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OMG Dew or something else?


SteveBz

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Hi People,

So I managed to get out last night for the first night in a month.  Humidity 95% and water dripping off everything.  However, I was confident that my trusty 18W dew strap at the entrance to my OTA and the triple fan on the back of my Newtonian primary would do the job, both home-made, natch.  But here's what I found:

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Is it dew? I couldn't see anything.  What do you think I should do?

Regards,

Steve.

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6 minutes ago, Davey-T said:

Doesn't look like dew, maybe high level cloud not visible to the naked eye or light pollution.

Dave

But stays in the same place all night.  Even when I look at different places in the sky.

Edited by SteveBz
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Maybe light leak around primary - I guess you've already ruled that out. I got caught out once when I repositioned my laptop and screen light leaked into primary end of tube.

Edit -  Hmmmm. would vary then when pointed to different parts of sky somaybe not

 

Edited by Tommohawk
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42 minutes ago, Tommohawk said:

Well that looks like off centre vignetting - also the collimation is a bit off. So I'm just wondering if something has slipped slightly, and the banding effect in the calibrated image is caused by the flats no longer being correct?

 

I had just corrected the Dec guiding with a belt upgrade (only for the Dec), but the RA guiding was quite horrible last night, so I think what looks light collimation may be guiding error (ie stars stretched east west).  Is that what you are talking about?  Let me show you the first 2 frames of the night (calibrated) compared with about an hour later.

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Autosave004.thumb.jpg.8e673d00c9ae4b12ed6cc0d5a9c6305d.jpg

Edited by SteveBz
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I don't think the collimation issue is huge. It's just that if something has shifted the flats might not work properly... though I've assumed possibly incorrectly that you're using library flats.

Are you using library flats, or do you take new flats per session? 

If you're comparing lights through the session I'd suggest do this without flat calibration  because if the flats don't match it might make comparisons tricky.

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20 minutes ago, Tommohawk said:

I don't think the collimation issue is huge. It's just that if something has shifted the flats might not work properly... though I've assumed possibly incorrectly that you're using library flats.

Are you using library flats, or do you take new flats per session? 

If you're comparing lights through the session I'd suggest do this without flat calibration  because if the flats don't match it might make comparisons tricky.

Yes, library flats.

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20 minutes ago, Tommohawk said:

I don't think the collimation issue is huge. It's just that if something has shifted the flats might not work properly... though I've assumed possibly incorrectly that you're using library flats.

Are you using library flats, or do you take new flats per session? 

If you're comparing lights through the session I'd suggest do this without flat calibration  because if the flats don't match it might make comparisons tricky.

I also have an old DIY dew heater for an old viewfinder (50mm), made from resistors in a ladder.  I thought I might just cut it down to size for a secondary mirror dew heater.  As it's a ladder, I can just trim it and it'll still work.

Steve.

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Well dew on the secondary can certainly be a big issue, and I routinely use a low power heater. BTW keep the power low as possible. I got problems with odd star shapes with overheated secondary, and was only using a couple of watts.

But, looking at your uncalibrated sub, I'm 99% sure the odd effect you note in the calibrated sub isn't dew, (though you may have few also)

Well, 90% sure. Ok ...maybe 50/50 

 

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15 minutes ago, Tommohawk said:

Well dew on the secondary can certainly be a big issue, and I routinely use a low power heater. BTW keep the power low as possible. I got problems with odd star shapes with overheated secondary, and was only using a couple of watts.

But, looking at your uncalibrated sub, I'm 99% sure the odd effect you note in the calibrated sub isn't dew, (though you may have few also)

Well, 90% sure. Ok ...maybe 50/50 

 

OMG Dew or something else? - Imaging - Deep Sky - Stargazers Lounge  This is the one that makes me think it's dew.  Single frame completely calibrated 1) after 30 mins, 2) after 90 mins.

Steve.

Edited by SteveBz
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If you don't use a dew shield I'd start there - they're cheap and will also help reject stray light, so can't hurt! Reaching for heaters is the last resort imo. Secondary dewing is definitely possible, coma corrector is where I mostly pick up dew so a heater around the optical train can help...

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9 hours ago, Tommohawk said:

Well you may be right - certainly my newts pick up dew on the secondary really easily. Usually if I suspect dew I remove the camera and look down the focuser with a torch. 

Good idea.  I almost never take off the camera!  I have an aversion to it.  Really, I suppose I should make a mark to show where it goes!.

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8 hours ago, discardedastro said:

If you don't use a dew shield I'd start there - they're cheap and will also help reject stray light, so can't hurt! Reaching for heaters is the last resort imo. Secondary dewing is definitely possible, coma corrector is where I mostly pick up dew so a heater around the optical train can help...

I'd forgotten about the coma corrector. I was thinking it can't be the sensor because it heats up, but the coma corrector!  Hmm..

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57 minutes ago, SteveBz said:

Good idea.  I almost never take off the camera!  I have an aversion to it.  Really, I suppose I should make a mark to show where it goes!.

Well if you use library flats leaving the camera attached makes a lot of sense to be fair

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So this morning I've just put a secondary heater on, which immediately dried out the central mirror holder.  However, the primary was also sparkling with condensation! 

More to do I think.

Thanks for your help.

Steve.

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What I would like - given I have a light cover on the back of the primary that provides a pretty good seal - would be to mount a big pile of silica gel in the tube when stored so it's at least starting out dry. Storing outside, it doesn't get a good continuous dry period very often, and with foggy nights the air trapped in the tube after sessions is laden with moisture...

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