vlaiv

I had similar issue recently. Did windows calibration, it helped with image processing, but it took some time to adjust to new gamma, brightness and contrast settings during day / business use (I program for living, so most of my work time is spent reading / looking at plain text). I also noticed that there is significant difference in results of my processing depending on the time of the day the processing was done in. If I process during evenings / early night time when there is no natural light (we tend to use really soft light just before bed time) images just look too dim during daytime. And if I process during daytime looking at the image in "night conditions" just makes noise pop out. All of this was more pronounced before calibration, but to some extent remains an issue even after calibration. I might add that my monitor is fairly decent, not professional level, but not some entry level stuff either - it's DELL U2311H with IPS panel.

Not sure about this, but I think that spherical would be a problem with mirror distance / spacing if they are collimated properly. If not, wrong spacing can amplify astigmatism which is caused by miscollimation. Don't know if this is going to be much of a help, but I did collimation of my RC8" - visually. It only needed collimation of secondary (quite a bit) - but I managed to do it under 15 mins. I collimated on in focus star image by inspecting airy disk / first ring - until it was more/less perfectly concentric. Still haven't tried it on larger format sensor (4/3 is hopefully coming soon), but for 9mm circle it's perfect.

Should be normal - but only far from optical axis - it's signature of astigmatism (inherent in RC design). Depending on your sensor size it should be noticeable in the corners, but not near the center.

I'm by no means expert in either conducting nor interpreting Roddier test, but I did two of them in order to check optical quality of my imaging scopes. First is TS (GSO) RC 8". Test was performed last summer, camera used was ZWO ASI 185mc, under real sky. Seeing was ok, but not great. I've heard that quality of test does depend on seeing, but in general obtained results are either that good or better - not worse. This makes me very happy since Strehl for this scope comes out to be at least 0.94. Test was conducted as per instruction manual for Win Roddier, and with use of that software (again, as I've said, I'm no expert so I followed the manual to the letter). Here is result of the test (screenshot): For the second test, conditions were not that great, a bit worse seeing than in case of the first test, this one was conducted in second half of October, 2016. This time I did deviate a bit from manual. I shot defocused star again with ASI185mc but used R, G and B channels of stacked frames to make three separate tests - one for each channel. OSC camera is probably not best choice for such work - I guess mono cam with filters would be better way to do it (there is some overlap in bayer response curves - over all three channels). Also I suspect that B result is most heavily impacted by not so good seeing, still I'm happy with results as well. Even in conditions of not so good seeing and with OSC camera, I got "diffraction limited" performance in Blue - Strehl 0.8. Green gave 0.94, and the Red was best with 0.98. My main concern with this scope, given it has traveled quite a bit in the Post - it was sent to the wrong country, and I had to wait 3 weeks for it to return to Germany and then be shipped again to correct destination - was that of possible damage in transport. I was really pleased to see that collimation was spot on, both visually and in tests. Here are results in Blue, Green and Red (again screenshots): I actually wanted to do proper reviews for each scope, but unfortunately I did not have good weather / time to do extensive work with them - only couple of recording sessions with each - in less then satisfactory conditions. From these tests and those few imaging sessions all I can say is that I'm really pleased with quality of the scopes. All comments are welcome, and I hope someone will find this information useful.

Ok here it is: 1. original FOV orientation 2. After 180 around RA axes 3. After 180 around DEC axes 4. Actual line DEC rotates around - it's angled to what it should rotate about because RA and DEC are not 90degs to each other. Diagram shows exaggerated angles just to make it obvious. So 1 - initial FOV orientation, 3 - final after flip - clearly FOV has rotated.

Just repeat diagrams with DEC not being 90degs to RA - it will show rotation.

Well if you have proper polar alignment and RA and DEC are orthogonal (I edited my post above, I think it is plausible explanation for field rotation - RA and DEC not being perfect 90 degs), any offset will be corrected with motion in DEC and RA - these do not create FOV rotation.

Yes, I see what you mean, still not sure if that would result in frame rotation. It might, but if we reason that 180deg rotation in each axis is flipping FOV (in either vertical or horizontal direction), not sure if it would lead to FOV rotation more or less than 180 degs. Pointing error will surely be present - GOTO would suffer no doubt. Center of FOV might end up in wrong place after flip - offset by some RA and DEC amount but not sure about rotation. Lets think of it this way. Instead of having tube pointing only in one direction let's consider tube that is hollow and points forward and backward - empty OTA for sake of argument, we are looking at central line - line of sight of OTA both forward and backward. 180 deg rotation around any axes that crosses OTA central line will result in flip. Yes, indeed - if we flip on two axes that are not orthogonal we will end up with rotation that is not perfect 180. This is possible solution to problem. So if RA is not orthogonal to DEC - frame rotation will happen! This case is not even hard to imagine. On my HEQ5 when I did adjustment of backlash and general maintenance - at certain point in procedure DEC axis is adjusted only on one side of RA (next to the worm, the other side, down where weights bar is is held by bearing in place), by moving couple of millimeters back and forth - enough to deviate from 90 degrees.

Also - rotation of FOV for 180 degrees after flip is just apparent rotation - it's not real rotation so it can rotate a bit more or a bit less - its two times mirror image - once in vertical, once in horizontal - two flips combine to give same result as 180 deg rotation. This is due to 180 degs RA and DEC movement when performing meridian flip.

I gave it a bit more thought and here is my argument: Properly polar aligned setup: Any movement in either RA or DEC or combination of those will not introduce FOV rotation. Cone error (up down) is movement in RA. Sideways movement of OTA is DEC. So anything that can happen to scope is combination of RA and DEC if properly aligned. Except tube / focuser / camera rotation. So possible causes of field rotation would be: improper clamping of tube - so it tilts left right (looking down the tube with mount being at the bottom) under gravity when lying on its side (just before and after meridian) - but I guess this is not case - easily spotted, one would feel ota being loose on a mount. Any kind of focuser rotation, camera rotation due to gravity and uneven distribution of weight in configurations west / east of pier. And of course - polar alignment. We know that field rotation is problem for long exposure if polar alignment is not good. So to me this is obvious reason for field rotation. Mind you, although points before and after meridian flip are close on sky, they are 180 degs apart in both RA and DEC when meridian flip is performed.

Not sure that I'm convinced with this argument. Cone error - in essence same thing as pointing to a different part of the sky. Only important in the terms - goto / mount / computer thinks that scope is pointing to a certain place, and the scope knows it's pointing somewhere else. After meridian flip, frame will be rotated 180 degrees and point where scope is pointing will be "rotated" 180 degrees - meaning it will flip cone error to other side - it will not contribute to FOV rotation - only offset - this is corrected with RA/DEC offset (which does not rotate FOV). Orthogonality of camera being X/Y orientation to RA/DEC? - no impact on meridian flip as it rotates FOV by 180 no matter how it's oriented - if it's 30 degs to "horizontal" (being RA) - after 180 deg rotation it will continue to be 30 degs to horizontal.

So it has nothing to do with actual exposures, its just scope orientation pre/post flip?

Yes, I'm thinking about small error, also depending on polar alignment error - it could be present on one side and almost none on the other side - look at two circles - place where they intersect - tangents are at the angle, further away from that point tangents are more and more parallel - 90 degs from intersection - tangents are fully parallel. Contributing to this would be guide scope alignment. Are you guiding with guide scope or OAG? Also what is length of your exposure? If long, check for field rotation inside frames. Make sure that guide scope is aligned well with main ota. You can easily do this by slewing to a bright star and checking fovs of both cameras - imaging and guiding - star should be centered in both.

No, I was wrong, it can't be due to mount being tilted. Only way to get angle between frames is due to polar alignment, or lack of it. One way to explain it would be - misalignment is such that on east side circles are aligned almost ok, while on the other side they start do diverge.

Just a guess here, what would happen if mount is not perfectly level? if it's tilted to one side? could this be a cause for slight rotation of field pre/post meridian? If mount is a bit tilted to west or east then meridian line on sky and meridian line of mount will be at a slight angle. Now when I come to think of it, yes, this might be a reason - sky / stars flip around sky meridian and sensor flips around mount meridian, on one side - east of meridian - consider them aligned, but on west side frame is rotated with angle 180 degs + difference between sky meridian and mount meridian. Well frame rotates 180 degrees but stars inside frame rotate a bit more. I made a mess out of explaining, let me try to make a diagram.

I think it is viable option for public outreach. Take for example following setup: Projector, white screen, fast system with matched cmos of new generation (low read noise order of ~2e) and appropriate software Proposed setup: F/4 newtonian 8" or 6", HEQ5, ZWO ASI185 / 224, laptop and software to autoguide while stacking short exposures and doing stacking and auto development in the background - I think most bright objects can be viewed in under a minute and even less - with exposures in range of 1-4s (software takes exposure, corrects guiding for next exposure, aligns and stacks frame and does basic stretching to display faint parts) - yes it will be noisy in first couple of exposures but by the end of the first minute - I think one will have decent image - probably better looking than image at eyepiece to inexperienced observer without dark adaptation - and it can be shown to bunch of people at the same time.

All sorts of things come into play when choosing guiding setup. It is not only focal length that counts, it is also the size of pixels both in imaging and guiding equipment. With increased focal length of guider scope you get more precise guiding provided your mount is up to it and the skies allow but trade offs are: Smaller field of view to pick guide stars from, longer exposure needed to get good SNR on guide star. Many people use off axis guiders that operate on same focal length as imaging train but use guider cameras with smaller pixels. I have read somewhere that you should aim for 1/3 - 1/4 in arcsec/pixel ratio between imaging and guiding setup if using guide scope. For example if using scope of 800mm focal length and have same pixel size on both imaging and guiding camera you would like to stay above or equal to 200mm focal length for guide scope. You can guide 1000mm focal length scope with 160mm guider provided that your imaging camera has twice as large pixels as guide camera. Sensitivity of guide camera and level of read noise also play part in this equation because of way that guide programs work. I'm in favor of fast guide scope with large FOV. This can have benefit of having multiple guide stars to guide in the same time. Not sure if this is implemented in any of guiding programs but I feel that it would be of great help in beating the seeing - perturbation of star position due to seeing is rather local so if average drift across multiple stars is taken - one expect for seeing effects to be smaller.

According to ZWO, Gain of 350 should give you the least read noise for that model. Any gain setting above 200 is good - aim closer to 200 to get better dynamic range. Set exposure time as low as possible to get at least 50% (you can go lower than that but at expense of SNR), higher values are better (like already suggested 75% of histogram, provided you don't clip histogram or have too long exposure for given seeing). I usually leave gamma at 50 (it is only digital so no real benefit, and certainly so for planetary imaging), white balance at 50 for both blue and red (again no benefit since it is digital control and can be adjusted later after stacking). One thing to do as well - there is brightness setting (at least for 185mc, but I guess it is same for 224), when you decide on exposure length, cover telescope (as if taking dark frames) and look at the histogram. If it is clipping to the left, increase brightness setting. Minimum values of your dark frames should be just above 0. Do take at least 256 dark frames and process your recording in pipp (to do dark frame subtraction - this will remove both dark and bias signal). For DSO there is no real benefit in going over 135 with Gain, so keep under. For shorter exposures stay above 60 for gain, for longer exposures you can go below 60 with gain. There seems to be a switch in read mode around gain value of 60 that has effect on read noise. Last tip - don't use bias frames - take as many dark frames as you can at exactly same settings - without touching any of controls - just cover telescope with a cap. It seems that these sony cmos sensors have some internal calibration that is applied whenever you change either gain or exposure length.