vlaiv

Seeing yes - look at above examples, but transparency - no. Or rather - you'll have to work harder to asses it. Transparency affects light throughput. One way to measure it would be to look at series of threshold star magnitudes. Say you know that in steady clear skies you can reach 13.1 mag star with your scope. You would need to examine stars around 13mag to see which one you can see - say you see 12.8 on particular night. Transparency knocked those 0.3 mags off. Problem with above approach is that seeing also affects what can be seen. If turbulence is strong - star image is spread over larger area and peak intensity falls down (that is why we see stars sparkling at night). You'll need experience to be able to "subtract" effects of seeing on max magnitude and relation to transparency. Be sure that you account for atmosphere number - look at reference stars near zenith where atmosphere number is close to 1. If you have imaging gear - than it is easier - you need to do photometry on reference star. Seeing is easier with camera too - take 2s exposure and measure star image FWHM (take multiple short exposures and note average FWHM and standard deviation to be more precise of how steady seeing is).

Oh, don't worry, I did not mean that you could get it wrong - M42/T2 in astro use is exclusively 0.75mm pitch. M42x1mm is used only with lenses and old cameras. I just gave info if in the future you run into old M42 lens - that one won't screw in properly with astro gear.

Not always M42 x 0.75mm is T/T2 while M42 x 1mm is not. Later is used for camera lens. If you see M42 lens - you won't be able to screw it in T2 thread as pitch is different - 0.75mm vs 1mm.

Well, in the end, I must say I was wrong. In addition to those two things mentioned - I forgot to consider diffraction grating. Energy in spikes does remain the same - there is no difference there, but difference in gap size is telltale sign. Dispersion depends on grating density and that means on distance between gaps - or thickness of grating. With thinner spider vanes - diffraction spike is "stretched" - it is longer and since it has the same amount of light - that light is spread over larger surface and becomes less bright. Similar thing happens with spectrum on diffraction grating - use more grooves per mm - spectrum gets more resolved - or spread over larger area - but also becomes weaker. For visual that is definitively a plus - you want weaker looking spikes even if they are longer as this means better contrast on planets. For long exposure AP - well, I'm not sure which one is better. Here we stretch our data and faint things become equally visible. Maybe shorter spikes are less "damaging" to the image here?

First, sorry for slightly derailing the thread, after all - it is about your video more than diffraction spikes. I completely understand why you mentioned in your video that thin spider vanes produce smaller spikes - it is iterated over and over as, in my opinion - a marketing trick. Here is screen shot from Skywatcher website: And another one probably referring to the same thing, although this one makes even less sense: Not sure how secondary mirror support can be diffraction limited, but hey, sounds cool, right? Back on the issue of spikes - I don't think people will notice rainbow effect while observing - for several reasons. "Gap" spacing in spike for each wavelength depend on the wavelength. Since we observe roughly 400-700nm there will be quite a bit of overlap in those gaps - as 400 is very close to half of 800. At those light levels - we don't really see colors and gaps in spikes will simply blend into one long spike. Add a bit of seeing to all of that to blur the things further and there you go - single nice uniform spike. If we want to know if there will be difference in brightness - we must look at energy distribution. More energy in spikes means higher brightness and more obvious and intrusive spikes. Sim shows that there is rather small difference - about 0.07%, or 7 parts per 10,000 - no way we will be able to distinguish that as we usually notice about 7-10% change in brightness (amount of light). I could also talk about dual slit experiment and Heisenberg uncertainty principle that I used as basis for intuitive conclusion that spider vane thickness has nothing to do with spike intensity - but don't want to further derail the thread. Anyway, that is my 2p worth on this topic.

Out of interest, I ran sim of thin vs thick spider vanes to see energy distribution and sim does not really support this position. I created two apertures - one with thin and one with thick vanes (thickness is enlarged by factor of x2) - can be thought of 1mm vs 5mm spider vane thickness comparison. CO is 25% Then I produced PSF of both systems and normalized it in total energy and then I measured encircled energy close to star. Reasoning being - more energy in star - less energy in spikes. Turns out that thick vanes produce more energy in same area around the star - which means that there is less energy in spikes themselves. It looks like thick spider vanes actually make spikes less pronounced. There is one thing that thick spider does - it "clumps" wavelength into more discrete chunks. This is monochromatic light so it is obvious - but for real light that is full spectrum - diffraction spikes will be solid. Thick spider vanes make them more "rainbow" like in images (clumps for different wavelengths are in different places): This was taken from image by @Allinthehead As far as I can tell - Tak 130 epsilon does not have very thin vanes:

You should really bin x2 your data in this combination. Natively this combination gives you 0.87"/px (oversampling) and binning it x2 will give you 1.74"/px - and that is rather nice working resolution for 120mm scope on most nights. There seems to be an issue with the scope: Either thermal management or serious spherical aberration. Cores of stars are visible - but so is extended skirt. This can happen if you combine subs that are in focus and provide sharp core and those that are out of focus. During the course of the evening as scope cools down - it contracts and focus changes. If you did not let the scope cool properly - it will produce such effect. It can also happen if there is rapid temperature drop and you don't refocus. Check first and last subs to see if focus is changing between them. If not - then your scope might have optical issue.

Exceptional image!

In that case - I stand corrected, I believe those authors certainly did calculations and that their claims are correct. Actually - groove density has large / measurable impact on diffraction spikes. https://www.dpreview.com/forums/thread/4454760 If there is counteracting action - it is not enough to offset increase in diffraction due to longer edge Not sure that it will be visible although effect will be real. If we take spider to be 1mm wide and say 15mm deep and we take 1° of incident light angle (2° FOV) - that will make total thickness be cos(1°) * 1mm + sin(1°) * 15mm = ~1.2616338 mm. That makes increase in thickness of only 26%. Doubt that change will be visible to eye. We could make it more by using scope like 130PDS and very wide field eyepiece (up to 2° of incident angle) - but I'm afraid that coma will be much worse and will distort both star image and spikes. Maybe with use of Paracorr or similar visual coma corrector (one from ES?).

By that logic - bright star should change appearance of its spikes as it moves across the field of view, because spider thickness changes with incident angle. Take dob and let bright star drift across the field of view and observe if diffraction spikes change depending on position of the star in FOV. Spiders tend to be vertically thick to provide enough support: Another example would be Bahtinov mask - you can't make spikes brighter by making slots wider - but you can make spikes bigger by adding more grooves - length of edge will put more energy in spikes.

Probably. Crab is small object and you need larger telescope to properly image it. With 200mm lens and DSLR - it will be less than 100px wide in the image. Even with serious gear - you can't expect it to be much larger - at best around 350px However - you should be able to detect it without too much trouble with that setup - it is bright enough.

Indeed - one must make target look the same for comparison purposes. You can't really show AFOV on computer screen because actual field of view on computer screen will depend on how close to computer screen you are. In that sense - both views are relative but can be "normalized" to common thing - in this instance target size.

I'm not entirely sure what you plan on doing but that Baader FfC sounds like overkill for whatever you are planing to do. If you want to take snaps of what you are observing - how about simple micro stage clamp and camera with zoom lens (if you want to zoom in/out)? I think that you can leave clamp attached to scope as you observe and then just attach camera to it and snap the image. https://www.firstlightoptics.com/adapters/baader-microstage-clickstop-digital-camera-adapter.html After that, it is simple matching of eyepiece AFOV to camera lens min magnification. For example, if you want to use Canon 18-55mm with APS-C sensor size - you'll want 70° AFOV eyepiece. At 18mm setting that lens captures about 73° along diagonal.

Same FOV - but different AFOV and magnification. FOV represents how much sky will you see and how will object appear in relation to that background sky. It's a bit like watching the same movie on your smart phone and on large cinema screen. Main character will take same proportion of the scene - but view on your phone will be small and very narrow but one on cinema screen will be huge and span almost all of your field of view. Above compares how much of the scene you'll see and what the size of the main character will be in relation to the scene - but it does not show whether you are watching it on your smart phone or 100" plasma screen 6 feet away

Same thing really - you can let the star be slightly defocused - it won't affect performance (at least I think so - maybe they changed centroid algorithm lately?). In fact - seeing affects less centroid calculations if star is slightly defocused. On the other hand - when you are working with longer focal length - star image is going to be spread over more pixels any way so no need to do it for that reason (to cover more pixels - that is only with large pixels and short focal lengths).

No need to add spacer - 44mm is max distance before prism starts to act as aperture stop - if you can place OAG closer - just place it closer. Only thing that really needs to be closer to sensor would be filters and next thing can be OAG.

It's ok,j Just make sure you don't stop down light to your OAG by placing it far from primary sensor. Short FL scopes tend to be fast scopes and with fast scopes - you need to place pick off prism close to main sensor to get full amount of light. Say you image at F/5 and prism is 9mm wide - in order to "fit" F/5 beam into 9mm aperture - you need to place it 5 * 9 = 45mm away from the sensor (in reality - a bit closer to get at least 2-3mm full illumination). That is only issue with using OAG with fast scopes - everything else works fine. If you pay attention to above advice, then no - ASI120 will work perfectly fine at F/5. People guide with ST80 - which is F/5 400mm FL scope - that is very similar to your OAG setup To me - it's easier. No differential flexure and setup is lighter. It also gives better guiding results. Only issue is focusing OAG - but it's not hard and you also have to do it with guide scope - so no real advantage there. Once you dial in OAG / main sensor distance - focusing is done at the same time when focusing main camera - so nice bonus that you focus only one scope. Don't know really - to me, OAG is actually better and cheaper solution than guide scope, but some people still prefer small guide scopes.

Heat capacity of glass is higher than most metals and comparable to aluminum and magnesium and glass elements in eyepiece are often heavier than barrel - it's unlikely that metal will pose more problems than glass. Of course they do - triplets much more than doublets and aperture plays a part too. I guess people often say that because they use relatively small aperture size refractors - those cool down quickly compared to say similarly priced MCT of 6-7" of aperture

You can use this to quickly check for tilt or any issues - SharpCap and artificial star. Slew scope so that star image ends up in each corner and use focusing aid that measures FWHM. It should be roughly the same in each corner - if not, you have tilt.

I think that you are right - focuser seems to be the most likely candidate for tilt. Not sure if I can offer any sensible advice on how to adjust it as I don't have much experience in that department.

I've seen color fringing on a few occasions when this scope was used for imaging with OSC sensor. I was surprised on how bad it looked. It was so bad that I thought scope was mistaken for FPL-51 model. Don't know if this is general thing or those particular models were suffering for some reason. Also don't know what the performance is like when doing mono + filters. I'm sure that for visual it is very good and color free scope.

Well, it actually does not have that much OIII: OIII in this image is that faint bluish background glow - you have exactly the same signal in your image above. Btw, imaging in moon light is going to be damaging to your data regardless of the fact that you are using narrowband. Ha/SII usually feel the least impact by the moon - but that depends on filter bandwidth (3nm are better of than say 5nm or 7nm)

What seems to be the problem? OIII is faint and not very structured in the target you are imaging. You need to collect your data and stack it before you reveal the faint signal. If you are imaging at 1500mm of focal length - you are working on quite high sampling rate and you should bin at least x2 if not x3 all your data. Here is about an four hours worth of OIII data on Bubble nebula that I shot with ASI1600 at gain 139 and using 4 minute subs at 1624mm of focal length binned x2.

Just thought of it - what is your focuser like on that scope, could it have any play in draw tube? If your FF/FR is screwed in on both sides, I doubt that it is causing this - you need to check for anything that might move and focuser is next suspect.

Yes you would. Say you compare 60% and 85% QE sensor. That is 41% increase in quantum efficiency. That rates to 41% longer imaging time. If you can see the difference between 4h and ~ 5:40h of total imaging time - you'll see difference between 85% QE and 60% QE. (maybe not as much as you expect - but it would be there). Most important thing about this sensor is it's size. If you can provide it with fully illuminated and corrected circle, then it will be faster than other sensors. This is because you can pair it with bigger scope to get the same FOV as pairing smaller sensor with smaller scope. Larger scope of the same type will have larger aperture and will collect more light. Simple as that. Don't get this sensor if you plan to use it on same small scope - like 80-100mm unless you really want to do wide fields.