SharpCap Forums

Tim, Brian

Thanks for the input.

I came across a rille in Virtual Moon Atlas which is 1km wide. Marked on my image which shows the equipment having captured a 1km wide feature.


The night in question was my second session with the new Barlow. I have come to learn that any new piece of kit (Barlow, reducer, filter) is going to take some time and effort to get the best results. I live on top of the Pennine Hills where currents from east and west collide. My imaging is always going to be affected, with steady atmosphere being less common.

The session comprised 19 SER videos captured over approximately 30m. The % autostacked by Biggsky were
49 18 35 42 44 27 29 33 21 32 30 21 22 24 54 90 83 43 27
This indicates how poor the conditions were. On a good night I can have data such that 70 - 90% of frames are stacked.

Focusing was hard, with small craters appearing and disappearing randomly. It was a case of averaging out and hoping for the best.

I have noticed that pushing the histogram to 70% (with the Barlow) as opposed to 60% (without Barlow) seems to give better results with the 610nm filter. I still have work to do with the IR685 filter to work out the best settings.

About 1 year ago I had started looking at % to stack in Autostakkert and after some experiments had moved from stacking 10 - 20% to stacking all frames with > 50% quality. I was regularly stacking 80 -90% of frames, tested on data going back to 2019. BiggSky and the mathematical decisions made by MATLAB seem to support the 'stack more frames' strategy (also helps with deconvolution). I am now of the mind that 'stack 20%' = ' throw away 80% of data', if the data is good enough then stack it.

With the Barlow I think I will have to capture more mosaics and crop to get the best framing on objects. Working with 2000 frame captures, ~35s for 2000 frames, will ensure that I do not get caught out with the ever changing lunar illumination (particularly near the terminator). I always try to balance the histogram across the captures thus eliminating problems when stitching panels.

I have seen on Cloudy Nights that people with C11 SCT's can capture features that I am not able to. I can live with that. For me a larger scope would also require a mount upgrade - money I am not prepared to invest.

The last year has seen an improvement and consistency in my lunar capture/processing (biased view) and hope that continues.

Dave

Dave,

1 km, impressive! So there you go, you are resolving down to 0.55 arcsec at Dawe's Limit of your aperture:

R = 116 / D = 116 / (8 * 25.4) = 0.57 arcsec

Brian

1km = 0.621 miles.

Equipment costs: C8 SCT + AVX mount = £800, Skyris 236m = £100, 1.5x Barlow = £120, Baader 610nm filter = £65, JMI Motofocuser = £200. Total = £1285.

Hours spent learning - best not thought about!

Dave

Thanks Dave,

That's a really useful post - and now we can all profit from your hard won (and expensive) learning .

I do wonder with the Biggsky % stacking algorithm whether a high percentage stacked always means that they are all good? In some of my cases I think that rather it meant that they were all equally mediocre - but the principle you stated that deconvolution is often better fed on more data rather than a thin slice of just the best data is correct I think. I have noticed the same general tendency with using the Russell Cronan Blur Exterminator software for deepsky.

The detection of the rill was really interesting. It feeds a bit into a discussion that I have been having with planetary folk in Reading Astronomical Society, It was put to me that planetary imagers often detect features at a resolution better than The Dawes or Raleigh limits of their telescopes. I was a bit sceptical - thinking that there can be no information to be resolved back out from within the Airy disc - but the examples proffered were the Cassini division and indeed some rills on the moon. Thinking about it -- maybe it's not really a question so much of 'resolution' (i.e. the normal analysis of resolution relating to two close point source stars) so much much as merely 'detection' -- and maybe that does work when there is a big change in contrast -- i.e a dark thin line sitting within an otherwise bright continuous feature even if the width of the dark line is less than the diameter of an Airy disc?

Be that as it may -- they are great images you have there!

It's a good point about focusing that you raise and rather credible as the real hidden advantage of the Barlow - otherwise just getting the pixel size smaller should work equally as well (although with a penalty of smaller well capacity).

In fact optimising focus is usually my biggest concern and difficulty with lunar imaging. Recently I have taken to hopping to an adjacent star and working hard to get focus just so (depending of course on air steadiness) and then hopping back to the moon to image for half an hour --then worrying if the temp has dropped too much and hopping out to a star again etc..

Thanks Tim.

The C8/AVX plus camera if new would have added another £1200 to the total.

I need to try the Barlow with my 183C/M cameras (pixel size=2.4µm) to see how that combination works. If I use 1920x1200 I can achieve 35fps (full frame 5440x3648 leaves me at 9fps which is OK for mosaics).

For focusing in SharpCap, I zoom in to say 125% and use a crater with some feature on the floor to try for the best image. Once back to auto, I use 'Highlight Over Exposed', up the exposure until black patches are highlighted, then back off the exposure until there are no highlighted regions. Target exposure is ~5ms which might mean having to up the gain. It is all variable with different filters. With atmospheric turbulence, I have found 2000 frames seems to produce decent results at the cost of 4.5Gb per SER file captured.

Capturing several images of the same target in rapid succession can lead to some surprising variances in quality.

I never tried using a star for focus but with a star at infinity and the moon only 250,000 miles away I wonder if that is a good strategy?

Dave

Tim

I asked David Biggs about using a star for lunar focus. His reply was:

Regarding infinite focus distance, a 2000mm FL is at infinite focus at 2km. So a moon and a star are in the same focal position. I autofocus on a close star then slew back to the moon.

2000mm being the focal length of my C8 SCT (2032mm to be exact). One for me to try out.

There is no end to learning in this hobby ...

Dave

Your images are always incredible.
Keep them coming.
Frank

Thanks Frank.

Dave