rotatux

Yes I'm often (wheather permitting) using a 130PDS on a Nexstar SLT mount, an alt-az one just to be clear. It's perfectly usable for visual, and a bit limited for astrophoto: My copy of this mount is a bit erratic and only allows 20s subs most of the time, 30s up to 40s when lucky. Other copies or mounts may be better, YMMV You can turn the tube in the rings to place the focuser straight upside, to line up the field of view to be more natural (up/down and left/right wise) for Alt-Az and frac/cass users. Only drawback is the vixen plate screws forbid the direct insertion of the plate in the mount: the plate must slide in the mount so one of the screws blocks the operation. I simply unscrew the front side of the plate and gently insert it (with the tube weight on my hand) before re-screwing it, trying not to change the plate alignment. If you need more info about this config just ask (I thought a few others would be using the same, but apparently not the case). EDIT: after reading the whole thread you (OP) appear to not have the same mount (and potential problems) as me; Your mount problably has better tracking than mine, so keep hope. However keep in mind the tube length of a 130PDS will prevent going to zenith or high altitudes, just like me, unless you use a long vixen plate and use it to offset the tube in the mount (with additional counterweights to compensate).

My camera can't disable that auto-clean feature. And, the dust on my sensor as I checked it seems totally opaque, so I don't think flats would take care of it (or badly); "Capture dithering" in my setup would be more complicated than just a power cycle every now and then. I'm half surprised that flats work so well with others, but seeing some flats shown here I saw most dust is transparent contrary to mine (because of my smaller pixels ?). That moving dust might also explain why I failed to make working flats so far :-/ but living well without them as you see :)

Some recently processed-during-rain images taken from february (yes I take my time :) ) First is my first acceptable horse head and flame ... at least I think so, since previous attempts didn't have enough signal. An EQ mount changes the game (coming from Alt-Az) by allowing to regularly break the 30s barrier. No darks or flats so it has some dust mites which didn't show with alt-az (because that latter moves erratically), so I should remember to regularly switch off/on the camera to let the auto-cleaning supersonic waves to move them. Second is my first HDR, with M42 as classic subject. Came out nicely out of a relatively short exposure, and /me/ being still junior and maybe underequipped at processing. Both captured with Olympus E-PL6 on 130PDS with SW CC and dydimium filter on Omegon EQ-300 tracking RA from a sky 50km from Paris, France (Bortle ~ 4), processed with Regim 3.4 and Fotoxx 12.01+. What do you think ? Exposure: 12 × 60s × 2500iso. Exposure: 19 × 10s × 2000iso + 10 × 30s × 3200iso + 6 × 60s × 2500iso (total 14mn)

A bit deceiving: the title says "unguided" which seems to suit well with 30s, and while unguided ≠ stacked nothing in the description would forbid 30s unguided stacks, except there's that damn parenthesis to clarify and remove most hope...

If you have it on a motorized (goto) alt-az mount, that's also fine for imaging within some constraints (and techniques to learn) -- as this thread proves. But depending on how you travel and how much space and weight you can afford this could prove too much, in which case the LX2 or self-made "barndoor trackers" are good and lightweight solutions (but limited in precision and hence focal range).

It's a nice image with regular stars on the whole field, but I can see the diffraction pattern of a closed diaphragm on bright stars... so are you sure it was wide open ? Anyway my collection only grew from trying several lenses to find an astro-correct one, and I will sell most of it ASAP but no Zeiss yet :)

My copy of the 200 (same "bokeh monster" variant as yours) is very soft at F/4, I must close nearly at F/5.6 to get decently shaped stars (esp. on borders and corners). This is why I ended getting another 200 (Olympus OM) which is fine wide open at F/4 (though it suffers another aberration). About 135mm I've got many, 2 M42/Pentacon F/2.8 (long and short) that are both good at F/4 (the long variant is a bit better), and 2 F/3.5 from Minolta and Olympus which I barely tested but need to be closed at F/5.6 (Minolta much better). Having to close is normal I think, the shortest the focal the bigger aberrations need to be corrected by closing; Only long focals would be kept wide open, at 200/4 you can find many good ones (don't know at 200/3.5), at 135 I didn't encounter any yet (but maybe some F/2.8 from Minolta, Takumar or Pentax) -- though if you have budget there's the Samyang 135/2 which is a wonder from what I have read everywhere.

Nice start. I've got the same 200, just wondering what aperture you used, is it full open (f/4) ?

Use an algorithm which selects best samples before combining. Such as those with "sigma clipping" or "range clipping" (just a wild guess, I'm not a DSS user). They will make a *huge* difference. When you have bad data (does not seem your case), use more selective parameters. From your number of frames, best is probably "average sigma clipping" because with a high number of frames, averaging statistically recovers more signal depth than median. Median is best with low number of frames because it converges faster (than average) to its target value, at the price of resulting precision (it would recover at most 1 bit of signal depth, when average could recover many more). Once you use the right algorithm, only way is more data, as Ken said. Most post-processing algorithms (such as median and other linear and non linear filtering) will only smear out your image, loosing detail; Some however will help, I personnally use wavelet denoising, but I'm too young on the subject to give advice.

That Sigma APO seems to fare surprisingly good ! (I like your Orion very much) How much did you have to close the lens' aperture ? or was it full open at 5.6 ? Seems only the DG version is available new on the market... did you hear how it compares (optically speaking) to your D version ?

Speaking of results, here's what I could catch during my last september holidays in 3 nights — Yes I'm the multi-subject-per-night kind of man :-P. Kind of tribute to this marvelous photon hoover (details on separate pages in my gallery) Night 1: details M31 Night 2: details M8 Laguna details M20 Trifid details M17 Omega details M16 Eagle details M33 Triangulum Night 3: details M45 Pleiades details Gamma Cas Unfortunately no clear night during christmas holidays -- apart the one before I leaved PS: sorry if layout is messy, editor/preview was a bit hard on me...

In fact I have seen that cropping also zooms in the remainging image, and so magnifies inner star trails to the point the whole image is mostly unusable. So you may be better off keeping exposure limited to a smaller value and take+process more subs. It's actually a combination of altitude and azimuth, but yes globally more alt = faster alt-az-relative rotation = less exposure. Rotation starts at a given speed low on horizon then raises to a common maximum speed (lowest possible exposures) at zenith. Azimuths East and West give the lowest rotation speed on horizon, while South and North are higher. There's a PDF with graphs depending on your latitude somewhere in the thread... PS: I don't have it at hand, but it should be pinned somewhere as the question comes in frequently.