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About vlaiv

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    Sub Dwarf

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    Novi Sad, Serbia
  1. vlaiv

    Field flatness of refractor

    It looks like my guess was wrong, here is quote from Telescope optics website: "As the two relations imply, Petzval radius is independent of object and image distance." Here is link to actual page (goes in depth explaining field curvature): https://www.telescope-optics.net/curvature.htm From above I would conclude that it is perfectly ok to adjust field flattener indoors as described above provided that distance of FF to sensor only depends on actual curvature being corrected (and I suspect that is the case, since curvature varies with FL - this is why recommendation is given for different distance depending on scope / FL).
  2. vlaiv

    Field flatness of refractor

    Don't know if I can be of any use here - I have no idea if flattener distance will be affected by range to target. My guess (and I need to emphasize it is guess only) is that it will be affected in some way, at least by "speed" of light cone. Closer you focus your objective, slower light cone is (if you bring it to focus on "front" side of the lens you should get parallel rays exiting on back side of the scope - equivalent to F/infinity ratio - beams won't even converge). Some field flatteners have different recommended distance that they best work on depending on scope / FL you use them with. Not sure if close focusing changes field curvature in any way (making field flatter than when focused at infinity) - might also be the case. May be that you can do it that way, but I suspect that distance needs to be a bit longer than 6m - same as collimation on artificial star - I can't remember but there is relationship between minimum distance and scope focal length of F/ratio - can't remember which. If you are really keen on getting this sort of thing done indoors, it can be done, but I'm afraid you will need two scopes - one acting as collimation device - you use small screen (phone might do?) as a source of light and place it at focal plane of collimating scope to create parallel beams exiting in front (turn both scopes towards each other - front to front). Of course this technique requires collimating scope to already have pretty flat field and be of good optical quality (think opticians and mirror flats ).
  3. 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).
  4. But I did consider buying one at first - until I calculated total costs and saw that it is quite easy DIY solution - very nice for a small project in winter months when weather does not cooperate. At the moment I think I'll look into building L bracket - lego way - out of two or more pieces. Retailers that deal with stepper motors and "robotic" accessories have generic mounting brackets - so maybe I can figure out some way of fashioning suitable L bracket - bolt here nut there ...
  5. More I think about it, more I'm in favor of "in line" approach vs belted side by side configuration. That would mean removing both fine focusing knobs, but I guess that would be ok as it's "non invasive" operation - I can mount them back without much hassle.
  6. Thanks for that info. I did look at usb focus as an option, but I can't justify the cost at the moment. My dealer has kit listed at around 180e without holding bracket, brackets are around 85e and additional motor is around 90e That would total to about 570e with VAT and customs fees - pretty steep. I believe that I can motorize both focusers for quarter of that price if I choose DIY route.
  7. With flats you are not correcting for CCD wavelength response, but rather difference in "QE" of optical path and that one is independent of wavelength (or should be, any dimming due to vignetting should be ND type of dimming - simply shadows. Not sure how "grease stains", or icing, or fogging up respond to different wavelengths). So for example you have 30% QE in Ha and you have 60% QE in your flat field source (let's for argument sake assume that dust distribution and vignetting is absolutely the same between these two). Pixel being illuminated by 50% vs pixel being illuminated at 100% for 30% QE has the same ratio as for 60% QE. Lets take 100 photons as example - first pixel in Ha will have 15e, second will have 30e (30% QE, first being illuminated with only 50%, second - 100%). You want to correct that with source that has 60% QE, so flat will produce value for first pixel of 30e (60% * 50%), while for second one 60e (60% * 100%) - but you don't care about absolute values of flat - you care about ratios between pixels, so your flat is in effect: 1/2 : 1 - being (50% illumination, 100% illumination) - it is scaled version just representing ratios and any QE dependence to source is lost. Unless by second order effects you imply already stated - umbra / penumbra dependence on wavelength (extremely small effect to be noticed, I believe - as diffraction vs wavelength around edge produces angles so small that over distances in optical train they can't project on imaging plane very large component - it will be much less than single pixel).
  8. No relationship what so ever (well there is, but it is consequence of other things, rather than number of stacked frames). Here is break down of what you need to know: 1. Pixel count in width and height of sensor will tell you size of image on a computer, but will also tell you print size depending on selected DPI - this part, I'm certain, you understand well. This holds unless you use binning or drizzle or other processing techniques that increase or decrease pixel count of resulting stack (advanced stuff so don't worry about this if you don't know about it yet - these are special cases of data manipulation) 2. Number of stacked images has no relation to pixel count (or width and height of resulting stack in pixels). Each sub in stack is aligned, so there is almost exact correspondence between each individual pixel in all stacks - they end up in pretty much the same place. If anything stack of subs will have less total pixels than pixel count of sensor - but not because of stacking process, rather because frame shifts between subs (tracking / guiding error, or deliberate dither), or is rotated (multiple session or not perfect polar align) and in the end you end up with cropping away edges that did not stack perfectly (not all subs have pixels in that particular place after align). I'll try dumb analogy here - you have a bank account - multiple deposits to this account increase total balance (signal in the image), but don't create multiple accounts for you (you will not end up with more pixels than you started with). 3. There is a way that you can make "poster sized" images (this relates to print size - or large pixel count image), even with very small sensor (in terms of pixel count) - Mosaics. In this process you image multiple panels in the same way you would image single panel target (multiple subs, calibration, stacking, ....) but you take images of panels that sit next to each other in the sky. When you complete each panel, you simply "stitch them together" to produce larger image - this process increases pixel count (adds all pixels from each panel) but does not do what stacking does - it does not improve signal in panels further. It's like taking any two images and putting them next to each other (either horizontally, or vertically) - you end up with "larger" image (more pixels). Of course there are technical details like: some parts of panels need to overlap (close to edge) so you can align them properly - meaning total resulting pixel count will be less than sum of individual pixel counts. You need to combine them prior to stretching and post processing, and you need to equalize signal strengths and background levels - so you get seamless transition - but all of that should be handled in software.
  9. Yes, that is correct. It also gives you a tool to convert between photon count and energy (because it can be viewed with appropriate transform as photon density function - transform would be just to apply energy of photon - wavelength relationship).
  10. I just realized that I will not be as straight forward as I thought. Although both focusers are of the same name - TS 2.5" - they differ in fine focus knob and travel per turn. On RC I believe that fine focus knob will have to be replaced with either pulley or coupling, depending on mounting position (below or to the side). On TS80 APO fine focus knob already has proper grooves that I can use to put belt on. I was hoping to leave focusers "intact" and just mount motors to them, but it appears that I will need to mod at least RC focuser. I've found good "summary" resource on this one that helped me understand differences between mounting options: https://astrojolo.com/astrolink-4-0-mini/focusing-stepper-motor-solutions/ According to that page, coupling to fine focusing shaft is pretty good option. Did some calculations and it looks like there will be enough resolution with 200 step motor and fine focus reduction to be able to go as low as F/4 - which is enough for my needs. Now I just need to find where to source pulleys, belts, couplings (I can do it online, but would rather like to inspect items for suitability prior to purchase).
  11. Yes, there are some guiding issues (or tracking if you did not guide), have a look at montage of two stars from each of five frames: second and third show elongation in same direction - probably RA, while last two are a bit larger - first frame is "spot on".
  12. Here is result of quick stack - I did not look at individual frames (might be that there is an offending one since stars are misshapen in stack): But result seems quite ok in comparison to stack from DSS
  13. There is something seriously wrong with stack Just look at histogram: All the values are clustered and distinct - almost like there is only 16 distinct values in the stack (not sure how can that happen, I'll download subs now and do a quick stack of it to see what happens).
  14. I think it is processing artifact. Either there were some strange star core clipping due to hot pixel removal routine (cosmetic correction in DSS), or very funny curves were applied to final stack in stretching. It will not show as much on single sub because of noise, but I got something similar looking in gimp on single sub, like this: Your single sub looks just fine on its own and does not look like there is anything wrong with star shapes
  15. I think that simplest way to put it why units needs to be like that is to observe what spectral flux density curve is and what you do with it: In order to get total energy flux between certain wavelengths (in certain band) you need to integrate spectral flux density, and integration is summation of term spectral_flux_density * d_wavelength and result should have units of Wm-2, but d_wavelength has units of nm so we are "short" of one nm-1 in spectral_flux_density term and it needs to be Wm-2nm-1 in order to produce correct units after integration.

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