almcl

1 hour ago, Shibby said:

I believe 16 is the "magic" number, so stacking 20 should work OK with a sigma clip stack.

Unfortunately that's not been my experience.  I have had stacks of 25 in which DSS failed miserably to remove hot pixels, despite quite large dithers every frame and just the other night it failed to Sigma clip a single satellite trail from a stack of 70.

Of course, other stacking software may perform differently...

They look like hot pixels.  

If you are not tracking, the stars progressively move across the frame and when each frame is aligned the hot pixel appears to move.

You might be able to eliminate them with a Kappa-Sigma clip stacking algorithm, but if you're using DSS you will probably need a good deal more than 20 frames for this to be effective.

Is there a reason you are using GPINT as your mount type rather than  EQMOD ASCOM HEQ5/6 ?

Thanks all for the thoughts, most helpful!

1 hour ago, DaveS said:

The "Good to Stargaze" app has transparency and seeing. How accurate they are may be debatable.

I see what you mean DaveS,  at the moment we have milky haze up to 45° above the horizon but it still says 'good to stargaze'.

1 hour ago, Second Time Around said:

Then I use Zoom Earth that shows infrared satellite pictures for night time cloud.  It doesn't make forecasts but shows the actual sky at 15 minute intervals and going back some time.  This has been accurate all week.

Go to https://zoom.earth/places/united-kingdom/

Thanks for that  link 2ndTime, hadn't seen that one before although I have been following your record of forecasts, which has been interesting. I'll have to give that a bit more study.

1 hour ago, Elp said:

Metoffice detailed map view is generally reasonably good, you have to manually change the hours on the slider and it will update accordingly, allowing it to play automatically doesn't give you the time to really scrutinise it.... It's a bit hard to notice as the grey blends into the green land masses but the misty cloud is visible. 

Is that the UK observations map? with the satellite visible and IR options ? (I find their info is pretty good but the website is a bit of a nightmare to navigate.)  

The last few nights have been clear according to all the forecasting apps I usually use, but the transparency of the air has been rubbish in my Bortle 6/7 corner of Shropshire.  A milky haze suffused the sky and even bright stars were barely visible.  My previously ultra reliable OAG, which has never had any difficulty picking twelve guide stars, struggled to provide even one.

Convinced that this was down to some sort of haze/cloud even though the Met Office, Meteo Blue and Clear Outside were all insisting the sky was clear, (the realtime hi-res infrared image on Meteoradar does shows a hint of haze, but is difficult to interpret over land) I started searching for an atmospheric transparency predictor, and while I found one for North America, (www.astropheric.com) I can't find anything similar for UK/Europe.

Does anybody know of one?

2 minutes ago, Catanonia said:

Need to check again, thank you

it's a while since I did it, but I think you have to down load the minor planet data,  in the solar system editor in plug-in, but I use very old version of Stellarium (18.2) and it may be different in more recent ones.

1 minute ago, Catanonia said:

Can I ask, where did you find this information.

You can find it in Stellarium :

11 hours ago, johneta said:

... that one done with your SW200P in your gear list?

Yes, indeed, with the Canon 700d, luckily (and unusually) I got more than one clear night. 

I guess things are similar on South Island although perhaps the Otago peninsula (if you're near there) doesn't suffer as much light pollution?

One thing I found with ICE is that if it won't link all the images at the first attempt, linking just two or three and, after re-naming them so that they're in the right order, joining the new images together often worked.

My 6 panel mosaic of the Rosette nebula was constructed by this method.

The effect is sometimes referred to as the keystone effect and is often noticed when tilting a camera to get tall buildings in frame (lots of web articles on the subject). 

Not sure of the best way to counter it in Photoshop as I don't use it, but there should be 'straighten and crop' function somewhere?

Rather than Hugin, if you can find a copy, Micosoft's ICE does a better (and quicker) job, particularly with straightening things.

Here's what my image editing software did to your image:

The tree on the left still isn't quite vertical though.

Interesting, thanks. I'll have to investigate that a bit further? 

How do you match the different flats from the various sessions to their respective lights?

ASTAP is another free alternative.  It doesn't do multi session stacking properly, unlike DSS and it is mind numbingly slow (over an hour to process an image that DSS can handle in 5 minutes) but the results otherwise are OK.

If you find Siril  too bad (and I hate script and CLI based software) then Sirilic gets round some of the problems, although it spends ages copying files from one directory to another and then then doesn't tidy up afterwards.

I cooled my Canon 700.  Not being brave enough to cut into the case for a cold finger, I applied a Peltier Cooler, heatsink and fan to the back of the camera in the space where the fold out screen sits.  

Although the results were not spectacular in terms of absolute cooling - it reduced the reported EXIF temp by 10 - 12°C, it did produce stable temperatures.  Unlike the uncooled sessions, all the lights were within a degree of each other and, possibly more importantly, it was possible to take darks at the same temperature, something I had long given up on before clamping the arrangement to the back of the camera.

What does PHD2 report the oscillations as? 

Your AZEQ6 mount is capable of guiding at around .6" (or at least my 6 year old one is) and you are imaging at 2.15 " per px so even in moderate seeing should produce OK stars?

Unlikely to be dithering, as that should be taking place when the imaging camera is not taking pictures and also should be in random directions.  

Not sure what your set up is but if the RA and Dec oscillations are similar and you are using a guidescope  it could be flexure. 

Hard to tell from the x4 reduced pngs, but the Rosette doesn't look as bad as the Heart?

3 hours ago, Acrab67 said:

Is the one you use the new model that comes with the rubber base?

Mine arrived in July last year and had no rubber base. 

I didn't realise you were visual; never used mine for visual, only imaging.

+1 for the Skywatcher model.

 I use it on my AZEQ6 with the SW 190MN which is quite a heavy beast and haven't noticed any stability problems.  It does need the central clamps to be done up really tight to avoid disturbing the PA when slewing at speed, but have had several OAG guided sessions with PHD2 reporting average oscillation at 0.5" or just below.

If you are using Deep Sky Stacker you may need rather more lights (25+) for the Sigma Clip stacking algorithm to work.  

If you are guiding (not sure from your equipment list) dithering may also help, although again you need quite a few lights otherwise you get a cloud of coloured dots rather than a line.

6 hours ago, The Lazy Astronomer said:

There's nothing fundamentally wrong with DSS, it's easy to use, but it is essentially adandonware, having not been updated in I-don't-know-how-long.

That's not quite the case.  DSS is still being developed, the latest edition is 4.2.6 and this was published in May 2021. 

Dave Partridge who posts here as Perdrix is maintaining the code and there is an active discussion group https://groups.io/g/DeepSkyStacker/topics

The detail in the core is just fabulous, Rodd and the strong star forming regions, emphasised by the Ha, really pop.

Wonderful image as is.   

You may be able to get them to stack by going to the reference frame (the one with the highest score) and manually deselecting all the hot pixels and non-stars using the star select tool.

I tried this on your screen shot jpeg (never likely to be satisfactory) and got a fair number:

All shot with ASI 2600MC and Skywatcher 190MN

Bubble Nebula, NGC7635

M81

NGC 7538

M106 and friends

Ghost, Sh2-185

29 minutes ago, Stuart1971 said:

That is a lot of drift for 60 second exposures even at that FL, so yes you PA is not good at all

I don't think you can draw that conclusion, the drift is from 80 exposures each of 60 seconds, or 4800 seconds (80 minutes) in total and, until we know the direction of drift, it isn't possible to say if it's polar alignment, incorrect tracking speed or something else. 

I think that to answer the original question we need to know what direction the drift is in.  Is it along the RA axis, the Dec, or some of both? 

Plate solving a single sub should provide this information, but a trailed one is a bit too difficult.