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Alexis

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  1. Alexis
    No reason to buy the 40mm. If you had an SCT or Mak and wanted to confine yourself to 1.25" eyepieces, then it would make more sense to add one, but you're not in that target audience.
  2. Alexis
    Well, I only agree because for me it's too late ;-).
  3. Alexis
    See? It's faster for me to ask you ;-). Thanks for the illustration.
  4. Alexis
    Agreed. I haven't been paying enough attention to what sensor was used here. Edit: yeah, for a cooled ASI2600 colour I doubt it's that relevant. Here, though, readout noise might be relevant, no, since we're just looking at one sub? Or is it low enough for it to be irrelevant given the signal in both the filtered and unfiltered image? [I'm sorry if the question sounds stupid, I don't know the particulars of the different sensors that well, and you're much better at this than I am 😉 .]
  5. Alexis
    That depends on the source of the noise and what dominates. If sensor-induced noise dominates more when you filter because you suppressed more signal, you might see better performance with longer exposures. Of course if there is readout noise in the sensor you still need enough exposures to get rid of that as well so for a given total integration you need to keep the amount of subs high enough as well, so it's not a simple subject. Given it's not simple I like experiments -- the proof of the pudding etc. We're totally in agreement about shot noise in the picture, of course.
  6. Alexis
    That's already a pretty good rage, to be honest. I'd be looking at a wider eyepiece to max out the field in 1.25" -- which one would depend on which Celestron 114eq this is. If this is a 114/900, a 32mm Plössl, but on a faster scope (e.g. a 114/500 or 114/450) I'd prefer a 22-24mm 68° eyepiece (if you can spring for one). Otherwise, I'd tend to replace the eyepieces you use often but find uncomfortable with flavours with a bit more field and eye relief (I find 6mm Plössls a bit harrowing; only my Pentax XP with less than one mm of eye relief is more challenging).
  7. Alexis
    I don't only try, I use it regularly (the advantage of having a filter slide is that it's farily easy to just try -- you have little to lose). It replaced an Omega Optical GCE and is a lot better. See above -- effectiveness varies with the objects. Granted it also depends a lot on the sources of light pollution -- it's spectacularly ineffective against metal halide/mercury vapour (we can "test" that at one of my darkish sites at less than an hour's drive: on Saturday the neighbouring village has a football match; luckily we stay up a lot longer than the duration of the match.) but quite good against the ubiquitous sodium street lighting which is really everywhere in Belgium. Which is why I was slightly suprised at the lack of effectiveness in photography. Although in our skies it might still help more than in this example -- when they use a fast scope and expose 120s, most people here don't get an image that is as neutral as the posted examples, but a nice HP sodium glow. It'd also be interesing to see if in _this_ example the SNR would still be worse if the exposure for one light frame was longer (at least with this sensor).
  8. Alexis
    Correct, and I've used it as such as well (also to yield much less than 2x -- if you screw it onto a 2"-1.25" adapter it's a very convenient 1.5x-ish barlow for most 1.25" eyepieces.) And it has filter threads for 2" filters too, which sadly the BarAdv does not have (and having to screw filters in between the BarAdv and eyepieces is a pain in the behind, certainly given the T2 thread of the BarAdv that sticks out means you need a 2" extension to make it filter-safe).
  9. Alexis
    Agreed, and it does explain something important for a photosensor that is less important for me when I use it visually (I'd be hard pressed to see anything above 610nm with night vision) -- but you're right that even I see less improvement for yellowish objects like the M81 arms than some other stuff, even visually. But you also saw a lot less signal for green (1.9x), which for these objects is quite important, and that's more puzzling. The red I expect to want to see most here is the H-alpha in M82, but that shouldn't suffer too much (and it takes quite a bit of stacking to pull it out of the noise anyway). Not doubting either the images nor the data that comes out of them (if you just try to adjust levels on the two FITs it's quite obvious to the eye as well that your data is correct) -- just trying my best to understand the causes.
  10. Alexis
    If you get a 2" BarAdv from A-P that fixes itself -- you can install it as a screw-on barlow that disappears into the focuser and makes the combination more focuser-friendly. you do have to rack out the focuser somewhat more, though, so you lose part of what you gained that way too (depending on how good the focuser is).
  11. Alexis
  12. Alexis
    Thanks for the welcome -- but if you look at the filter specs (and I just looked at it through a pocket spectrometer as well -- that matches) you wouldn't expect a drop of signal of a factor 2. All that being said, even visually there's a striking difference in how effective it is on different targets. NGC891 -- not much. M81 -- also not much. M101 -- a lot more. M51 -- even more. NGC 253 -- also quite a lot (but then that's down in the soup where I live). Globulars are quite a bit better in my really polluted backyard too.
  13. Alexis
    That's (given the filter) what I'd only expect if a large portion of the light pollution were mercury vapour (which the D3 purposefully ignores). But given the spectrum of M81 and M82, I'm baffled by the loss of the signal's amplitude too, though. I'm not that suprised the results are so-so given the target: the arms of M81 are faint and yellowish, and even visually (I do use a D3 visually) it's one of the objects that doesn't respond well to any filter.
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