vlaiv

It is not as horrifying as it sounds. Check out this video: https://www.youtube.com/watch?v=Ylb7xnc_03U

Btw - if you look at images on that last link - you can see transition from purple to green close to focus:

+1 Baader Contrast Booster - it does work and has neutral tone compared to other filters (maybe a bit warm image). Green/Yellow is companion to violet / blue as far as refractors go. It all has to do with spherochromatism and best focus position. Prism influences spherical aberration of the scope (at least I think so), and much of fringing is not down to defocus but also due to spherochromatism. There are two important graphs that you need to look at in order to understand what is going on and how prism / mirror swap changes levels and color of CA. First is of course this diagram: This shows what colors will be out of focus depending where you put your focus point. If you focus at 550nm - and we usually do that because green part of spectrum carries the most luminance information - and we are most sensitive to sharpness in luminance (so that means green), then it can clearly be seen that both red part and blue part of spectrum (700nm and 400nm) will be out of focus. Violet part will be the most out of focus at 400nm. This is "standard" mode of achromat telescope - and what it is usually optimized for. If you change focus position - you will bring red and blue in focus - but in that case - central part of spectrum will be defocused: Now look at spectrum: That central part is green and yellow light. If you shift your focus - color of chromatic aberration will change. We often don't see this as sharpest image is when we focus on green light mid spectrum at about 550nm. Now let's look at one more graph (it is also present in above graph in top left corner - but it is not as significant in that model - we need to look at fast achromat for that): In perfect achromat telescope - this graph would look like just bunch of vertical lines. Each line here represents one wavelength. Stronger the curve in the line (deviation from vertical) - more spherical aberration there is at that wavelength (spherochromatism). If you look first diagram / top left corner - you'll see that for F/10 lines are much straighter. That is why we say that these short achromats often suffer from spherical aberration (while long ones don't) - it is really spherochromatism. You might have significant spherical aberration in green part of spectrum and prism can correct that a bit. With prism - you will want to focus on on green line and then you get nice violet fringing. When you remove prism - you remove that correction and now green has spherical aberration again - and with spherical aberration - you can't find good focus - it's all blurry. Then you instinctively focus to different line - one that is straight and will produce sharper overall image - and this means that green is now a bit out of focus (see that focal plane position) and you see green / yellow fringing. You can test this by using mirror diagonal and then refocusing even if image is a bit blurry (don't look for perfect focus - look for different type of fringing as you refocus). There is a way to correct this, but it is advanced technique. It involves changing distance between doublet lens. There doublets are usually separated by small bits of metal or similar little shims. You can replace those with slightly thinner version (or simply file down thickness of existing ones). It is trial and error method and you need some way to asses spherical aberration - best would be some sort of narrower filter (maybe using CCD green would be enough) and Ronchi EP and star testing telescope. You can see results for example here: http://interferometrie.blogspot.com/2017/06/3-short-achromats-bresser-ar102xs.html There is good comparison of 3 different short achromats and results of testing for spherical aberration. Here are shims that I mentioned for example. You can see that doublet lenses are real close as there are Newtonian rings showing.

If I'm reading this right - everyone is suggesting to go with tubes instead of fins at the bottom? Like this: and not like this:

https://www.cloudynights.com/articles/cat/user-reviews/telescopes/ritchey-chretiens-dall-kirkhams-and-other-designs/meade-20-rcx-r1513

I have no idea what that is . T section steel is clear but not sure what would "angled strut" made out of it going from just below top of the pier down to concrete base - look like

If you fix your primary and dot from laser is not in center after that - then yes, it is down to focuser (I'd try several focuser positions to see if it makes a difference in where laser ends up - even without fixing primary).

Ok, I re did calculation again from scratch and it checks out. I got 0.166" for 4.35N load on 200x200mm steel pipe with 6mm thick walls

Could do that, but is it really necessary? I've heard people using sand for that, and some think it is good idea and some think it's not necessary as frequency is too high.

No unless I weld them myself (or someone does it for me) - but I'd rather not mess with that. 200x200mm is largest cross section and 6mm is thickest walls that I have (easy) access to. According to this calculator : https://www.engineersedge.com/calculators/section_square_case_4.htm difference is not that big - less than double deflection between 6mm and 10mm walls

What would be advantage over say ES 6" MN for NV user? Can NV users exploit very larger field stop?

That is what, like £5000 worth of fun?

I was thinking of drilling 4 holes in base slab (which is rather big and heavy piece of reinforced concrete - about 1x1x1 in meters) and putting M16 threaded rods in those and using some sort of chemical anchoring agent (there are several options, I'll ask for the best). I think I can easily get at least 20-30cm deep holes for those. 50-60cm square base plate cut out of 10mm steel and pipe welded to that with triangular fin supports at corners. That is way too much. Even if we reduce load by factor of x100 to only 0.5Kg, we will still have 0.0015mm of deflection. If pier height is 1600mm, then angle of deflection is 0.2" Well, that is not bad - but that is only 0.5Kg of load. How much is wind load on 0.2 meters squared? According to this 6m/s wind speed https://www.engineeringtoolbox.com/wind-load-d_1775.html produces 4.32N of load which is equal to 0.44Kg of load. Ok, I'm sold - that sort of deflection in direct 6m/s wind is acceptable for pier stability.

What criteria is that based upon. Is it just structural stability / load bearing capability? People that advocate larger "diameter" (or side in square case) of pier often consider minimal deflection at usual wind loads. I don't have a first clue of how I would calculate such a thing, but I guess you probably can. If I have square pipe with side 20cm that is 1.6m high and wall thickness is 6mm - what would be angle of deflection if wind of say 6m/s blew perpendicularly on 60cm x 30cm OTA cross section? I personally think that pier is not the weak link and that scope / mount connection is weak point, or maybe there is some "elasticity" in the mount that will respond to this wind load - like backlash or whatever, but for sake of argument - can we calculate deflection of the top of the pier if everything else is rigid?

https://www.teleskop-express.de/shop/product_info.php/info/p14000_TS-Optics-152-mm-f-5-ED-Rich-Field-Refractor-with-4--RAP-Focuser.html Something is just odd with this combination. To be honest, I'm drawn to it as alternative to 6" F/5 achro - as it will have much better objective lens, less CA and consequently better contrast, but look at that price tag! @Louis D I think you once mentioned that you miss having fast ED doublet even if it has a bit of CA. Maybe this is the scope that would suit you (except for the price tag).

ED doublet will have much smaller issues with star bloat / CA. There will be some - but not nearly as much as fast and large Achromat scope. RGB will no doubt work fine. With LRGB - you'll either have to handle a bit of star bloat in your luminance layer with some star reduction techniques or you can take Astronomik L3 luminance filter. That L filter is a bit narrower then standard luminance filter and will remove farthest parts of spectrum thus taming any small bloat produced with ED doublet scopes.

I think I'm currently preferring this solution. 20x20cm square pipe - 6mm thick walls and 10mm base plate that will be at least 50-60cm on a side. I'll add some 10mm fins as well - probably to the corners of steel pipe. From what I'm reading 8"x8" might be a bit small for that height - but that is what I have readily available without actually building/welding pier myself from steel sheets. I was offered hollow / lattice type steel pier as alternative - something like this: What do you think about that approach?

I think that piece that fell off had air pocket issues and as far as I can tell from discussion - they were not quite gentle when separating wooden mould from concrete. Rest of the structure does seem solid and good enough. Observing deck is 10cm and it looks 20 because there is cross beam underneath it. I'll post some better images tomorrow.

I think it goes at least 80cm but closer to 1m. That sounds like a good idea. Not sure if it's going to be doable without wrecking the observing deck first as there is not much room beneath it to comfortably work. I guess smaller person could get under it and operate drill, although length of drill + drill bit could be an issue? Will look into that option - but yes, it sounds good.

Here is an update. Not sure what I'm going to do - but I'm not going to keep that pier that is poured at the moment. While showing the problem to main builder - he proposed that we do - exactly as I proposed above - to anchor pier and pour additional segment on concrete. Then I complained about the finish of edges and asked if there might be air pockets inside or anything - and I managed to separate chunk of pier about 5-6 inches long and 2 inches wide with my hand (maybe I'm exceptionally strong and not being aware of it - moving pier, crumbling concrete - single handed ). Thing is crumbling on its own. I absolutely have no idea what could have happened to cause all of this - but anything further is no go until I figure out good solution and find people capable of doing it. One solution would be to do steel pier. I looked up and I have easy access to square pipe 6mm walls 200mm side. I can also have base plate 10mm steel cut to size and make triangular supports out of same material. Only problem would be anchoring it to the base of the pier. I'm thinking pouring sort of thin slab of concrete - 10cm with rebar and epoxy thingy to hold it glued to base with M16 threaded rods sticking out of it so I can bolt down pier base?

It can. Chromatic aberration is easier to control with mono camera. With OSC camera - you don't have a choice - you will always record all information at single focus point. With mono camera, you can refocus with each filter, choose to use only RGB imaging instead of LRGB imaging thus only recording part of spectrum at it's best focal point or do narrowband imaging which does not really care about CA at all. Single wavelength can always be brought to perfect focus (and that scope has good spherical correction so spherochromatism is not issue either).

Does it look like this: when you are near the focus? That would suggest that your telescope is not properly cooled and you have thermals inside ota.

That is very good thing to do. Starting off with equipment that you already have and adding simple star tracker is cost effective way to learn the ropes of AP. You'll need to learn and practice things like calibration, stacking and processing of your images. Working at low focal lengths (btw even something considered long focal length in regular photography world - like telephoto lens of 300mm is only "starter" FL in AP world) is not going to put too much strain on things like polar alignment and precision of tracking. You might also want to get a book that explains different things you need to know in AP world. Often recommended is this one: https://www.firstlightoptics.com/books/making-every-photon-count-steve-richards.html

Hi and welcome to SGL. Take your time and do the research. Astrophotography is quite different from regular photography and that tends to confuse many with experience in regular photography when first starting out.

Hi and welcome to SGL. It is quite possible that your collimation is not spot on. This shows on higher magnification, while views at low power will look ok. One way to check that would be to insert 9mm eyepiece, turn your scope to Polaris (it is easier if star is not moving) - put it in center and then slightly defocus it both in and out focus. You should see concentric rings each side of focus - something like this: In focus / slightly defocused. Notice how things are concentric. If you see something like this: then your collimation is slightly off and it's blurring your views. There might be other things as well that impair your viewing - like thermal issues. Is your scope properly cooled down? How about local thermals - are you observing over rooftops of adjacent houses? They are probably heated this time of the year and that creates turbulence above their roof tops. There could be just seeing issues - unstable atmosphere as well.