vlaiv

Actually - it helps if you think in terms of fixed grid in space. Imagine that you have 3 galaxies in a row. There is 7 grid spaces between first and second and 3 grid spaces between second and third. Such setup represents expanding space - as long as you don't equate grid space with constant physical distance. If one grid space is variable in terms of "kilometers" and depends on the moment we are talking about - this setup represents expanding universe. Conversion factor between "grid space" and actual physical length is called scale factor and is main variable of the Hubble law that can be expressed like this: Or in another words - Hubble parameter (rather than constant) is equal to how scale factor is changing at some time divided with value of scale factor at that time.

It is very hard to depict what is seen at the eyepiece when looking at computer screen image. First - you need to be at exactly "prescribed" distance from computer screen in order to make Jupiter image on the screen appear the same size in the eyepiece - and that will depend on pixel pitch of the screen used. Second - there are various cognitive effects that come into play. When looking at the eyepiece - one just sees patch of the sky and a planet. There are no reference points to be able to judge size. You are correct - 9mm eyepiece with 700mm FL gives about x78 magnification. That is actually plenty of magnification for target like Jupiter. If current apparent diameter of Jupiter is ~36" - then x78 magnification will make it about x1.5 larger than the full moon when viewed with naked eye. We can see some features on the full moon - and yes, you should be able to see some features on Jovian disk at x78 as well. At least main belts. Here is handy way of knowing if you have focus right - you already know that Jupiter will look like large circle when out of focus. You can use that circle to roughly judge where is correct focus position. As you change focus (rotate focuser knob) - diameter of that circle will change - it will either grow or shrink - you should go in direction of shrinking the circle. At some point - circle will be the smallest and if you continue rotating knob in same direction - it will start to grow again. This point where circle is the smallest is place of actual focus. Once you reach it - pay attention to the detail on the planet and try to tease out detail - tweak the focus as needed to make features on planet sharp. Do be careful - there is spot just before perfect focus that focuses on disturbance in our atmosphere rather than planet itself - if you focus on that - you will see actual air - much like warm air over fire - distorting the planet. You don't want to focus on that - try to focus on actual planet, but do understand that atmosphere will make it difficult as everything will dance around more or less (depends how calm the atmosphere is).

Any chance you could limit size of microstepping? If I recall correctly - they have very high resolution - something like less than 0.1" per micro step. Maybe going to 1/32 instead of 1/128 would be beneficial although it is lower stepper resolution as far as angular movement goes?

It is hard to tell. Both FPL-51 and FPL-53 have potential to behave in a certain way - but will they, depends on what is the mating element and how well the glass is figured. If we assume that both are reasonably well executed, then there are two parameters that we need to consider - one is F/ratio of the scope and other is aperture. For slow enough scopes - there won't be significant difference. For small enough aperture - there won't be significant difference. I know that there is difference between the two at 4" F/7. I think there might be difference at 80mm F/7 At 60mm F/6 I simply don't know if there will be significant difference. Even if there is some difference - use of special UV/IR cut filter like Astronomik L3 (which is advised for fast doublet anyway) - should even the playing field.

That is technically not a backlash. Backlash is motion in mechanical part of the system. What you've described is more like latency in stepper motor due to lack of torque - sort of torque build up. I agree it is bad for guiding - but can also be dealt with by using beefier stepper motors that have enough torque, or using some sort of transmission prior to worm.

Yep. That skews results just a tiny bit, and in principle can be accounted for with multiple iterative measurement (of the top of my head). When you subtract dark - you inject back a bit of read noise - which should be removed, but you can only remove it if you can measure it - and in order to measure it, you need to have e/ADU - see the problem? If you want to be really pedantic about it - this is procedure: 1. Measure light signal and noise like above and derive gain 2. use derived gain to measure / calculate read noise and subtract that from measurement of step 1 - return to step 1 to get better e/ADU value (repeat several times). You would measure read noise (combined with dark current noise) like this: Take number of darks that you are going to use to calibrate lights and split into two groups - stack first group and stack second group and subtract the two and measure standard deviation - then use that to calculate noise level. If you used sum stack - divide measured stddev value with square root of number of subs. If you used average - it's a bit more complicated - divide with square root of two then multiply with square root of number of stacked subs.

Can't really see well on that video - is the whole mount (or rather RA part) moving when you push on CW bar or is it just CW bar? In any case - that's not looking good - there should be no wobble like that and I don't think it is caused by springs in spring loaded worm. That motion is "perpendicular" to springs themselves and should not be related to them.

What is your guide exposure (how long is it)? Try the following: Shoot one or several subs with your regular guiding settings and then use longer guide exposure - like at least 4 seconds and shoot another set of subs. Compare FWHM values between two sets of subs. If there is no significant FWHM difference (or even FWHM or second batch is smaller) but RMS got down - then it is the seeing that is at fault.

You are way off with your focus there.

Why don't you perform your own measurement? Measuring of the gain depends on simple fact - shot noise is related to the level of signal if both are expressed in electrons. Say you have 100e of signal - you should measure 10e of noise (minus read noise and other detail that we won't get into now). Say you have some e/ADU value which you don't know - let it be 3.7 in this case. you will get 370ADU and standard deviation of 37ADU You need to use those two numbers to get your coefficient. sqrt(370/C) = 37 / C 370/C = 37^2 / C^2 370 = 37^2 / C 370 C = 37^2 C = 37 * 37 / 370 = 3.7 There you go. You only need to take "flat" at certain gain setting and measure average ADU value and stddev in ADUs to derive gain constant.

I can see two benefits with this design: 1. No need for careful spacing 2. Tilt is effectively removed from the equation

Ok, so if you are using iOptron commander - it is different from using EQMod. You won't be doing PE curve preparation - mount will do that for you, or rather iOptron commander will do that for you. In this case - it must know what the needed corrections are and you need to start guiding when you hit PEC record. Commander will track where mount is supposed to be - but it will also track corrections and from the two will derive PEC curve. You need to keep it like that for at least couple of PE cycles - let's say 5 cycles - that is 2000 seconds total, so just a bit more than half an hour. Once you are done - when you next start RA tracking, regardless if you are guiding or not - you can hit PEC playback and it will apply recorded correction to the mount. It should keep that PE correction until you record a new one - so you should be able to reuse it very session by playing it back. As far as 1.2" RMS - that is quite a lot, and there are couple of things that you should look into: 1. Do above advice on balancing your setup 2. See if your clamping connection is strong enough for the scope - you might need to improve that 3. You should have virtually zero backlash since you have spring loaded gears - make sure you've tuned that properly in both axis 4. Make sure your tripod is sturdy enough and have been placed on ground where it can't move - don't put it on soft ground like sand - or if you do - try to "dig it in" a bit so it is rock solid. If it has extending legs - don't extend them fully - you don't need your setup to be at "observing" position for imaging. It can be lower than that.

That is true - except you made an error - it is 13.8 billion light years in every direction rather than just 13.8 light years. Phenomenon is not strange at all if you understand that universe is 13.8 billion years old and that light that is further away than that did not yet have time to reach us. There are some additional details to the whole story that might not be essential in understanding that the edge of observable universe is 13.8 billion light years away (or, just to make things unnecessarily complex - 46 billion light years away in co-moving coordinates ).

We do know one important bit related to seeing. In the same seeing, larger aperture will resolve more than smaller aperture. How much more, that I agree, we don't know , but we do know that it will resolve more, regardless of how poor seeing is - if its the same for both scopes.

Just run PHD2 like you normally would - except that you disable guide output. There is a checkbox somewhere in the settings that you should uncheck. PHD2 will continue happily doing its thing, but not corrections will actually be sent to the mount. Afterwards - you just take guide log and use that in PecPrep. Here is the setting I'm talking about: just remember to turn it back on after you've finished recording pec data.

I haven't tried that one (that I remember), but in essence - it should be close in performance as it relies on same things. Same way you can hit "auto" in PecPrep to get the curve (it tries to remove noise and insignificant harmonics and isolates important stuff) - it can be done in PHD2, after all - they have access to same raw data (PHD2 can record position prior to issuing correction). There are only two things that are different: - predictive pec needs some time to learn and it won't give you best performance from the start of the guiding sessing - PHD2 relies on corrections being followed to the letter - and that might not be the case. In each new round of correction, in order to get "uncorrected" behavior of the mount - PHD2 needs to mathematically reverse last correction and figure out where the mount would be without correction issued (so it can get accurate reading on periodic error). Problem is - corrections are not perfect. Sometimes they overshoot, sometimes they undershoot - no way of knowing how much of a correction was taken up by the mount (backlash and other mechanical issues). For this reason I would give slight edge to the PEC recording and preparation as it can use multiple PE periods to establish good mean periodic error and correction will be applied from the start regardless if one is guiding or not.

Point of PEC is to produce correction of the mount as mechanically is - without any external aids - like guiding. This will make mount run smoother and will put less strain on guider system once you start using it. Guide corrections will mask true underlying shape of PE curve and produced PEC will be sub optimal (even if you apply it with guiding only).

Depends on sensor used. Any decently sized sensor paired with such scope will easily have nice FOV to frame galaxies or planetary nebulae for example.

Those numbers don't paint pretty picture. It looks like ST80 is almost stopped down if used with regular diagonal mirror. I went with following numbers: Draw tube length - 135mm (last image with 1.25" visual back). Draw tube inner diameter - 43mm (which makes sense if it has M43x1 inner thread - but also looks like that from images). Any diagonal mirror that has more than 80mm optical length will stop down aperture - or rather, focuser tube will act as aperture stop. If we go by this: and assume that shortest path diagonal mirror has something like 70mm of optical path - that gives us 4mm diameter fully illuminated circle. Interestingly enough - illumination level at edge of 1.25" field stop is not significant - in fact, I don't think it can easily be seen by human eye. Above is simulation of what edge of 1.25" field stop is seeing with 70mm optical length diagonal. Outer circle is objective lens and smaller circle (or rather arc) is edge of the focuser tube. Area between two is effectively blocked, but all the rest is available. Does not seem like much (not sure if I can easily calculate percentage).

Everything about those results disagrees with published specs. At gain 0, SharpCap measured: 4.12 e/ADU, 16K full well capacity while camera claims that it has only 8K full well and that 2.1 e/ADU. Specs say that unity gain is at gain 68 while sharp cap places it somewhere above 121

Can anyone measure length of focuser tube on their ST80 (preferably stock / not modified version of focuser)?

Essence of long exposure imaging is to attain certain level of SNR for your data. You can't show the detail in the image if it is polluted by too much noise (all you'll see is noise rather than signal). Photons that you mention being collected by sensor - are in fact collected by individual pixels and each pixel is like a bucket - it captures certain amount of photons. More photons pixel captures - larger signal and larger signal - better SNR. This is directly related to imaging scale. Say you have nebula that is covered by 100 pixels in one case or by 400 pixels in second case. That is the difference between say 4" F/5 and 4" F/10 telescopes with same camera mounted. Double the focal length - and you double resolution in both height and width - you quadruple amount of pixels needed to cover the object. Aperture did not change. Target did not change - they both emit and collect same mount of photons in total. What has changed is how many pixels those photons are divided among. In first case all photons are distributed to 100 pixels and in second case - same amount of photons is divided between 400 pixels. In second case - each individual pixel gets 1/4 of photons compared to pixels in the first case. Just by simply changing the focal length of instrument - you changed level of signal and hence SNR dramatically. You say that you understand over and under sampling and that is the part of the story. Over sampling is bad as it wastes SNR. Under sampling is not as bad and is needed for wide field work. Matching focal length of instrument to camera has to do with making sure you are less likely to over sample and more likely to properly sample, so that you always get best SNR possible for detail captured. There are other ways you can "adjust" things - like changing pixel size instead of focal length (or in combination) - but that is advanced stuff.

Are you sure? I was referring to this bit here: not the thread that black 1.25" receptacle screws in (one on the inside of silver draw tube).

Sensor size does not play major part here, nor back focus - as long as you focused properly, and I'm guessing you have (no sign of defocus stars). Yes, there is genuine concern about over sampling - but over sampling itself won't produce bloated stars. What over sampling will do - is to make everything bigger - stars, galaxy - everything in terms of how many pixels are used to cover - but relative sizes of things in image will stay the same. Here is a comparison between that image and one taken with similar equipment (F/6 8" newtonian and QHY5LIIc camera): I resampled images to be roughly the same size. Notice difference in sharpness of the galaxy and also relative sizes of objects in two images. Galaxy is roughly of the same size - but stars are x2-x3 larger in left image. This is either due to very poor seeing or spherical aberration - it is not due to over sampling as it sampling has been reduced to adequate levels. In fact, I think that focus was as good as could be had under circumstances as there are no defocus artifacts (doughnut shaped stars) that should be obvious at those star sizes if issue was defocus.

Out of interest, what is ID of focuser tube and how long is it? I recently did some calculations for 80mm scopes (F/7.5 in my case, not as fast as F/5) - and it turns out that it is quite hard to illuminated larger field with regular focuser specs. I suspect that situation might be even worse for ST80 given that it is F/5 and has focuser that is probably around 40ish mm in diameter (If I recall correctly - it has T2 thread on end of focuser tube, so focuser tube can't be much wider).