Thoughts on which imaging rigs to concentrate on

[Gina]

I'm optimistic for tonight if the clouds part   I have processed last night's 7 subs of 120s and 45 subs of 150s in PixInsight using an approximate set of darks but no biases or flats.  I then saved the image as 16bit unsigned TIFF and transferred it to my Win7 desktop with Photoshop CS5 in which I applied histogram stretch and cropping - here is the result.  These are full resolution - pixel for pixel, though first one is zoomed in to a smaller area.

Edited September 17, 2016 by Gina

[Gina]

The 150s subs were a bit too much for the PA and show a bit of the oval star syndrome so I'm going for 120s subs tonight with full calibration sets if the sky clears.  I already have 100 bias frames and 33 dark frames.  I might stop the darks if the sky clears before they're finished.

[Gina]

Sky cleared after the 66 darks count so I went over to Lights.  Removed the scope cover and set up on the Crescent, putting it near the corner of the frame so that the Soap Bubble comes near the centre.  Found the focus had changed so needed to adjust that.  Now running 120s lights with g500 and t-26°C.

There are bands of cloud passing over so a lot of the subs are poor or unusable but I'll leave it running and see what I get.  No problem from frosting of the sensor now nor condensation so the new desiccant tablets seem to be working.

Here the latest light frame seems to be in a clear patch of sky   Full frame, histogram stretched.

[RichLD]

Glad you got the icing problem sorted Gina 

[Gina]

Thanks Rich   Wish the clouds were as easy

[Gina]

About half of the lights so far are either totally duff or pretty poor.  About 15 out of 35.

[wimvb]

Great results. A clear soap bubble in those short subs. (Referring to your earlier image.)

As for stretching, you could try PI masked stretch followed by histogram transform and round off with exponential transformation + curvetransform. These tools should help you with faint nebulosity.

Edited September 17, 2016 by wimvb

[Gina]

Thank you Wim   I'll look into those.

[Gina]

Overnight I captured 166 Ha subs of the Crescent and Soap Bubble some of which are fine, others with no DSO at all and some mediocre ones.  Makes sense to cut out the totally duff ones but what about the mediocre ones?  These show a full range from almost fine to just a little of the Crescent showing.  So should I let PixInsight decide what to do or should I arbitrarily dump the poorest frames?

I should add that to go through 166 frames looking at each individually is a long and tedious job, of course, so what I really want to know id how good is PI at rejecting poor frames?  Maybe I should repeat this question ing the software section - PixInsight thread...

Edited September 18, 2016 by Gina

[Gina]

No reply to PixInsight question so I decided to press ahead with the whole 166 lights and see what PI did with them.  During registration it rejected 29 of the 166 leaving 137 to stack/integrate, which it is now doing.

Edited September 18, 2016 by Gina

[Gina]

Had a reply in the software forum with a recommendation of which PixInsight processes to run for this.

The result from running the BPP is decidedly poor - much worse than my previous run - so I does need poorer lights removing from the stacking.  This could be done manually or using some tools in PI to measure the quality of each light sub.  PI seems very powerful but so much to learn - which I knew   Far more powerful the Photoshop.

[Gina]

Looking at individual subs I can see one problem - the focus has changed during the run of several hours.  I guess this is due to ambient temperature change or maybe due to flexing in the rig.  Once I sort out some adapters to use the SX mini FW it might be better.  Also, I could avoid the drift and field rotation due to less than perfect PA.  During the 5 hour run the view of the sky moved by about 400px in X and 250px in Y or about 10% of the frame size.  Rotation was 0.18° according to PixInsight.  Individual frames only showed very slight oval star shape.  It might be possible to improve PA a bit more with drift alignment.

[Gina]

I've been given a link to a tutorial on using a tool in PixInsight that will check each sub and work out how good it is and give it a rating and hence a weighting to control how much it contributes to the final result.  I shall be applying that to the subs I have and see if I can get a better result. 

In previous imaging runs with this new camera all the lights were of similar quality and I just used the lot with equal weighting but this last lot were with varying cloud.  Of course, I could  take the view that the latest imaging run was a duffer due to periods of light or heavy cloud and wait for the next clear night sky, but to be able to make use of nights with intermittent cloud could add to the number of viable imaging nights.

Edited September 18, 2016 by Gina

[Gina]

I've been carefully examining the optical path components used the other night for the Crescent and Soap Bubble viz. Esprit scope, OVL OAG, EFW2 and ASI1600MM-Cool camera.  Also, comparison of the OVL OAG and the one I bought from ebay a few days ago.  The OVL OAG finger screws that hold the two parts together don't hold them very accurately and there had been movement.  This would explain the change in focus. 

The new OAG although very similar is slightly different.  Instead of notches in the slotted part the new one has tapered sides and tightening the screws pulls the parts together such that a flange on the slotted part rests up against the other part, fixing both alignment and focus.  Changing to the new OAG should improve matters. 

I was expecting an adapter and a couple of other bits from FLO by now but they seem to be delayed in the post.  The weather forecast for the next five days is dire and unless it changes there will be no chance of any imaging in the foreseeable future so there's no hurry except that I want to sort this out ASAP

[Gina]

Dumped some bad frames from last imaging run and got this - cropped to Soap Bubble area.

 

[Gina]

Now used Blink in PixInsight to reject yet more poor frames, copied good ones to another folder and applied StarAlignment plus ImageIntegration.   Result coming up...

Edited September 19, 2016 by Gina

[Gina]

Here's the full frame uncropped.  This was using 93 of the 166 frames captured.  In spite of dumping all the bad frames this result is still poorer than the previous run with mainly 150s subs.  The sky was definitely not as clear and there was the focus shift.  I may try combining both runs but really it wants a new run with the gear fixed!

 

[Gina]

With the new OAG + EFW2 the measurement from field flattener to camera front is 50.5mm so the back focus is 50.5 + 6.5 = 57mm.  This compares with the specified 54.9mm so that's only about 2mm too much.

[Gina]

I don't think this is the best rig to capture the Soap Bubble - the MN190 would be better.  2.5x the focal length of the Esprit.

Here is the above image cropped to a rectangle 2.5 times smaller, showing the coverage of the MN190 with ASI1600MM-Cool camera.

[Gina]

The MN190 would also be the one to use for the Crescent Nebula.  I would put the DSO in the centre of the frame though - of course   The limiting factor will be the seeing though as the resolution would be around half an arc second or less.  This might be partly alleviated by using short exposures perhaps but whether it will be possible to reduce exposures enough is another matter!

Edited September 19, 2016 by Gina

[wimvb]

That's still quite impressive, considering that big bright blob that's high in the sky most nights.

As Chris noted earlier, it may be worth it to wait for the moon to play nice.

BTW, if you want to play around with some noise reduction, here's a PI recipe that works really well on small scale noise:

 

Create a clone of your (linear) image and apply the STF stretch permanently using histogram transform.

Make a preview in your linear image, that only contains background. Read the std deviation in the statistics tool (under Process | Image | Statistics)

Open TGVDenoise

Check CIE L*a*b mode

uncheck the apply checkbox for Chrominance

Go to the Lightness tab

Set Strength to 1

Set Edge Protection to 1/10 of the std deviation you read off the statistics

Leave Smoothness as is

Set Iterations to 300 to start with

Check Local Support. Expand the panel and choose the stretched clone of your image as support image

Before applying to your image:

Create a preview around the soap bubble and apply STF to make it visible

Apply the TGVDenoise tool to this preview

Make small adjustments to the edge protection parameter, until you get the noise reduction you want without losing detail.

When satisfied, crank up iterations to 500 or more. Apply to the whole image, and get a cup of tea while the tool does it's magic.

Here's a reference to where I got it from

http://wimvberlo.blogspot.se/2016/07/noise-reduction-for-dslr-astroimages.html

Cheers,

[Gina]

Thanks Wim - I'll give it a go

[Gina]

I think I have pretty much proved that I can capture the Soap Bubble  - it just remains to refine everything to improve the image.  If we get any clear night sky in the next week I think I shall try something else that can make use of the 3nm Ha filter (the only thing viable with a bright moon out) such as a yet wider FOV than the 135mm f2.5 lens provides, by which I mean the 55mm f1.8 lens.  I might ditch the FW and put the 3nm Ha filter right on the camera with the adapter provided.  That will prevent the vignetting that I would probably get with FW and such a wide aperture.

There are so many things I want to try with the new camera that I don't want to spend too much time on each one at the moment.  Once I find out what this camera can do with the various FOV options I shall get down to some serious imaging runs.

Edited September 19, 2016 by Gina

[wimvb]

2 minutes ago, Gina said:

I think I have pretty much proved that I can capture the Soap Bubble  - it just remains to refine everything to improve the image.  If we get any clear night sky in the next week I think I shall try something else that can make use of the 3nm Ha filter (the only thing viable with a bright moon out) such as a yet wider FOV than the 135mm f2.5 lens provides, by which I mean the 55mm f1.8 lens.  I might ditch the FW and put the 3nm Ha filter right on the camera with the adapter provided.  That will prevent the vignetting that I would probably get with FW and such a wide aperture.

You certainly have. I think that this definitely proves that ZWO are entering the DSO/CCD arena with their latest model. Now all that remains is for us to wait for a full frame model that doesn't break the bank.

Thanks for sharing your experiences, Gina.

[Gina]

Thank you Wim - your advise has been invaluable   And yes, I think it proves the viability of using the ZWO camera with a half decent refractor.