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Need guidance

Filroden

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Another short clear window in the early evening let me practise set up and alignment of the new AVX mount again. I ran through my new alignment process, including calibrating the StarSense to the OTA. I then repeated the process and the handset reported a final PA accuracy of 30" in Dec and 3' in RA. However, (and with clouds rapidly moving over) I managed to take another sequence of 30, 60, 90, 120 and 180 second shots to test tracking. Here's the 180s (which is heavily affected by high clouds), which is consistent with the others but shows the trailing best:

NGC2239_H_180sec_2017-03-09_193100_1x1_-20C_fpos_5773_g300o50_0001.thumb.jpg.b72de33ed591a812a2ed86e81dd03f68.jpg

Q1. I roughly measure the trail to be 14 pixels, or 26.6" at my pixel scale, so 0.15" per second. If correct, my maximum unguided exposure would be 13 seconds to stay within my 1.9" pixel scale. Would a PA error of 3' give this sort of trailing?

Q2. I also noticed (or believed I noticed) that the mount seems to move when tightening the mount bolt. I think that this further tightens the accessory tray which pushes on the legs more. I noticed this when I calibrated the StarSense on Betelgeuse. Normally, my goto would then be bang on centre but when I slewed back to Betelgeuse it was a little way outside my crosshairs (using SGPro). I wonder if this is causing me problems and whether I need to attach the accessory tray at all?

Q3. I also use anti-vibration pads beneath the tripod feet. Could they also slip when adjusting the mount?

Q4. Whilst the length of trailing seemed proportionate to the exposure time, the direction was not always consistent, particularly in the shorter subs. I'm assuming this could be the affect of many things: PEC, wind, seeing. Is that right?

I did get reasonable 120s exposures the previous night, so I know it's possible. However, I'm quickly concluding that having tested unguided I now need to quickly move onto a guided set up (skipping over drift aligning though I probably should learn how!). I have a ZWO OAG and ZWO ASI224 so time to bite the next complexity bullet. As it's likely to be cloudy until Monday at the earliest I can start working on that now.

Spacing

I think I've just got my spacing right for the ASI1600MM-C.

IC0443_H_120sec_2017_03_08_202514_1x1_20C_fpos_5829_0001_c_mosaic.thumb.jpg.45fa105eb0038e619b47ed0f918aaeea.jpg

Which for my reference was 66mm for the Skywatcher field flattener plus an additional 1mm for the filter (1/3rd of the 3mm filter thickness on the Astrodon 3nm HA).

I achieved this spacing with:

IMG_1354.thumb.JPG.c05975250ff2d2165dcbd26c913d4e01.JPG

The OAG has a spacing of 16.5mm and comes with a M48 adaptor so I'm hoping I can just swap out the 9.0mm FLO M48 to M42 adaptor and the 7.5mm Baader T2 extension tube.

How good does a polar alignment need to be for guiding? Would it be happy with the sort of accuracy I was achieving above or would I need to drift align and improve it further? I'm hoping that a single StarSense alignment routine will get me close enough that guiding will take over (having also read that guiding seems to like an error in PA so that it only has to correct in one direction). Tune in next week for what I expect to be a frustrating first attempt :)




2 Comments


It might be better posting this in the beginners imaging section rather than in a blog.

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You're right. When I started writing this entry in my blog I'd intended just to talk about my latest experience but as I wrote it, it became a series of questions.

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