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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).

F/ratio plays significant part in performance of CC - SW CC is good for F/5 scopes, although I'm not sure how much SA there is with that F/ratio. Scope used in comparison of coma correctors is F/4. 8" F/4 has focal length of 800mm - and that puts it in right spot for 1"/pixel with ASI1600. F/5 8" with x0.9 reduction will provide 900mm focal length - that is 0.87"/pixel with ASI1600 and I would personally rather be on north side of 1"/pixel than below it - that would mean ASA x0.73 CC - and that one will not provide fully corrected field for ASI1600. It will for ASI183, and I think that would be really good combination, but then again I would bin ASI183 as well for effective resolution of 1.36"/pixel rather than leaving it at 0.68"/pixel. There is characteristic signature of SA on star shapes - most SCTs have such stars (SA depends on wavelength with SCTs, and also with primary/secondary distance that changes when you focus) - almost "button" like rather than point like - smooth vs sharp so to speak.

After reading posts like this one: http://www.astrofotoblog.eu/?p=856 Not really convinced in perfromance of coma correctors and fast newtonian systems. Granted 200PDS is not that fast at F/5 - but slower the newtonian - longer the tube and harder on the mount and guiding it is.

I agree with you on that one - aim for final resolution, but this is where I found RC to be the best match. Let's say that we want ASI1600 and 1"/pixel resolution with 8" aperture on HEQ5. That means focal length of about 800mm. There are couple of options to go here: 1. F/4 Newtonian with native pixels, or F/5 with reducer CC 2. F/8 RC with binned pixels. For option 1, we need good CC. I'm not sure that I've found CC that will correct coma to good degree but not introduce SA or something else, over large enough field. Best CC that I've seen in terms of correction would be ASA x0.73, but it has 17mm corrected circle (or there about). It also means F/5 scope - and that will add its own complexity in terms of bulk being put on HEQ5. Somehow I can't seem to find good 8" Newtonian candidate for 1"/pixel - maybe I just have not searched enough. With option two we have nice compact design that sits well on Heq5. It is flexible in terms of resolution / focal length as there are reducers for it - x0.67 or x0.75 (I would say that any reduction past x0.72 will not work well on this RC and asi1600 due to corrected field - x0.67 reducer can be "spaced" to x0.72 - x0.75, but there is also very good reducer - Riccardi FF/FR - x0.75). Binning ASI1600 in software is exactly the same as using camera with larger pixels and having larger read noise - one of 3.4e read noise - still better than most CMOS cameras out there in terms of read noise. Only drawback for RC is FOV, and for galaxies it is of course less important. So if we go by aperture at resolution (around 1"/pixel), I think RC is very viable if not the best option in 8" class to be mounted on Heq5 (with given camera). Being true mirror system it has other advantages - broader use as astronomical instrument - for spectroscopy, photometry and astrometry - I mean it is most widely used by professionals in scientific role because of its characteristics.

Hi, yes, I ended up upgrading it precisely because of lack of threaded connection. Stock focuser was otherwise quite usable - never had slippage issues with it, but I try to keep my imaging train light. I plan to use that monorail focuser on my SW Evostar 100 F/10, just need to get suitable adapter to it (OTA tube to M90). This is the one I upgraded to: https://www.teleskop-express.de/shop/product_info.php/info/p6970_TS-Optics-2-5--Rack-and-Pinion-Focuser---holds-Acc--up-to-6kg---travel-53mm.html I also have this extension before focuser, screwed into OTA: https://www.teleskop-express.de/shop/product_info.php/info/p2773_TS-Optics-50-mm-Extension-Adapter-for-M90x1-thread.html At the time of my purchase it was not included into delivery (I have TS version - regular one, not carbon fiber), but from what I see, TS includes it now with their RC units. I also added rotator and suitable thread adapter (because need to mount 2" filters and reducers): https://www.teleskop-express.de/shop/product_info.php/info/p9781_TS-Optics-360--Rotation---Thread-Adapter---M63-to-M68--M54-and-2-.html + https://www.teleskop-express.de/shop/product_info.php/info/p6400_TS-Optics-Adapter-from-M54x0-75-to-M48---T2-Focal-Adapter-for-M54x0-75.html

+1 for RC 8" - I have such setup and very pleased with it. I also bin x2 in software for around 1"/pixel. It is on edge of HEQ5 capability - in terms of guide performance, you need nicely tuned mount to be able to guide at around 0.5" RMS total. Scope hold collimation very well - I setup each session and I've collimated scope only twice (well, one could say that it was single collimation, because I did not get it spot on in first round, so I had to repeat the next day).

You can't lock the mirror since it is not movable. Problem with 6" and 8" models is that mirror cell and focuser attachment are in one piece. There is tilt mechanism for focuser - it is "squared in" by design and manufacture. This as a consequence has a problem if too much weight is hanging of the back of the scope (focuser included) - as it will move the cell together with the mirror. I've checked my scope and one of the collimation screws was indeed a bit loose (1/8 of a turn) - but I think I now wrecked collimation - I forgot that smaller screws are collimation screws and larger ones are locking screws so I tightened up smaller screw (instead of just making sure cell is locked by larger screw). Will need to check it under stars. Don't know if I'm over weight limit, really should not be - using ASI1600, OAG, and filter drawer - all of those are fairly light components (in their class). Putting lightweight step motor and bracket shouldn't really be a problem. I've decided on "serial" configuration, and made sure both fine focus knobs are removable - and indeed they are, simple set screw. Fine focus shaft looks like 4mm (or maybe 3mm I need to finally get myself a caliper one of these days) - and this seems to be problem on its own . Checked all local retailers and they have elastic shaft couplers in every possible combination except one that I might need (3-5mm or 4-5mm, neither is available, every other combination under the sun is in stock).