Focussing.....help!

[Marky1973]

Right, third time lucky I think - I've only done this once, but am hoping it will help all future stacks - and thought it would be useful for others who come across the same issue!

Found a post on Cloudy Nights with the same problem and the suggestion was turn of the hot and cold pixel correction in cosmetic section in DSS Stacking Settings. I guess I should have searched sooner, but I was convinced I was struggling with focus. But last night I used the focus aid in APT to get a best value of 0.64 and then the Bhatinov aid which said the focus was close, and then went for it - spent about an hour with PA and focus - the the point that, after 3 hours, Polaris was still just on the target arc in the polar scope....if only it hadn't been midnight and flippin' freezing, I might have played around with some longer subs.

Anyway, I was playing with 60s subs @ 1600ISO. I've stacked and processed (gently) in DSS, just to see how the image turns out. I am not sure I like the output from the 1600ISO images, so will probably go back to 800, but it was worth trying! 27 out of 30 images stacked - I lost one by the neighbours driving past the scope at an ungodly hour in their massive 4x4 (long story, but the front of the house is south facing, so I have to set up on the drive for Orion.....probably only a matter of time before the police come calling.....)

Anyway....

So, first image - stacked in DSS with Hot and Cold Pixel detection and removal on in the Cosmetic Tab. Remove hot pixels is also on under the Dark Tab

We get halos, which can easily be seen when zooming in.

The second image is exactly the same, but has the setting deselected in the Cosmetic Tab, but still selected in the Dark Tab

No Halos!!!

But what happens if we also turn the Hot Pixel setting off in the Dark Tab?

Well, not a lot on first glance, but it certainly didn't make it any worse.

Anyway, I still have a bit of learning to do with focussing, but did make some strides last night and now don't have to worry about the halos, so I can concentrate on taking some nice photos!

Thanks to everyone for their help!

Onwards and upwards!

[Marky1973]

Well, after posting the images they now all look the same when you click on them, the halos in the first image aren't that obvious.....but the original JPEGs on my computer clearly show they are there in image one and not in images 2 and 3.

I can't edit yet, so sorry for the useless images. I won't post more, but please take my word for it that they do disappear when the Hot/Cold pixel settings are adjusted!

Cheers

Mark

[frugal]

Well, after posting the images they now all look the same when you click on them

That is the problem with JPEG compression. Try cropping a smaller section and using PNG instead as it is a lossless compression format.

[Marky1973]

Yeah, that's the problem isn't it - you solve one problem and find another!  Thanks for the suggestion....I've gotta go do house stuff now, but will have a look later to see if I can post something more useful!

I just noticed you are local - did you manage to enjoy the clear skies last night Frugal?

[uhb1966]

It's about 50% of my scope and mount! ;-)

I have the beta version of GIMP which I think works with 16 bit images, but I may be wrong. I am slowly playing around with StarTools which does seem to give some better results, but I don't really know what I am doing half the time! Will try the Pixinsight trial as well sometime when I have a chance to watch some tutorials.

Sounds like I need to avoid DSS and stick to unprocessed images in StarTools. I believe I will be better off with a FITS file and telling StarTools that the image is Linear/Bayered and Non-White Stretched, if I leave it alone in DSS - my 1100d is unmodded.#

I'll have a play tonight!

Thanks everyone!

Gimp 2.9 runs with 16+32 bits and even floating point precision. It's still in dev. I can give you a link for a Mac binary.

[Marky1973]

Thanks UHB, I have 2.8 and 2.9, but I do find myself jumping between them as some aspects are more friendly on one version that the other - either that I I'm a numpty still getting used to it all! :-)

[uhb1966]

I have them both, too, and an old 2.5.9 for those special moments :-)

[jefrs]

My laser alignment tool arrived yesterday and it shocked me as to how far the collimation was out on our 130SLT.

Fiddly little job done but images are so much sharper now.

I only got to test it in daylight because last night we got big heavy clouds blotting out everything. Daylight however was tack sharp whereas it had been washed out with halos.

[Roy Challen]

Frugal - Yeah, I have been locking - although you wouldn't believe how long it took me to figure out what the similar knob was for on my first scope when I got it last year....at least I was prepared with the refractor! :-) The mechanism on the 70ED seems pretty solid, but I'm not taking any chances - however, the added weight of a cat on the end (even with the locking nut tight) probably didn't help!

Hmm strange, I have the same scope and although I do lock the focus, I haven't found it strictly necessary even with a 60D with battery pack attached - unless pointed straight up - and it doesn't feel overly stiff when unlocked.

[ollypenrice]

A few focus related things... Firstly don't use a bright star. Use quite a faint one. The FWHM calculation fails when it can't use a full bell curve because the star is saturating the chip. This gives the bottom of a bell curve with the top chopped off flat. You can usually read the brightness of the star and it must be below your camera's ceiling, whatever that is. With CCDs it's usually 65000. Secondly try longer subs or use integration as suggested above. This averages out the seeing for a more stable reading. Thirdly, if it still bounces around wildly it's useless. Don't use it. It never worked for us on our biggest scope and we went Bahtinov on that. On the smaller ones it's fine and we use it all the time.

Also I've never know a focus lock that didn't affect focus, including Feathertouch and Takahashi. Fortunately neither of these needs a lock. The trick is to use it sparingly and keep it in a little tension while focusing, then tighten it but keep checking.

As others have said, focus and guiding are the be-all and end-all of capture. There's not much else involved.

Olly

[frugal]

I just noticed you are local - did you manage to enjoy the clear skies last night Frugal?

You must be in a different part of Gloucestershire than me last night It was cloudy or mostly cloudy here untill at least midnight. Fortunately I had a delivery of woodworking tools to keep me occupied.

[Marky1973]

Roy, the Focus lock doesn't feel essential with the scope, I would agree - my main issue the first time around was the pesky cat that yanked on the cables - I suspect the lock wouldn't have helped in that situation! I do also worry, as suggested by Olly, that just tightening the knob will send the focus off. 

Practice Practice Practice!

Yeah, I'm near the centre Frugal - nice clear night last night and another one forecast tonight... but I'm stuck with a cold and throat infection and really don't feel like making any effort with anything....

Even typing this post was t

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