vlaiv

With increasing interest for astro photography, question is often raised about suitable equipment under restricted budget. Many people can't afford what would most consider adequate start of mount for serious AP (HEQ5 class), and there are some cheaper alternatives on the market but I think we lack proper understanding of performance of such mounts. Even higher priced mounts rarely come with graphs depicting mount tracking performance (such as obtained by high resolution encoders coupled to RA shaft). I wondered if there is a way to qualify generic mount tracking performance, and by generic - I mean, out of the box, unloaded, with no external influences - like wind, seeing and all other things that hamper mount performance additionally. A way to measure p2p periodic error and also smoothness of tracking. This would be considered as "baseline" mount performance, and real time usage would probably yield performance worse than that. While parameter like - "mount will perform better than ... " could be viewed as generally more useful one - I think knowing baseline is also useful - and the way of measuring it is nice "indoors" project for cloudy nights Idea is rather simple, but it looks like it will take some creative thinking to get it to usable level. Initially I thought of following: Take a laser pointer (collimated light beam) and mount it on top of dovetail clamp in some way so it is secure - pointing to the side, so that beam is close to level. Take telescope and camera (equipment that most imagers have so they can readily test their mount) and place telescope with camera attached so that laser beam initially coincides with optical axis (or close enough). Since laser light is close to collimated (probably diverging very slowly) - it will act as a distant point source when focused and project a spot on sensor. Tracking of scope is then engaged and sequence of captures is taken. Data is later analyzed and centroid of spot is calculated for each frame. If light is bright, even very short exposures in quick succession can be recorded - providing very detailed graph. Spot should move slowly across sensor - one can even calculate the rate of motion for fully collimated beam. Sidereal rate is 15"/s, and from telescope focal length and pixel size - we can get sampling resolution. Let's take common setup - something like APS-C sized chip and short frac. Such setup often has 2"/pixel, so in one second spot should move about 7.5 pixels. If sensor has for example order of 3000-4000px across - that will let us record about 500 seconds of tracking. With use of focal reduction, most mounts can be sampled over whole worm period. I started researching into lasers - but it looks like that might not be viable option, I'm afraid that concentrated laser beam might damage the chip! Full power of laser (even very low power one, like 5mw) will be concentrated over few pixels - extremely short exposure will be needed to avoid saturation, and I'm really worried about damage to sensor. Does anyone have idea if laser would be feasible? Maybe strong ND filter, or even better - solar filter film - really small piece would be enough to filter out laser light to acceptable levels? Other ideas would be - artificial star - but then there is issue of close focus - most telescopes can't focus close enough to do it indoors? Another option would be artificial star with collimation lens? Or simple variant - eyepiece. One would place artificial star at field stop of eyepiece (involves unscrewing 1.25" nose piece) and testing against the wall - projected image should stay roughly of the same diameter - maybe diverging ever so slightly (less divergence - further out object appears to be). My worry is that in case of collimated artificial star (or simple tiny whole in some sort of screen) - there will be very large image on the sensor - whole setup will act as relay lens system with magnification (image - short fl - long fl - projection). Maybe that won't matter as we are interested in measuring center of the image as it moves - it does not matter if it's a single spot or a circle - as long as measure of position can be taken (like center of mass). Laser sounds more interesting because filter can be employed on sensor star (laser at 532nm and Baader solar continuum for example) to boost SNR if measurement is not done in absolute dark. Question is only - is it safe to use, and if no - can we make it safe? What are your thoughts at all of this?

I just discovered these: I've been thinking about doing something similar with planetary/guider cams - it looks like there is software to generate these maps. Simple photo tripod with 1/4 thread can be used to mount most planetary cams. Simple CS wide lens can be used - like those used to capture meteors. This gives much better info on sky lp - especially for imagers as it provides direction information - useful for selecting targets. One can even do sequence of images over the course of the night to do idea of changing levels of light pollution (lights going on and off).

There are couple of places (you can find them in links section on SQM-L/Unihedron website - check "Light pollution maps / databases" section). You can also submit your reading to lightpollutionmap.info You can also view submitted readings with additional info by selecting appropriate layer (color dots used as markers for location - click on one to get info on readings):

Same here, and it says M31 difficult with averted vision and MW invisible. I managed to see M31 on few occasions with averted vision (maybe twice or three times at the most), but I was also able to see hints of MW at zenith, probably more times than M31. Only once have I managed to see M31 dust lane with 8" scope from my location. These things vary with transparency, so LP is not the only factor. Aerosol optical depth extincts light quite a bit, and for my location I get variations from below 0.1 to above 0.5 - when southern wind blows it can lift Sahara sands and carry them all the way across Mediterranean Sea. More than once we had "muddy" rain because of this in past few years. This is why it is useful to check out aerosol optical depth forecast (finally managed to find the page, they kept changing it lately): https://atmosphere.copernicus.eu/charts/cams/aerosol-forecasts?facets=undefined&time=2019010200,3,2019010203&projection=classical_europe&layer_name=composition_aod550

You know how the saying goes: "one man's trash is another man's treasure" I found that website to be extremely useful tool once you understand it's limitation and application. Readings with SQM/SQM-L will certainly vary across nights and will be different to map as map probably represents annual average value, or at least average value of recording times (which might or may not be uniformly dispersed throughout the year). VIIRS data is actual recording taken via satellite of ground sources total illumination. Atlas data represents numerical approximation of sky brightness made by integrating ground sources illumination and atmospheric scatter - I think it is very clever that they managed to get it to such level of precision. One needs to distinguish sky brightness to other things that impact visual astronomy. Transparency, both local and high altitude can have significant impact even in very dark skies - this can make one observing site preferential to another one. SQM readings on particular night might vary significantly to this map. Amount of water vapor in the atmosphere can contribute to light scatter quite a bit, light sources are very dynamic, lights get turned on and off, it even depends on traffic density and road conditions if you have significant road network near by. Dry road and wet road have different reflection properties. Snow increases light pollution quite a bit. So many things impact this that it is quite a miracle that such map works and works so well - within mag 0.5 in most cases - that is up to 63% of base value. Brightness and Artificial brightness are probably ground luminosity recorded by satellite. First being just natural sky glow reflected of the atmosphere and ground, and second artificial lights on ground - what we think of when we say light pollution. SQM is magnitude per arc second squared and can be thought of as: if every arc second x arc second (square with sides 1 arc second) contained a star with apparent magnitude of XX without any other light sources in the sky it would be as bright as sky is now. Just for comparison, Vega is 0 magnitude star, and Jupiter has about 1600 arc seconds squared of "angular surface" when largest. Every 5 magnitudes is x100 less light, so mag20 star is 100,000,000 times fainter than Vega (hope I got number of zeros correct ). It should consist from natural sky brightness (zodiac light, milky way, stars ...) + atmospheric scatter of artificial light from the ground. When there is no artificial part, natural brightness is at 22mag. This number is more meaningful to imagers than observers because it can be used in SNR calculations. It is also useful to guess visibility of some faint objects if you compare that to surface brightness of those objects - this is what Contrast Index in Stellarium for example represents: Ratio of brightness of target to brightness of background sky. Surface brightness in Stellarium is given in magnitudes per arc minute squared, and for conversion one needs to subtract add 8.89 to get magnitudes per arc second squared. Do be careful with surface brightness in stellarium - it is average value and real value can vary quite a bit - think galaxy core vs outer parts - core is way brighter than outer parts. Ratio from above info represents how many times that particular sky is brighter than natural unpolluted sky in zenith. Altitude and coordinates are self explanatory, me thinks.

Not really if you think about how magnitudes are calculated - it is ratio of brightness to a reference point (0 magnitude). Just because natural sky has brightness of mag22 - that does not mean narrow band can't have much lower brightness. Simple example would be as follows: let's take 400-700 spectrum and we approximate uniform brightness over that spectrum. Full spectrum under consideration contains 300nm. Now take 3nm narrow band filter. It will collect x100 less light. x100 is 5 magnitudes of brightness down. Such filter would measure mag27 if our source is mag22. Now take into account that light spread is not uniform and that some wavelengths carry more brightness than others - you can easily see that this difference can be over 5 mags for narrow band filter.

That is quite a bit of difference! Most that I've heard is like half a magnitude of difference between SQM reading and Light Pollution Info. Maybe your SQM meter is not calibrated properly? For example: This is what light pollution info provides: And these are set of measurements on star parties in 3 years: Btw this shows increase in LP levels over time - first measurement is done in quarter moon but is still the best.

Here it is, but it is worse now than 3 years ago - a lot of development / new construction right near me. Viirs data 2015 vs 2017: But I hope to move to: This year at the end of summer . We decided to move out of the city and looking a place to build a house (and obsy at some point) - this is my favorite location so far: Just click with your mouse pointer on lightpollution info map and there will be popup with said info

One could try progressively wider band pass filters, but even if we don't see pattern without filter, we would still be able to detect it - by light level around bright stars when no filter is present. One can measure background "scatter" levels further away from the star and right in vicinity of the star - they should be different in case of this effect layering on top of itself. I believe it should also be visible "by naked eye" as gradient that starts of at star then fades away further from it. This is what I pointed out as missing in no filter image above. I do have a "model" of how interference filters and ASI1600 can produce such artifacts due to micro lenses that is dependent on both wavelength and presence of interference filter (and depend on it). Mark (sharkmelley) stated that it is definitively due to sensor cover window as reflections produced come from source that is less then 1mm away. I did some quick (but also maybe inaccurate due to this) measurement of reflection diameter (first order) on one of my OIII images. According to this reflection distance calculation: http://www.wilmslowastro.com/software/formulae.htm#REFLECT Source of reflection is about 2.6mm in my case. Only thing that I can think of producing such reflection would be camera chamber window that is AR coated and should not produce pronounced reflection because of this on its own (unless of course AR coating is not adequate). What can happen, at least I think it can, is following: Interference filter placed close to camera chamber window (not sensor cover one) and parallel to it, can maybe create Fabry Perot filter configuration thus turning AR coated window in very effective blocking / reflection window - and this together with micro lenses then proceed to produce artifact. Light would pass forward thru filter + AR coated window because of angle and then get reflected of micro lens in all directions thus changing angle. This reflected light then encounters AR window + interference filter combo that acts as reflection filter and gets reflected back to sensor producing artifact. This is of course simplified explanation - in reality it would be multiple interference of light with itself with one component of the wave being reflected of interference filter.

Yes, I know they have about the same length (focal length at least, tube length depends on secondary size as well, and how much of tube there is after focuser - good figure, not often utilized in mass produced scopes would be x1.5 diameter), but size is quite different - base is also a bit wider and quite heavier in 8" model - around 16kg vs 26kg total (base + OTA). Not something that can easily be seen on video though

Some of the smaller models (usually designated as table top) need something to put them on, but 6" and 8" are properly sized to be placed on ground. Here is interesting thread about observing chairs (lot of recommendations) - something that you will need. 6" and 8" dobs are not really suited for standing observing (although they can be used like that, but you will bend your back quite a lot). Here is a short video (plenty of such videos on youtube) - showing the size of the scope and how it is assembled: It would be a good idea to browse thru some of those videos to get "the feel" for the size of the scope and what it looks like - both 6" and 8" - it can help you decide if you opt for any of the two.

Both 6" and 8" are fairly easily portable with even small car. Both tubes are around 1m and a bit long (1200mm focal length, but not all of it goes into tube length) - so fit nicely on the back seat lied down. Dob base can fit into most booths. You need an observing chair - look for foldable one or one you can take apart and put together with ease. Eyepiece case and you are all set. In reality such dob is about as portable as EQ mounted shorter dob - you need place to put tripod and mount and scope. You might not need observing chair for EQ mount - but it is much more comfortable observing while seated down. For really compact and portable design you need to look at folded scopes - here you will find that you will be limited by aperture even second hand for your budget - like Mak 127 you mentioned - it will gather about x2.5 less light than 8" dob. Mind you, nothing wrong with 5" Mak or SCT - very good scopes and very light for their size - Dobs are often said to be the best bang for the buck - meaning the most aperture at lowest price, and aperture is important for visual (as long as one can manage bulk).

Get yourself one of these used, but even new one is not so far out of budget: https://www.firstlightoptics.com/dobsonians/skywatcher-skyliner-150p-dobsonian.html If you can manege the size (in storage and transportation terms), look for 8" version used - probably the best starter (and sometimes lifetime) scopes for visual.

Hopefully we are not going off-topic, I just found that image online as example of people complaining at mentioned artifact (I believe it was posted on zwo forum, but can't be sure - forgot where I copied it from). We agree on most points - how certain halos were created, but what I wanted to point out is that "no filter" does not generate much larger (like we agreed roughly x50-100) effect - if anything it seems that there are no artifacts at all related to micro lens reflection. This leads me to conclude that your description does not account 100% for what is going on - clearly presence of filter and it's reflective surfaces in some way contributes to interference pattern. It is also very indicative that different people get different results based on type of filters they are using. Some even have this effect on R, G and B interference filters. Don't get me the wrong way - not trying to say that your analysis is flawed and that you are wrong - I'm just questioning if it's the complete picture of the issue. From above, it seems to me that it might be the case that type of filter, it's position in optical train and F/ratio of system all have "a say" in how pronounced effect is, or if it's there at all.

I appreciate your explanation, and certainly think it is plausible. I'm still not 100% convinced however (can't be helped ) What do you make of this image then? Ha and OIII clearly show patterns and filter presence (large defocused aperture image superimposed over star). Luminance also shows this, and what I believe is mild reflection from chamber window (enough light to make it detectable since it is AR coated). No filter has no filter signature, only halo that I would say is from chamber window (and some diffraction spikes that are probably from mirror clips and focuser or what ever - since this is newtonian scope). Not sure that there is halo from this effect though - unless I've mistaken halo around star to be from chamber window and it is in fact from this source - but I would expect it to be progressively brighter towards the star?

Thanks for explanation, and couple more questions if I may - we are talking about sensor protective cover and not chamber front window that is not being AR coated, right? By saying that it has been shown beyond doubt that reflection is off this surface, I presume distance from micro lens to this cover has been measured and also its thickness and compared to defocused star at certain F/ratio beam (defocus position being twice distance to first or last surface of this cover)? Also, by saying it is not filter induced, I can expect in pure mirror system and no filters when shooting bright star to have large halo around it with ASI1600 - even more so than in narrow band image (if we look intensity in small part of spectrum being roughly 1/50 - 1/100 of 400-700 continuum depending if its 7nm or 3nm filter, I can expect it to be about 100 times brighter than with narrow band)?

I agree with you, but question is where constructive interference comes from? It needs two parallel surfaces to form - and is usually in form of reflected wave that creates out of focus star image on sensor. It is definitively due to micro lens on sensor because out of focus stars create square / cross shaped pattern. So it can be between micro lens and sensor cover window, or it can be between micro lens and filter. Sensor cover window is AR coated, while filters are interference blocking filters - one is coated to pass as much light as possible - other is created to pass narrow band of light and reflect everything else - key emphasis on word reflect Simple experiment could show if this is related to cover window or filter - just move filter further away - if pattern grows in size and gets dimmer and possibly change intensity - it is related to filter. Another clue that it might be related to filter is to observe OP image and example that I posted. My filter is mounted very close to sensor, and my guess is that OP has regular filter wheel and filters a bit further away from sensor - based on size of reflections and distance between first and second order reflections - mine smaller and denser while in OP image larger and further apart (indicating greater distance between reflective surfaces).

If filters play no part in it, then similar artifact would be visible in red and lum channels as well (those pass Ha wavelength as well), but would not appear with different wavelengths of light - sensor to cover distance is fixed and interference happens when there is matching between wavelength and distance of reflective surfaces. Here is H-alpha shot of mine (ASI1600 + Baader H-alpha 7nm, pure mirror system): Effect is barely noticeable on mag 7 star Same star with OIII filter: Much larger effect And I have not seen this effect in any of my broad band images - even with very strong stars.

As above it is interference between filter or focal reducer / field flattener and micro lenses on ASI1600. Not everyone gets those, and it usually happens on bright stars. If you can, try changing distance of your Ha filter with respect to sensor - either a bit closer or a bit further away. You might also try removing field flattener or changing a bit it's distance (I know it will cause trouble with flat field, but just for testing purposes). This effect is probably dependent on spacing as well as angle of light cone coming in on sensor. Maybe some combination of spacing and light cone angle will lessen or almost remove effect.

Yes, you are right - it is rigid connection between the two (there better be ) - and going in circle - catching up is in opposite direction on sensor but in same direction along the circle.

Interesting. I do have one question - will it work as a guide scope? Raising this question because I can't get my mind straight right now if there will be impact of it sitting on the opposite side. Both main scope and guider rotate in same direction, but mount moving ahead in RA on scope side will mean lagging behind on guide side (or will it?) - wonder if calibration is going to take care of that.

I have that GSO (Revelation) x0.5 1.25" focal reducer, and here are some things that you should know about it: 1. Focal reduction depends on distance between focal reducer and sensor - greater distance, greater reduction - use this in combination with above diagrams - start with close distance and small reduction to see if you are getting usable field at all - then extend distance up to point where it still works. 2. I had to reverse lens in mine - it was set one way at factory but I found that for imaging it works better by reversing the lens in holder (don't ask me how I got idea to try it out - probably read something somewhere). You might want to try which way it gives you better image, by undoing retaining ring and just flipping it around. Use some sort of soft cloth to handle the lens - don't touch it with your fingers to avoid staining it. You can do this at prime focus of scope with your guide camera to avoid any OAG related complications.

I'll try to explain with diagrams, first just reducer physical length: Regular diagram: Diagram with reducer in place: As you see, in order to focus both guide camera and imaging camera at the same time, because you added focal reducer before guide camera and thus pushed it back, you need to add optical path between OAG and imaging camera. Next diagram will try to explain additional distance needed because of the way simple reducer works: This one is trickier to understand because reducer bends rays - this is why we need inward travel of focuser for simple reducers like mentioned two element 1.25" x0.5 reducer. Actual sensor will lie where bent rays meet to be in focus, but at the same time imaging sensor needs to be at a distance equal to where guide sensor would be without bending of the rays - further out. Hope this makes sense

Forgot to add, if you can, guide with ASCOM driver and higher bit depth, rather than native drivers and 8bit.

It will not work (at least I think so, below is why). All it can do is shrink available field onto smaller region of sensor, but ultimately field of view is limited by pick-off prism and more importantly opening in prism holder. I currently guide with a bit larger sensor - ASI185, which has diagonal of ~8.6mm at 1600mm FL - F/8 beam. I get vignetting on this setup, here is screen shot: Other things that might interfere with using focal reducer are: - do you have enough back focus to fit body of reducer between sensor and T2 connection of OAG? Additional distance must be mirrored between OAG and main camera as well. Focal reducer moves focus point inward - this means another change in imaging sensor position vs OAG (not sure if this will bring it back forward or does it need still being pushed further out - probably this second point). What about fixing problem of guide stars in another way? Things that you can do to guide on fainter stars: 1. Make sure your OAG is focused properly. Although you can guide on slightly defocused star, this is not a good thing if you want to use faint star for guiding. You want star light to be concentrated in smallest possible area to maximize star profile and SNR. 2. Position prism as close to light beam aimed at the sensor - just barely avoiding prism shadow on your imaging sensor. This means rotating prism so it sits next to longer sensor edge. Further out in the field stars become more distorted due to different aberrations like coma or astigmatism - look at difference between stars in above image - in right part they are more concentrated than in left part of the field. You want to pick up stars in the least distorted part of the field - again has to do with star light concentration into smallest region and SNR 3. At longer focal lengths you can use a simple trick because with small pixel guide camera you have enough precision - bin your guide camera output. It does not matter if it is true CCD hardware binning or CMOS software binning - it will increase SNR of stars and make it possible to guide on faint stars. 4. You can always increase guide exposure (up to mount limit), if you are used to guide at 1s or 2s exposures and can't find guide star - why don't you try 3s or 4s guide exposures? Sometimes in poor seeing I go as long as 6s or more with my HEQ5 (it is tuned and belt modded, and that is above max exposure that I would otherwise use on it but if seeing is poor, guide performance is not going to be the best possible anyway).