at the same position - this is atmospheric Fraunhoffer B line. After zooming into Ha line I have made several measurements in BASSProject software: And here are the calculations. Last step was correction to Earth orbital and rotational motion (calculated in BASSProject Tools). The actual values in SIMBAD database are 242 and 38 km/s. So the error is more less equal to my expectations (about 1/10 R). One thing that I should do is to determine systematic error. But I will do another attempt when 1800 lpmm grating will arrive (it is already ordered). I am pretty happy with these results at the moment Although I think spectrum SNR is too low for this kind of work, and it probably also negatively affected the accuracy. Another thing to improve is position of the line of interest. I should have put Ha line in the centre of FOV and focus there. http://simbad.u-strasbg.fr/simbad/sim-id?Ident=BD%2B37+1458 http://simbad.u-strasbg.fr/simbad/sim-id?Ident=hd43094
  9. I have used both a flat mirror from cheap 1.25" diagonal, and also zero order as Eric mentioned above - both solutions worked ok. As a collimation tool I used DSLR camera with 50mm lens set manually to infinity. And I have been watching slit at the Live View at 10x magnification.
  10. Thanks Jim! I will do some final tweaks and clean up, and also need to add some small diffuser to Relco starter to make calibration more reliable. Then I will post or link whole project here. At this moment I am very happy with LowSpec results. Resolution is pretty high (over 1500 at the red part with 600 lpmm gratning), it is fairly easy to setup and calibrate. I use QHY163M as main camera, ASI290MM as guider, all is attached to 10" SCT and there is IMX224 camera with 50mm guider as electronic viewfinder. One tricky part (for me at least) was to align guiding optical path. Guiding lens is fast and needs to be set very precisely to avoid coma and/or astigmatism. Now it is time for me to decide what kind of work it should do. I am not yet 100% sure, but I ordered 1800 lpmm grating to check it out I already had few nights with LowSpec (with and without Moon), and my almost all spectra are at my blog https://astrojolo.com/spectra/
  11. Yeah, I already have problems with such solution - I wrote about it at separate thread https://stargazerslounge.com/topic/348122-relco-sc480-starter-lines-position-depends-on-bulb-orientation/ . I need to redesign it to use some diffuser.
  12. I use LowSpec spectrometer in the (mostly) remotely controlled roll-off shed and also I plan to use 1800 lpmm grating, so I decided to automate the device. The automation covers such functions: moving Relco starter in and out of optical train adjusting grating angle adjusting focus switching on/off Relco starter regulating 0-100% slit LED iluminator perform all the tasks above from the dedicated small software The components that were used: three small 12V BYJ28 stepper motors with integrated gear box Arduino Nano two ULN2803 universal drivers universal PCB, two 5.5/2.1 DC sockets, 5 and 2 pin JST XH connectors, some cables and wires 5mm red LED with 15 degree light angle as slit iluminator M5x100 bolt that replaces M5X80 bolt in focuser M6x40 bolt with M6x20 cylindrical nut that replaces micrometer screw to regulate grating angle 3D printed elements - casing, gears, etc All gears and software were developed and assembled step by step, so actually there was no serious problems during whole process. Currently Relco starter is controlled with small relay, but it will be replaced with 12V inverter, so no 230V will be supplied to spectrometer. After one and half night session all worked well, and I didn't need to visit my shed to refocus or change spectrum range. And that was the idea behind it If someone is interested in this solution please let me know, so I clean up all files and share STL objects to print, schematic, Arduino sketch and control panel software. Panel is for windows system only, but the communication protocol with Arduino is pretty simple, so it can be rewritten to other systems as well. Or it can also be controlled using serial port terminal as well. RR Her spectrum below - captured already with modified LowSpec spectroscope:
  13. Thanks for quick response - I didn't realize that I would need RV standard stars for high accuracy RV measurements. There is whole lot of things to learn. I will check again if that shifts in my setup are caused only by bulb rotation and if they are repeatable. I will fix that bulb in the holder so it will not rotate, add some small diffuser and then maybe the line shift will be more stable, so it can be measured with RV standard stars.
  14. In my LowSpec spectroscope I placed Relco SC480 starter bulb to be able to do absolute spectrum calibration. Last night I made some first tests and tries to measure Doppler shift one of the fast stars http://simbad.u-strasbg.fr/simbad/sim-id?Ident=hd237090&submit=submit+id - 120km/s. Hovewer the results I achieved were far from true (about +30km/s when velocity correction was applied). I eliminated few possible error sources and noticed, that position of emission lines recorded from Relco starter depends on the bulb orientation (rotation along vertical axis). Starter is placed about 25mm in front of the slit, and is oriented parallel to the slit direction: I recorded three test starter spectra after rotating it 90 degrees in the mouting hole. The lines are significantly shifted depending on the starter orientation: This is screenshot from BASS Project software. All images were tilt and slant corrected in the same way and active region is the same for each image. Lines are shifted by several pixels (dispersion is 0.61A/px). I do not think I will be able to do proper calibration in such configuration. Am I doing something wrong? There must be a way to do proper calibration with starter bulb in front of the slit - many commercial devices work that way.
  15. I have ordered two small Nixie tube regulated converters, once they arrive I will let you know. https://pl.aliexpress.com/item/32926216972.html https://pl.aliexpress.com/item/32986857367.html
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