vlaiv

I've come across bunch of things that daytime photographers use that can be described as intuitive and helpful - but are misleading at best. One such thing of the top of my head is "50mm equivalent lens" for either APS-C or 4/3 sensor. What could be equivalent of focal length? And surely different numbers can't all be equivalent to certain focal length? It turns out that 25mm FL lens is equivalent to 50mm lens for 4/3 sensor and 30.86mm FL lens is equivalent to 50mm lens on APS-C sensor (and bunch of other equivalent FLs ). As far as I've gathered - photographers primarily think in terms of FOV on 35mm / full frame sensor rather than in terms of actual millimeters of focal length. 50mm on full frame will have same field of view as 25mm on 4/3 sensor or 30.86 on Canon APS-C sensor - that is why they are "equivalent". Other than that - you are absolutely right, focal length is distance between objective and point where parallel rays bent by lens converge into single point.

I use flat box like this one: https://www.teleskop-express.de/shop/product_info.php/info/p8241_Lacerta-LED-Flatfield-Box-with-240-mm-usable-Diameter.html except mine is older model. You can DIY flat box with LED strip and couple of pieces of matte plexiglass panel to act as diffuser (in fact if you google it - there are quite a few options for light diffusion).

Keep exposure time low, especially if you are using stationary tripod / non tracking mount. Something like 1/250th.

I use steel tripod from HEQ5 to mount AzGTI - it was just a matter of getting M10 to 3/8 adapter, so I guess you only need appropriate bolt - just make sure your mounting surface is flat (adapter I'm using fixes issue with polar alignment peg on top of HEQ5 tripod). Not sure about payload - I never loaded mine that much.

I think that you can do couple of things to improve guiding that will have larger impact than swapping the guide camera (although getting new guide camera is not bad thing - do use that new one). 1. Place OAG as close to sensor as you can. What is your imaging camera and what OAG are you using. DSLR, for example, is rather poor choice to be guided with OAG at F/4. This is because there is 44mm of distance between T2 adapter and sensor. Let's suppose that you have 8mm prism. At F/4 - it needs to be placed closer than 32mm in order for prism not to stop down the beam used for guiding. If you have DSLR - best you can hope is is probably around F/6 instead of F/4 - and that is if you place pick off prism right at T2 adapter. 2. Place prism as close to optical axis as you can. I'm guessing you are using fast newtonian since you have 800mm F/4 scope. Fast newtonians really need large secondary mirrors in order to illuminate large diameter - and often they don't have it. If you place prism too far from optical axis - two things happen: - you are in vignetted part of the field - so further light loss happens. This combined with prism acting to stop down the beam can result in guider getting (only F/10-12 or so of light) - you are in coma affected part of the field (even with coma corrector - far from axis stars are not going to be sharp). This spreads light from a star and reduces signal that each pixel receives (here having low read noise helps). Keeping prism closer to optical axis makes guide stars tighter - they have better SNR There are of course two additional things that you can do: 3. Increase guide exposure. Most people use 1s guide exposure as it is sort of a default value, but in reality, if your mount is mechanically sound - you should be able to use 3-4s guide exposures. You can examine your mount behavior when not guided to find out maximum drift rate. You can then use maximum error that you want your guiding to have and divide with max drift rate and that will give you how long your guide exposure can be. If you can - do PEC on your mount - that reduces max drift rate in RA. Good polar alignment often reduces DEC drift rate to very low levels - as far as that one is concerned - you can have 10-20s guide exposure no problem. Very crude methods for estimating drift rate would be to use polar alignment error calculator here: http://celestialwonders.com/tools/polarErrorCalc.html and for RA to measure P2P of periodic error and divide with worm period (then multiply that with 2). Say you have Heq5 and have 35" of P2P. Worm period is 638s. This means that in 319s mount drifts 35" (other 319 it drifts back) - and if that motion was uniform speed would be 0.11"/s. However, most often it is not uniform and in fast parts it is about x2 this value so 0.22"/s. I personally would keep such mount under 3s guide exposure as in 3s it can drift 0.66" in worst part of worm period. However, smaller P2P Ra error will enable you to use 4s for guide exposure. Another benefit of using longer exposure besides better SNR and more guide stars is that you give seeing enough time to average out - and you won't be chasing the seeing as much (that is why you want to use multi star guiding after all, right?). 4. Bin your pixels. That also increases sensitivity. You have plenty of focal length with OAG and you can easily bin your pixels. General rule is that you take your target RMS, divide it with 3 and multiply that value with 16. Your guide resolution should be that number or higher (lower number in "/px but higher resolution). Say you want to precisely measure your RMS down to 0.3" (which is great value by the way). Divide that with 3 gives you 0.1 - now multiply that with 16 and you have 1.6"/px. 2.9µm pixel size of ASI290 at 800mm will give you 0.75"/px. You can bin that x2 without any issues for 1.5"/px and you'll still be able to reliably measure centroid position for 0.3" RMS guiding. In fact - depending on the mount you are using and your target resolution - maybe even bin x3 your guide pixels. That will give you 2.24"/px and that is still good for 0.5" RMS guiding.

How so?

Well, question was asked in context of plossl eyepieces and there seems to be related discussion here: While part of answer does relate to actual focal length - as in what sort of plossl eyepiece would be usable on such telescope (and answer is all but shortest FL plossl eyepieces that are not generally usable due to short eye relief - there are much better alternatives in short FL range), I guess another part of question has to do with speed of scope - as plossl eyepieces are not best suited for fast scopes if one wants excellent edge correction. In that sense - both actual FL and F/ratio are part of answer.

That really depends on observer. I did not pay attention to edge performance of my 32mm GSO plossl in F/5 scope until I read that plossl eyepieces are not quite good for that F/ratio. After that, I payed attention, and sure - edge of the field shows little seagulls - but that really does not bother me when observing as I concentrate on the central section of the FOV. Some other people are probably much more bothered by this.

In that case, maybe you are more interested in speed rather than focal length. below F/6 - fast scopes F/6-F/8 - medium speed scopes above F/8 - slow scopes (above is of course "arbitrary" and you can say that slow scopes start at F/9 for example instead of F/8 - there is not hard line to be drawn). As far as Plossl eyepieces go - and eye relief, there is general formula - it is about 2/3 of EP focal length. You have to know what sort of eye relief you'll be comfortable with - say 8mm is ok for you, then you shortest FL plossl that you will be happy using is around 12mm (8/2 * 3). Say you want to observe planets with up to x200 - in that case, get telescope with 1200mm FL and barlow x2. F/6 is still very fine for plossl use. I have 12mm plossl as my shortest FL plossl and use it in my 8" f/6 dob - works very well and I have no issues with eye relief (but I don't wear glasses when observing - take that into account). You can still happily use plossls in say F/5 scope. I used plossls with ST102 with 500mm of FL. However, that is wide field achromat and I never really wanted to observe planets with that. Regardless - plossls performed quite well in that scope (and I was not really looking for edge of the field aberrations).

I'm not sure first part is true. Here are some things that you should consider and see if you meant any of that: 1. Simple eyepieces with few elements and narrower field of view work better in slower scopes (not necessarily longer focal length) - meaning slow F/ratio - like F/8 and above. This is because of angles involved. Fast optics produces shallow angles - so does wide field eyepiece designs. Only best eyepiece designs with many elements can deliver sharp to the edge wide field views in fast scope. One of the reason to prefer simpler eyepieces like kellner, plossl, ortho and similar is because they are cheap and they work well in "long focal length" scopes (but not because of focal length - but because of slow F/ratio - it is easier to have slow long focal length scope - because short focal length scope that is slow - would just not appeal to anyone because of tiny aperture. Say you have 400mm F/10 telescope - it is only 40mm of aperture). 2. It is easier to use plossl design at longer focal lengths because of eye relief. Eye relief with designs like plossl and ortho is function of their focal lengths - shorter the focal length, shorter the eye relief. At around 10mm and below - these eyepieces start to be uncomfortable for use. Say you want to observe planets with 1500mm FL scope - you can comfortably use 10-12mm eyepiece for ~ x120 magnification. However, if you try to use 600mm focal length scope to observe planets at x120 - you would need 5mm eyepiece. Plossl would be very hard to look thru at that focal length - you would almost need to press your eyeball against eye lens (something like 3mm of eye relief or something like that). 3. Some telescope designs extend eye relief. I must admin that I don't fully understand what happens here, but if telescope design has negative element in optical path - it extends eye relief of eyepiece used. This happens when you use barlow (it is negative optical element that diverges rays), but also with scopes like Maksutovs and SCTs - because their secondary mirror diverges rays (acts as amplifying mirror - similar to barlow). This makes short focal length plossls more comfortable to use as it improves their short eye relief.

Depends in which context you define long and short. Is it imaging? Then you have to look at "current" pixel sizes. Current pixel sizes are around 4µm. Just 10 or so years ago "regular" pixel size was around 7µm and another 10 years ago, more like 9µm was norm. Maybe you think visual, but what sort of visual? Planetary? Wide field? General purpose? Again - here you need to match that to usual eyepiece focal lengths and wanted magnification range. If you are talking about general purpose viewing - then I would say that you are pretty close to what would be considered short / long focal length. I think that you went a bit short on the long side. Maybe long is longer than say 1800mm rather than 1200mm.

Depends what you define as sensitive camera. For guiding, I think QE is much more important than read noise. Read noise is added once per exposure. All other noise types grow with exposure time. You can always control amount of read noise in comparison to other noise sources by controlling exposure length. Only when you can't use longer exposures - level of read noise becomes important. For example in lucky type planetary imaging. Here you need to lower your exposure length to freeze the seeing. Exposures are often in range of few milliseconds. Having low read noise camera becomes really beneficial in such cases. Noise math is not that complex at all. Read noise is modeled by Gaussian distribution, while other noise types (target shot noise, thermal noise, LP noise) are Poisson type distributions. Value of read noise is fixed per exposure, while other noise sources are equal in value to square root of associated signal intensity. Noise adds like linearly independent vectors (square root of sum of squares). What will you use this camera for and what type of sensitivity are you after?

I'm tempted to say that it won't be big issue - but I can't be 100% sure as I don't have experience to answer that question. I would rather someone else confirm that for you.

I think it will work without issues.

I did correct myself - few lines below that: When I researched DIY motor focuser - recommendation was to put it on fine focusing shaft due to precision needed. One of comments in the thread that I linked to - also pointed that out. However, ZWO EAF has rather decent resolution - of 5760 steps. Even if one revolution of coarse knob is 10mm of focuser travel - that is still 1.7µm per step. More than enough (critical focus zone is in tens of µm for fast systems).

Have a look here: https://www.cloudynights.com/topic/670574-autofocuser-slipping-and-not-working-zwo-eaf/ There are couple of pictures of setups with EAF. First - you can put it on coarse side if you get needed flexible shaft coupler. Second - EAF needs to be bolted down to the scope - which prevents manual focusing once it is installed (so you might as well put it on fine focusing shaft) Third - it is recommended for it to go on fine focusing shaft. Actually - scratch that last part - have you seen manual for ZWO EAF: https://astronomy-imaging-camera.com/manuals/EAF_Manual_EN_V2.2.pdf

I don't have experience with electronic focusers (other than planning to DIY one for myself) - but I do believe they should go on fine focusing shaft. That really depends on step of motors driving it and any reduction ratio in motor itself. Single step needs to be fine enough to hit critical focus zone. Mind you - if you put motor focuser on fine focus knob on the other side - well, why do you need manual focusing then, especially if you are going to use scope for imaging?

It really depends how stuck grub screw is. Putting another one on top of it should force it all the way down and possibly out if shaft opening is large enough if grub screw is not really that stuck - but question is - why did it get stripped if it's not stuck.

I think that easiest fix is to do following: - take a drill bit that is slightly larger than current grub screw thread - if it is M2 - take drill bit that matches M3. - drill out grub screw and old thread in one go with drill using above drill bit - take tap that is used to cut larger thread size (M3 in above example) and cut new thread - take new thread size grub screw to tighten coarse know onto focuser shaft

You should select ROI based on two things: 1. frame rate you want to achieve 2. correction of your scope. Some scopes are diffraction limited only on axis - or close to it and further away from axis have significant aberrations. Newtonian is example of that - coma gets worse off axis and with fast newtonians - very quickly, so they benefit from small ROI. What F/ratio is your scope? Your signature says it is F/7 but I can't find such reference online. Intes MK67 is always quoted at F/12 being Maksutov (I'm asking because of both your signature and @wouterdhoye mention f/7 for MK67).

I don't think you need a barlow with ASI178 and F/12 scope. First image is very good - other three are a soft due to over sampling.

I'm not sure if AS!3 can read multi image fits files and interpret that as color image. You can try to calibrate things in Pipp and keep bayer matrix intact. Then open in AS!3 and tell it to debayer using wanted bayer order.

Don't think there is AutoStakkert! 6, I know only of AS!2 and AS!3 - maybe you are thinking of Registax?

If I remember correctly - AutoStakkert has feature to automatically add alignment points. This is very handy for lunar shots as number of alignment points tends to be rather large. Here is a screen shot from another thread here on SGL There are rather big alignment points. Distance between alignment points is roughly the half of side of the square. If you look at above gif that I posted - whole gif could be covered with maybe 4 alignment points of above size - but in reality, we would need about 50-100 alignment points for that gif alone since size of tilt deformation is small. You can select alignment point size in AutoStakkert - by selecting number of pixels. Here is diagram roughly explaining how to select proper alignment point size: If you have a feature in your movie that bends and stretches so that in one frame - looks like elongated blob in the top, in some other frame looks "normal" and in a third frame - it looks squeezed - you need to cover size of that feature with 2 alignment points rather than one (left part of diagram - not right). In right part of diagram - large alignment point will stack complete feature as single thing and you'll get the blur of it shrinking and extending. In left part of diagram - each alignment point will "figure out" that in top part - stretched blob - left and right sides are further than they should be - that in the middle part they are roughly where they should be (mean position over number of frames) and that in the bottom part - they are squeezed - and it will try to correct distortion. You need roughly two alignment points per "wavelength" of deformation If you look at above gif again - you'll see that larger crater is deformed but smaller is just bouncing around staying roughly the same shape: In that image - I would try to make alignment points be roughly the size so that at least couple cover that larger deforming crater. This is all related to that isoplanic angle from above linked lecture - alignment point needs to cover number of pixels that add up to isoplanic angle.

Explanation is the same, although I can't remember the page itself - it is good example of a brief explanation of phenomena involved. Does not answer our question though - what are average exposure times that are needed to freeze the seeing - but it does point out dependence on used wavelength - and nicely explains why jet stream is responsible for such a poor seeing - due to wind speeds involved - it shortens coherence time considerably. There is also dependence on scope size vs r0. As you say - when scope aperture is less than r0 - dominant wavefront aberration is tilt - image stretches and deforms rather than blurs. It is also interesting to relate level of distortion in video like above to density of alignment points. Alignment points need to be dense enough to properly reshape back distortion created.