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That is really interesting - they quote peak-to-valley error of no more than 0.24 (for scopes up to 150mm of aperture) - that is just a tiny fraction better than 0.25 or 1/4th of wave P2V. What they are really saying - our scopes are "diffraction limited" or have Strehl of higher than 0.8. Maybe we are being mislead by low cost scopes that advertise as "diffraction limited" - maybe they are much poorer than that?

To be fair - point four I included because I'm aware exists - but it does not really figure in the way I look at things - which pretty much leaves number one as prime candidate. Herein lies the problem for me. I understand that there is difference, and I can accept that last few percent of performance come at very steep price. I just don't really understand how much difference those last few percent really make. I have to go by very conflicting data sets - we have personal experiences that varies but in general there is consensus - expensive = better and it can be seen at the eyepiece. On the other hand - theory suggests that you should be able to hardly notice - if at all. I did my simulations, but there are resources available online as well. Here are few sim images produced by Damian Peach: Top is 1/10th wave optics - bottom is "diffraction limited" - or 1/4th of wave optics. If that is the difference between 1/10th and 1/4th - I wonder if we would be able to see it at all between say 1/8th and 1/6th or perfect and 1/8th. Now, there are few explanations for this discrepancy: - observer bias - theory being wrong - application of theory outside its domain of validity I really doubt that theory is wrong, but we could possibly use it outside of domain of validity - maybe it is valid for recorded images but our eyes might see things differently and we did not include this into account. Then - there could be observer bias. Maybe people simply expect more expensive scopes to perform better? Maybe it is something completely different - maybe what we think are "average/good" samples are in fact poor samples and therefore very different than "excellent" samples? All of this is something that I would like to understand more before "deciding" if expensive is worth it. To me - if difference is there - then yes, some will have the money and be prepared to buy the best possible - and that is a good thing as those that produce the best will be kept in business and everyone will have a choice. But if difference is not there - how to justify it?

How about against FPL-51 or FPL-53 triplet that is less expensive? I was under impression that Taks are known for their quality of figure rather than absolute color correction?

Not sure if we should be thinking like that. Telescope can cost as much as a small car - or say sports motorcycle. Can we compare quality of product between the two? Even with all hand crafting - sports motorcycle requires more man/hours of precision labor to be made. Yet they cost the same. Economies of scale come into play here and standardization. You don't have to hand craft every bolt on motorcycle and so on. Chinese telescopes for example share components - like finder shoes, or focusers or scope rings and so on - that brings their cost down - but do they suffer poorer quality because of that? Not if QA is good enough. Even with high volumes one can assure satisfactory quality of product.

Can we break down why are expensive telescopes expensive? - Optical performance - Mechanical fit&finish - Value depreciation over time? - "Lust factor" / "availability" / "status" ? Any other point? Could we compare two scopes - say 4" refractor - pick expensive one (Tak will do ) and put it against say Starfield 4" F/7 ED in each of above categories both quantitatively and qualitatively? By quantitatively I mean also assessment of "worth" - or colloquially known as "but is it x10 better".

Incentive to jump high enough will be there

Can't you just leave phone screen permanently on? There is option for manual lock - not timed one. It does not really need to be phone, I used phone as an example - you can print out Jupiter image (again - I'm using image of Jupiter because we are familiar with that target) and shine a torch at it. You can also use any sort of small object with intricate detail, low and high contrast areas and so on as test target. Point of the test is: 1. place target far enough so that spherical is small enough 2. reduce impact of atmospheric turbulence (avoid doing it over concrete or water) 3. remove observers (bias) from equation by comparing images We need camera+lens and eyepiece to be able to record easily even smallest detail (hugely over sampling in such combination). Camera at prime focus won't be sampling at critical resolution for faster scopes (we would need barlow and so on). Scope + eyepiece is also closest to observing setup. This is interesting test for also comparing eyepiece sharpness - use same scope and put different eyepieces, or other parts of optical train - such as prisms/diagonals - shoot with/without diagonal and compare images.

In my case - solution was easy Instead of buying a new focuser I fitted old one that I was left with after upgrading focuser on one of my imaging scopes There are a few videos on youtube of stripping and re-greasing synta R&P focusers - like this one: https://www.youtube.com/watch?v=K89k3U9mPe8 You'll need to adjust it and you'll probably need to replace PTFE pads that are in there

It's much easier when you @ mention me rather than just "shouting" my nick Indeed, pixel scale is 0.95"/px. In my view that is still very high sampling rate to be useful on a day to day basis, but it can be used. I'm at that sampling rate (or very close at 1"/px) with my 8" RC and ASI1600 (when I bin x2 small pixels). Sometimes it works, but most of the time it is still to much. Here is an example of such resolution with heq5 that did not really work: it's ok image - you can see the target and all - but it looks blurry when viewed at 100% zoom (this is just crop and if you view it on computer it will be already at 100% zoom). None of above stars are pin points. Sometimes it works better: Notice difference in star sizes between these two images. There are nights when this combination is very poor because seeing is very poor: For general use - I'd say that it still very high resolution, but if that is something that you have on hand - well, go for it.

No need to go to Arizona desert to do such test. All you need is stretch of grass about 200m long where you can on the one end put a smart phone or tablet on a stand and on the other scopes in question. You'll then need very sharp planetary eyepiece (good ortho), good lens, about x10 focal length of eyepiece (this does not need to be very precise) and DSLR. A piece of black cloth acting as a shroud between eyepiece and lens will be needed as well. Put image of favorite planet - let's say Jupiter on the phone, aim the scope at the phone and focus image as best you can, put DSLR + lens at the back of the scope where eye would go. Make sure no outside light gets into lens except what comes out of eyepiece (use sleeve/shroud for that) and take image. Do that with every scope and only processing that you should do to images would be to reduce their size (resample them) to optimum sampling rate for particular DSLR - or even simpler - make Jupiter look right size on the screen and reduce size of each image the same. Then compare images for similarities and differences. Of course - you can also do visual assessment and testing against various targets - either on phone, or maybe some small piece of fruit placed on a tripod (so called nut test )

This points to a fault in scope, and yes, asking for replacement would be good idea as lens need to be adjusted but no means of doing that is provided for the end user.

There is another possibility here - you can also check for that. Do in/out focus star images look the same, or do they "change direction"? It could also happen that one side of focus star image is elliptical, while on the other it is normal. - if they stay the same - most likely cause is in your eye - if ellipse elongation rotates by 90° - then it is down to lens adjustment - if there is ellipse one side of focus but not the other - then it might be focuser sag / draw tube not being straight. ST120 does not have very good focuser and this can happen - you can try to fix it if there is play/sag by adjusting / tightening it up In any case - more experimenting is needed to pin point the cause.

Actually yes. Do you have astigmatism in your eyes? Astigmatism is usually function of pupil size. Larger exit pupil and more dilated your eye pupil - more likely that you'll see it. At low powers, exit pupil is very large with fast scope. At higher power exit pupil shrinks down. There are cases where people with astigmatism in their eyes can do planetary observation without wearing their glasses because exit pupil is too small for astigmatism to even show up in image. Similar thing happens during daytime use - if your scope seems sharp during daytime - it could be down to same reason. During daytime use - there is plenty of light and your eyes act as aperture stop - your pupils shrink and image is not affected by astigmatism even on low powers. Try changing observing eye, try observing with glasses. It looks like at least some of the problem might be down to this. As far as newtonian is concerned - there could be couple of reasons why you did not notice this before. It's probably slower scope so exit pupils are smaller. There is also central obstruction that removes central part of exit pupil so exit pupil again uses smaller section of your eye lens - less area / less distortion.

What is your eyesight like? Do you wear glasses usually? F/5 scope will have large exit pupil (unless you stop it down) - and it might be that you are seeing astigmatism from your eye. Try switching eyes to see what happens. Another explanation is that lens is out of collimation. When you say that you did collimation on the scope - how exactly did you do it. As far as I know SW 120ST does not have lens cell that can be collimated? Have you performed a star test? It is something you should do to see what the optics is like - or at least to check collimation.

Hi and welcome to SGL. Very nice set of images.

That is very nice article - haven't seen that one before, but I do have 4 out of top 5 . All but #15

According to this: it would not be very good - it would start too "early" at 470nm - about same place as #8 but would have gentle slope - ending the same as #12. Evostar 120 is rather nice scope for this sort of thing. You can use its 5" (or rather 4.72") of aperture on deep sky and then switch it to be 4" F/10 for planets. Just make 100mm aperture mask for it. You can also have 80mm F/12.5 if you want - by making 80mm aperture mask. With about 70mm - you won't need filter as you'll be hitting Conrady standard (F/ratio divided with aperture in inches >=5 - minimal/no visible CA), but you'll be limited to say x100-x150 magnification. 3d printed ones will likely be the best, but I've made them out of PVC pipe plugs as well.

Wratten #8 is closer to 495 long pass in its characteristics. Both start at 450nm and raise up to 500nm, but Baader hits 500nm at 95% transmission - with steeper curve and higher general transmission reaching 97-98%. Wratten #8 reaches only 60% at 500nm and peaks at 525nm. Wratten #12 starts beyond 500nm and reaches peak at 550nm. As cutoff filter - 495 is the best as it has the steepest curve. @Ian McCallum What scope do you have and how much are you bothered by CA? Have you considered stopping the scope down a bit together with wratten #8 filter?

I would not know. I found that screen shot online to try to help - I believe it is screen shot of CEM40 drivers where it allows for guide speed adjustment - but I don't know for sure since I never operated that mount. @Legion Of Andromeda should be able to answer that but as far as I can tell - he/she have not visited SGL since May 5th

Here is result of quick processing. I binned image x3, wiped background, did color scaling on B-V index of 0.63 star (from Stellarium database). Gimp was used for processing - I did stretch of the image, some masked denoising and some masked sharpening. I think there is some IFN captured as well, if I'm not mistaken, but it is very faint. Here is further binned x3 image to improve SNR. I also stretched it very very hard in order to try to bring out IFN to be compared by reference images: Here is reference image for comparison: Distinct inverted L can be identified in left bottom part of the image - and some IFN structure below M81 as well

If you worry about stacking in DSS - just stack with simple average method (nothing fancy) - that maybe won't produce best results, but will always work (you can't mess up parameters there): So no background calibration and average stacking mode.

This is red channel only stretched like crazy (vignetting really clips more than just shows in corners): Here is reference image of that region rotated and scaled to roughly show the same FOV/scale: I see a lot of captured detail, and keep in mind that this is camera is not modded for Ha part of spectrum.

Always use 32bit precision - but use 32bit float precision. I think that there are two options for 32bit - 32bit int and 32bit float. Second one should be used.

This one was a bit challenge to get working. Image was saved as 32bit integer in float point format - which means that most of the image was negative numbers in range of ~2,000,000,000. Gimp could not recognize it and it displayed completely grey image. I wonder if other software had similar issues and that is why you thought that image is blank / dark. In any case - loaded it up in ImageJ - I scaled image to 0-1 range manually. Did background removal as gradient is very strong. There is vignetting in the image - flats should help with that, but I guess you did not take any. I binned image x4 to bring up SNR and help expose faint signal. Above is the result after composing and stretching in Gimp. I did not do any sharpening or noise reduction. Data is good (apart from missing calibration frames) and focus is excellent - stars are tiny.

If you can't see anything in stack, that is ok. Stacks have very high dynamic range and you need to stretch your data quite a bit to really show what lies in the murk.