vlaiv

Framing is a bit strange?

All of them serve the same purpose - behave the same. You have slight defocus and it will show as asymmetry in spikes. If you can't see them properly - use brighter star, use longer exposure, or bin your image, whatever improves SNR. I doubt that one needs 1cm wide grooves / supports to have rigid B mask. I guess few mm wide is enough even for large masks.

Not sure how you plan to cut such thin lines . At some point I wanted to make very narrow slit - like 10µm slit (which 100 per mm) and that was simply beyond my skill by far. In any case, I suspect that it will start behaving like solid and reflect light. In fact - most of the things that we perceive as solids are very empty and we could argue that they are extremely fine gratings (crystals for sure) - some transmit light and others reflect it, but that depends on internal structure. Even if material was transparent and had very fine "molecular" level "grooves" in crystalline structure, I think there would no longer be diffraction effects. Diffraction happens because of different distance between light paths. Once slits are too narrow - difference in distance between adjacent paths vanishes and so does phase difference for interference. Wavefront will just pass undisturbed (transmission) or get reflected back (reflection), but there won't be diffraction.

I just did image search on google and came up with that image among others. Have no idea who makes it.

You reach a point where mask starts working as diffraction grating. You can see that as one gets more line density - spectrum starts to form in different orders, like in this image: With higher density you are both increasing length of diffraction edge and spacing of the grooves - second one just makes spectroscope of higher and higher resolution. Diffraction gratings have hundreds of grooves per mm and produce nice rainbow spectra - like StarAnalyzer. This is 100 lines per mm - very stretched out central spike of bahtinov mask. B mask has 3 distinct angles of grooves and that creates three different spectra at an angle.

https://www.dpreview.com/forums/thread/4454760

Low diffraction - small spikes : High diffraction - large and strong spikes: or this one:

William optics seems to think so However, their rationale that it will help with narrowband and make spikes stronger / more contrasty is wrong. Strength of spikes depends solely on length of diffraction edge - in this case, density of the grooves.

I personally don't use it - I've found that FWHM/HFR much more suits me and is more precise in my case, however I do know that it often happens that people forget it on the scope.

I was just explaining why most of things are black - that is just a "convention" because there is a need to suppress stray light which lowers contrast if not suppressed. In the case of B mask - difference is not noticeable in most cases as most people use relatively strong star to focus on and yes, B mask is removed later when imaging.

Color of mask has absolutely nothing to do with operation of mask - it is the edges and their angles that create spikes. There is no reason why one could not use colored Bahtinov mask. There is however issue of light reflection that could potentially lower the contrast. Most accessories that are in light path are died black for this reason - to minimize reflections and stray light, however B mask is not kept on during imaging (at least not by choice , but sometimes it may stay on due to forgetfulness ) so it really does not matter and one could use colored masks without much issues.

@FLO Any chance you'll be receiving those little guide scopes - https://www.firstlightoptics.com/guide-scopes/astro-essentials-32mm-f4-mini-guide-scope.html any time soon? It says 20-25 days and I ordered mine more than month ago ...

With that little dob, I feel that focuser is going to be somewhat problematic - it is very basic unit. Also, F/4 newtonian is going to have quite a bit of coma. It is certainly very portable solution. I would suggest 90mm mak only if you really need that difference in size and weight. Mak102 has a bit more aperture an almost exactly the same focal length (only 50mm difference). Two scopes will be very similar in performance but Mak102 will have edge in both light grasp and planetary resolution (again, small edge as it is 102mm vs 90mm). I don't really know much about virtuoso mount to be useful there.

Part 2 - simple gimp composing and stretch. 1. Load all three fits saved in previous part. 2. Do Color / Channels / Compose with RGB model and assign each image to corresponding channel 3. Do simple levels stretch - Color / Levels Move top slider until you see that M31 core is starting to saturate. Move middle slider to left, like this: 4. Do another round of levels, this time bringing left most slider at the base of histogram hill like this: 5 Do a round of Curves like this: 6. Due to artificial flats - there is distinct red cast on background everywhere except for galaxy. Select it and remove it by curves on red channel. And voila, here is resulting image:

Ok, I'll now do a step by step processing "tutorial" so I apologize in advance for long posts and plenty of them. Hopefully people will learn something from this. Tools used - Gimp 2.10 and Fiji latest version (Fiji Is Just ImageJ - one of those recursive acronyms - just ImageJ loaded with plugins. Probably stock ImageJ will work as well as I use pretty much basic things). You'll also need a plugin that I wrote. I'll include it for download: Extract_Background.class This should be placed in Plugins/Astro folder inside Fiji/ImageJ installation directory (create Astro folder inside Plugins folder as it won't exist by default). Restart ImageJ/Fiji in order for plugin to load. Part one - data processing part. 1. Open M31.tif in Gimp 2. Use Color / Components / Decompose menu item. Select RGB model and uncheck decompose to layers. This will create three additional monochromatic images - each channel will be as mono image, there will be red, green and blue channel image. 3. Export As each of those channel images - as fits files (Gimp 2.10 handles fits files) 4. Open all three channel fits in ImageJ (or Fiji, but I'll just write ImageJ from now on ...). Open red then green then blue (keep that order as will matter later on) 5. Use Image / Stack / Images to stack menu command, use default settings (just click ok). This should create a stack of images. 6. Use Image / Adjust / Brightness&Contrast menu item to display Brightness and Contrast window. This is "screen transfer function" window - it won't change data just display range, so you can move sliders to display different part of histogram of the image. 7. Use rectangle selection tool to make a crop section (first icon in main program window / toolbar). There are some stacking artifacts on the left side and on the bottom and we need to remove those. Make larger rectangular selection of almost whole image except dozen or so pixels at each side (top, bottom, left and right). 8. Right mouse click on image and select "Duplicate" from popup menu. Select duplicate whole stack check box. This will make duplicate of selected area as new stack. 9. Close original stack as we won't be needing it any more 10. Use Image / Transform / Bin menu item. Use default settings (X and Y factor is 2, Z factor is 1, method is average). This will bin data x2. This image came from OSC sensor and there simply is no point in having interpolated resolution which is empty resolution. If you want to get some more SNR at this point, you can bin by factor of x3 - just change X and Y value to 3. This will create smaller image but higher SNR. 11. Image / Stack / Stack to images menu command - this should split the stack again into individual images. I will describe next section on one image / channel, but it needs to be performed on all three images. 12. Select image that you are working on and run Plugin / Astro / Extract background menu command. Uncheck produce background map (you can leave it checked if you want to see what pixels algorithm considers to be background) and leave produce gradient map checked. Leave default parameters of 3 and 10. This command will produce new image that will be just gradient image. 13. Hover mouse over gradient image and examine pixel values - displayed in main window. If gradient image values in the corners, where gradient starts and ends are very small values - like something e-7 or something e-8, you will be done with this procedure, if not, you'll have to repeat it. Depending on type of gradient, it usually takes something like 5-6 round of removal to get nice background. 14. Process / image calculator. Select first image to be image you are working on (red channel for me at the moment of writing) and select second image to be gradient image (gradient image will have same title with "Gradient map of" prepended). Select operation subtract and uncheck create new window and check 32bit precision. this will subtract gradient from our image (it will also change how image looks if you set certain range on screen transfer function so you can inspect image by selecting it again and changing sliders in brightness and contrast). 15. Close gradient map of .... image. If at step 13 you found that gradient is still significant, select again the same image and repeat procedure (go to step 12). Else, proceed with step 16. It took 4-5 rounds to get this value for red channel for me. I can now proceed with next step. 16. Again select image that you are working on and do Plugin / Astro / Extract background. This time uncheck produce gradient map and check produce background map. Leave other parameters the same. This will create black and white image where "foreground" pixels will be white and "background" pixels will be black (255 and 0 values - nothing in between) 17. Select the image that we work on and run Analyze / Measure command. This should bring window with basic stats for the image. Important ones right now are Mean, Min and Max. If one of those is missing, run Analyze / Set Measurements and check ones that are missing then run Analyze / Measure again. Note minimum value in the image (Min). for me, it is -0.0017009.... 18. Process / Math / Add and enter any number that is slightly larger than this - I'll go with 0.01. We want all values in our image now to be positive, so we want any number that is larger (by value) than minimum value (negative one) in the image. You can run Analyze / Measure again to see the change: and close results window now ... 19. Process / image calculator. First image is our image that we work on, second image is background map of that image. Operation is add, check create new window, check 32bit precision. This will create new image. This image will be our artificial flat. You can right click on it and rename it to something meaningful like "working_image_flat" - in my case "m31_red_flat". Close background map of ... image now. 20. On this new flat image run Process / Math / Macro and type this line in: like this: This will set all values that are "foreground" pixels to non existing value (Not a Number). 21. Run Process / Noise / Remove NaNs and enter value of 100 on this flat image. This will take some time to complete. It just fills in missing values with median of surrounding pixel values (in circle of 100 pixels). 22. run Process / Filters / Median with value of 10 on this flat image. 23. Run Analyze / Measure on this image and note max value 24. Process / Math / divide and use max value that you measured in step 22 25. Select image that we work on and Process / Image calculator. First image is our working image (red channel), second image is our flat image. Operation is divide. 32bit precision and uncheck new image. This will apply our fake flat. After that you can close flat 26. Again do one round of background removal (steps 12 - 14) to remove offset that we added for artificial flat to work properly. Now repeat above with two remaining channels (starting at step 12). Just a note - if Process / Noise / Remove NaNs does not "close" the image completely - run it again until there are no "missing" pixels in the image. 27. Image / stack / images to stack - this will create stack again. 28. Find star with color index 0.48 in Stellarium . 29. Find matching star in your stack and make circular selection around it - like this: 30. Image / Stack / Measure stack, you should get something like this: 31. Unselect and Image / Stack / Stack to images to split images 32. Divide each channel image with corresponding measured value (Process / Math / divide and then enter corresponding number) 33. Image / Stack / Images to Stack and then Image / Stack / Measure stack 34. Process / Math / Divide - take the least out of Max values. Process all three slices when asked 35. Again measure stack, and note highest of min values 36. Process / Math / Max - set to 1, Process / Math / Min - set to highest of last measured min values (in my case -0.02878031 as it is highest of the three). 37. If you measure once more after this you should get same min / max values for all three channels like this: 38. Save each as fits ... Part one done

I think you've done yourself a disservice by watching astro photography videos first. People often form expectations by looking at astro photos these days and become very disappointed by the views they get in their telescope. Here is another video that you should watch to level your expectations: https://www.youtube.com/watch?v=jI7IPPmu76U Pay attention to telescope size (top left) and light pollution level (bottom left) that are displayed next to each comparison. There is similar video that deals with planetary views: https://www.youtube.com/watch?v=Z6NIBBldy8U Now that we have that out of the way, scope that I would recommend that you get is this: https://www.astroshop.eu/telescopes/skywatcher-maksutov-telescope-mc-102-1300-skymax-102-az-eq-avant/p,56249 That is probably the lightest scope that will be suited for solar system objects and bright DSOs, that will fit your budget and provide you with most aperture. Scope with tripod has 7Kg and it can be used both in equatorial and alt azimuth configuration. You can add tracking motor later (very good for planets) when you use it in EQ mode. If you are concerned with often mentioned "narrow field of view" - I'm just going to say this - you originally considered scope that has 1.25" focuser and 1200mm focal length. This scope has 1.25" focuser and 1300mm of focal length. Difference in field of view with the same eyepiece is minimal. Here is an example of comparison of widest fields of view that both scopes are capable of: As you can see, difference in FOV is minimal. Another alternative is to get the same scope on AzGti mount. I'm mentioning this as you said you would like to do some astro photograpy. That is probably the cheapest way to get both in to AP and visual. https://www.firstlightoptics.com/sky-watcher-az-gti-wifi/sky-watcher-skymax-102-az-gti.html That mount can be converted into EQ mount that you can use with your DSLR camera to take long exposure photos. For example to something like this: You only need to provide equatorial wedge (that can be purchased - or you can make something yourself, I used ball head in the image above), counter weight bar and counter weight (again, you can purchase that, or DIY one - like I did above from M8 threaded rod and some big washers).

Only major issue that I found with this data is lack of flats. This is processing that I did in ImageJ and Gimp 2.10. If you wish, I can do a step by step guide, but I'm not sure if you'll be interested in that since you use PI and PS.

Most people use multiple scopes. Scopes that are good for imaging are not necessarily good for astro photography and vice verse. I have 5 scopes at the moment. Two of them are AP scopes and 3 of them are primarily visual scopes. 6" ED or APO refractor is certainly very nice visual scope and I would possibly like to own one, but I would not use it all the time. Best scope is one that you use the most often. Such scope is very heavy and not easy to handle and setup. It's worth having if you have permanent setup, but can become chore to setup each time very quickly and can even kill off some of observing enthusiasm. Telescope that can "rival" 6" refractor is certainly something like 8" F/6 newtonian - on a dob mount. Why not get that for start for general observing. It is such a versatile scope and it won't eat much into your budget. You'll learn your way around the sky and practice your observing skills (you actually see more as you hone your observing skill). Very easy to setup and to operate and carry short distances (for longer distance transportation people have different solutions) as it breaks down into two pieces. Mesu and 10 Micron are top tier mounts. Most people use much cheaper mounts and make good images with them. I think that very good platform for you to learn AP would be something like 4" APO refractor with decent dedicated cooled astro camera and EQ6 / CEM40/60/70 class mount (cem60 is no longer produced for some strange reason). There is so much to learn in AP that you don't need to rush into it - get to know basic concepts by reading a book or looking at youtube tutorials. Reading/searching thru AP threads here on SGL is also great source of knowledge. As you start gaining understanding of AP concepts, I'm sure you'll have more exact questions and people will be happy to answer them and help you out.

RMS can be thought as average for collection of errors. I've written about it above - it is square root of average of squares as regular average would lead to 0 error if there was balance between negative and positive values. Important part is not how RMS is calculated - important part is what it represents - average error. When guiding, RMS value will be average error of pointing of the mount - where it should point in comparison to where it is pointing. This is calculated by examining position of a guide star on sensor over period of time. Ideally - guide star should stay still - in the same place. This would mean perfect guiding. In reality what happens is following - software captures image of the guide star, calculates its position - compares to previous/expected position and if finds error - adds to running average and issues correction. Later being important part for guiding while former just giving us some indication of how well our mount is tracking. Remember, guiding corrects for error after it has already happened - larger that error - more impact to the image. Registration RMS is something entirely different. Again, it is average error of some sorts, but this time it represents measure of how well two subs match after alignment. In order to align two subs, software analyzes each sub for stars - finds the stars and their centers (and also other statistics on the stars like their FWHM values). After it has found stars in each image - software tries to find best matching transform in order to align stars in both images. It calculates something like - shift 3.23px to the right, and 1.12px up and rotate by 0.1 degrees or similar. It is actually represented by matrix (the transform, and whole process is done by least squares method). Stars won't align perfectly in two images. There are many factors for this: - there is noise in both subs and due to noise it is impossible to accurately determine star center - there is finite precision in sampling of both images (finite number of pixels) - again that limits precision in finding star centers - there is certain level of "lens" distortion and other effects in the image that impact different stars differently (as you have seen on your images - stars in center are round but those at the edge - astigmatic). As you dither (either purposely or "natural" dither due to shift in the system of some sort) stars end up in different parts of the frame and have different intensity of such effect on them. This changes their "center" somewhat (there are symmetric aberrations and asymmetric aberrations - later change star center) and this leads to error in alignment. - due to seeing and star "jumping around" somewhat, actual star center can change position between frames (although seeing averages out in long exposure - there is always some residual error, small but still present). In any case, after registration is performed - there will be some error in alignment of star centers - that is registration RMS. In your case - this error being as low as 0.1 or there about just means when that particular frame was transformed, average error between where star should have ended up and where it really ended up is about 1/10 of a pixel. For all intents and purposes that can't be visually perceived. These two RMS values are not related (they have one common component - seeing, but impact is so small that there is virtually no correlation between guide RMS and registration RMS). Part of guiding RMS is due to seeing and not due to mount tracking error, but we try to minimize that - otherwise we end up chasing the seeing instead of correcting for mount tracking error. If you suspect that your guide RMS is higher because of poor seeing - just increase guide exposure as that averages seeing out a bit more. Although many guide at 1s intervals - I've found that I like to use at least 3-4 seconds of guide exposure, but in order to do so - you need smooth(er) running mount (belt mod ). In the end FWHM value is related in part to guide RMS. With "perfect" scope, we can sort of write down very decent approximation of expected FWHM value: FWHM = sqrt(guide RMS^2 + airy_disk_size^2 + seeing_blur_size^2) (one needs to use corresponding values for each of these and relation between RMS and FWHM for gaussian is x2.355, also one needs gaussian approximation for airy disk as well).

That is a good question. I have no idea about that setup - and I presume it will be much more resilient to wind than larger scopes. I do know that 8" RC on HEQ5 does not like anything above about 3-4m/s. 1-2m/s is fine, or even a good thing - some wind will keep dew of the scopes and optics close to ambient as it is natural circulation of the air. That means anything up to about 7-8mph is acceptable for even heavier compact scopes on lighter mount. I think that small / lightweight scope on heavy mount working at lower resolution should be fine at up to 20mph but that is just an assumption / attempt at extrapolation.

There are ready made tripod adapters by Berlebach, so I got one to go with my tripod: https://www.teleskop-express.de/shop/product_info.php/info/p8483 They provide these for almost any mount type so its worth getting. It is extremely stable platform and when imaging on the ground, you can push it into the ground (has spikes) for additional stability. I did this to improve guiding performance. Mount itself is capable of handling 8" RC scope weight wise without any issues. Scope and camera and OAG and balance weights - total at probably less than 11Kg. I used almost 15Kg of gear on Heq5 and although it worked - I was not feeling comfortable with that. This is short scope so 11Kg is quite ok for imaging. Saddle plate is combined Losmandy / Vixen. Larger connection adds stability. Belt mod of course helps with guiding.

I think it comes from the same factory as Omegon one. Only difference being dual speed focuser (Opticstar model had DF) and case - and of course, price difference reflects that.

HEQ5 It's been modded quite a bit - belt mod, saddle plate and puck replaced, Berlebach planet tripod ... I now guide it at about 0.5" RMS on a good night.

I have 8" version from GSO, so I use that when I want long focal length.

That Omegon that I linked was the same scope Opticstar used to have in their lineup - and it was even better as it had dual speed focuser. For some reason they don't have it any more, but 90mm F/8.8 is certainly excellent scope for that money as well. I think one could mount it on AzGti as well.