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Autofocus, poor transparency and thin cloud cover?

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#1 Jon Rista

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Posted 14 June 2016 - 11:01 PM

I have a question for those who use SGP and autofocus. I started to tune my AF offsets for my filters the last "clear" night I had, as there is definitely some discrepancy between L and the NB filters. I think it might in part be due to the TSRCFlat2 I have in my imaging train, but not certain. Anyway, I set offsets for Ha and SII, then the clouds came in last time. 

 

I went to start configuring the offsets for OIII, and first decided to lock in focus with L. Since I had it preset from before, I figured it would simply be a matter of setting the L filter and clicking Focus for L. When I did that...my stars were well out of focus. They showed up as scintillating rings rather than points of light.

 

When I was setting my offsets last time, there was no doubt that the stars were in focus, I could clearly see it, and as I checked and rechecked and adjusted and fiddled, as it was the first time I'd used AF (and it's AWESOME...I don't think I'll ever be able to live without it now! And I've only used it twice! :p) So I'm racking my brain trying to figure out how focus could have shifted so much since the last time I configured the offsets. 

 

That night was poor in transparency. That's a common problem here in Colorado, we have rather thick inversion layers a lot of the time, and even if there are no clouds, it traps a lot of moisture. Is it possible that such an inversion layer messed with my focus? The discrepancy seemed to be quite large (I have my focuser set to a too-small step size, I believe, so the discrepancy in steps was around 320...I'll need to remedy this at some point, but not tonight.) Even visually, the stars went from pretty decent points from my focusing on the moon, to obviously out of focus. 

 

Anyway, I am resetting my offsets now, but I'm worried that I may have to update them again if the atmospheric conditions change...which is...a bit of a bummer. 



#2 Jerry Lodriguss

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Posted 14 June 2016 - 11:19 PM

Were you using the autofocus on a scope or your camera lens?

 

Do you have an absolute position focuser?

 

I seriously doubt an inversion layer is going to change your infinity focus.

 

But temperature changes in the scope can change the focus.

 

Jerry



#3 Jon Rista

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Posted 14 June 2016 - 11:21 PM

Oh, sorry. I am currently working with the AT8RC, Moonlite CSL, EFW2 with Astrodon LRGB G2E and Astrodon 3nm Ha, SII and OIII filters with the ASI1600.

 

I uncovered and let the scope start cooling down the moment the sun stopped hitting it, so it's been cooling down for hours. It's the CF scope. Outside temp is 66 tonight. It was 68 the other night. I would be surprised if a 2C difference in ambient temp would result in such a significant shift as well.

 

It's very odd...



#4 freestar8n

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Posted 14 June 2016 - 11:38 PM

For filter offsets with narrowband I use a bright star cluster that has many stars in it so the narrowband exposures aren't too long. I try to do all the filters as quickly as possible - and after going through them I go back to the original reference filter and make sure its value didn't change.

Make sure you are compensating for backlash, and the focuser is moving the right direction while taking the curve.

Also make sure there is no mechanical slop in the focuser. I once had a slightly loose set screw on the focuser knob and the focus performance was a disaster because it wasn't repeatable.

The offsets really shouldn't change for a given setup - but they could change after adding a reducer or something.

With my EdgeHD11 at f/10 the astrodon sloan and narrowband filters are very parfocal - so I don't need offsets. But with my c11 and reducer I did need offsets. So it does depend on the scope and its aberrations - and not just the filters.

Frank

#5 Jon Rista

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Posted 14 June 2016 - 11:49 PM

Thanks Frank.

 

So, apparently, my scope is still cooling down. It's been...over three and a half hours. I have gone through the LRGB cycle several times in a row now, running the AF routine for each. Every time I get back to L, and then run AF again with it, it's focus has shifted. It also seems that it is shifting fairly quickly, as instead of getting a pretty decent V like I was in the first couple of rounds, focus improves sharply throughout most of the sequence, and only starts to defocus again the last one or two steps. 

 

I purchased the CF version of the AT8RC in hope of avoiding focus shift problems...but maybe that was a bad choice since I live in Colorado. During the day in late spring and early summer, it can get over 90 degrees...then plummet into the low 50's or even high 40's at night. That has been particularly true this year...temp has dropped a couple more degrees just since my last post. Since the CF scope gives up heat more slowly...it seems as though it never stops shifting focus. :(



#6 freestar8n

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Posted 15 June 2016 - 12:17 AM

Do you leave the scope out under a telegizmos cover? If not I would aim to do that so it is pre-equilibrated as much as possible.

Is it fully exposed to the elements? If so - some shading from a beach umbrella my reduce the radiative cooling.

And make sure the focus really is changing due to temperature. Does the focus position return to where it was at the start of the previous night - when you start on the next night?

Frank

#7 Jon Rista

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Posted 15 June 2016 - 12:34 AM

I usually cover it in some padded blankets to protect from hail, then cover it in a rather thick silver tarp. It is pretty well protected from the elements, and open at the bottom to keep the air fresh inside. 

 

I am pretty sure focus is shifting due to the temperature. The outside ambient temp was just over 66F when I started, and now it is dropping to 57F. I have the Moonlite temp probe attached to the OTA, and that was reading over 21C when I started, and it currently reads 14.24C. So nearly a 7C change since around...9:40pm I guess, in two hours. I have been trying to determine the steps per degree, but the rate of temp drop is changing as well, which is making it rather difficult. 



#8 syscore

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Posted 15 June 2016 - 12:40 AM

"The outside ambient temp was just over 66F when I started, and now it is dropping to 57F."

 

That is a huge drop in the context of focus.

 

I have to refocus after every degree.

 

That being said, currently, the temp varies by only a degree here, but it is hot.


Edited by syscore, 15 June 2016 - 12:41 AM.


#9 Jon Rista

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Posted 15 June 2016 - 01:00 AM

Yeah...huge drop. Heh, that's Colorado. It's quite chilly now...it was quite hot during the day. OTA temp is now 13.7C. Still dropping, but a lot slower. I plugged in a steps per degree of 45, mostly a guestimate. 

 

I'm curious...does SGP adjust focus relative to temp while the frame is exposing? Or does it only do it between frames? 



#10 syscore

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Posted 15 June 2016 - 01:30 AM

On my moonlight, temp compensation is part of the ASCOM driver.



#11 freestar8n

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Posted 15 June 2016 - 01:48 AM

I think a reason I have little temperature change with sct - and I'm not alone in that regard - is that I have a dew strap on the front. I guess your RC does not. That might be worth a try.

I'm in Melbourne now and the temperature is more stable - but in NY there could be big swings of temperature and focus was fairly steady.

I think sgp will change focus based on temperature - but some focusers have that built in.

The main way to use sgp's focus routine is to re-focus periodically. That shouldn't take too much time.

You can also have it re-run the focus routine after a certain temperature change - rather than at a certain time interval.

Frank

#12 syscore

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Posted 15 June 2016 - 01:53 AM

"I'm in Melbourne now and the temperature is more stable"

 

You are only minutes from me. Unless you meant Australia.



#13 freestar8n

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Posted 15 June 2016 - 02:50 AM

You are only minutes from me. Unless you meant Australia.


Yep - Australia it is. Currently. About a 24 hr flight from you.

Frank

#14 spokeshave

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Posted 15 June 2016 - 04:43 AM

Jon Rista, on 15 Jun 2016 - 02:00 AM, said:Jon Rista, on 15 Jun 2016 - 02:00 AM, said:

I'm curious...does SGP adjust focus relative to temp while the frame is exposing? Or does it only do it between frames? 

SGP will only apply temperature compensation focusing between frames. As Frank pointed out, most ASCOM focuser drivers allow continuous temperature compensation. When doing long exposure narrowband, I have mine set up for both. The focuser makes continuous tiny adjustments and I have SGP set up to do periodic AF routines.

 

Temp compensation can be tricky and I have not found it to be terribly reliable with my gear. Part of the problem is that the temp probe only reports the temp of the probe itself, not the tube. It helps to tape the probe to the tube with foam tape. This helps insulate the probe from ambient air and lets it report a more accurate temp representative of the tube. Still, every time I generate a temp coefficient, it is different from the last time. So, for me at least, temp compensation will not reliably maintain focus throughout an imaging session. It is good enough, though, to keep focus tight during long subs when the temp can change a good bit from sub start to sub finish. SGP can then touch up fine focus between frames.

 

Tim


Edited by spokeshave, 15 June 2016 - 04:44 AM.


#15 TimN

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Posted 17 June 2016 - 05:49 PM

Jon, Are you sure your problem is pure focus or could it also be not allowing enough time for the mount to settle after dithering. Just a long shot but worth checking. I use a Carbon AT8RC but leave it outside in observatory. I haven't had a problem with the new (.15) SGP focus routine. After midnight I probably only need to refocus once until end of total darkness - around 3am. Are your individual focuses ok but the OTA is just cooling that fast that you can't take a 5 minute sub with it still in focus?

Edited by TimN, 17 June 2016 - 05:51 PM.


#16 Jon Rista

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Posted 17 June 2016 - 06:19 PM

I don't think it is dithering. I tried to settle at 0.5 originally, and it wouldn't ever settle. SGP does not seem to actually cancel the dither, even though I enabled the "For x seconds" setting and set it to 10s. IT just sits there, waiting, and PHD is trying to settle at 0.5 and it just can't seem t do it, even though after only a few seconds it's back to tracking along the same as it was before dither. I upped that to 1.0, and it would eventually settle, but it took FOR-EVER. I tweaked that to 1.2, and it now settles, although it takes a lot longer than I used to be able to settle in the past (and I was settling at 0.65 previously.)

 

My individual focuses are wild. It's a sawtooth graph on a slope most of the time. Rarely get a V graph. I had V's for a short while when I first started, but I haven't been getting them lately. The graph will usually end on a down trend at the leftmost iteration, rather than on an uptrend, which is why I think the scope is cooling and focus is shifting faster than AF can keep up with. For a while I thought that maybe the focuser was slipping, but I checked the tension on it, and it seems to be fine. I started increasing my exposure times for AF, and that helped reduce the extremes, but I still get inconsistent results for each AF exposure, and the trend is always downward from right to left. I've manually forced reattempts a number of times, to see if I could get the AF routine to catch up to the cooling rate of the scope. But those second and third attempts are usually worse, with just wild results across the graph. Something is definitely wrong, though. My very first attempt at AF, the steps were around 4500. It settled around 4430. That eventually shifted to 4333. Then it was around 4200. It has been consistently shifting down, night after night, and I am now around 1700. I've double checked the tension on my Moonlite. It's pretty darn tight, tight enough that I'm worried it's too tight. I dunno...something very odd is happening...

 

I actually think my mount may be tracking poorly as well. I've had better than usual seeing lately. My stars usually scintillate so much that the brighter ones exhibit a full spectrum sparkling rainbow effect, however lately I can barely tell that they are twinkling at all. MeteoBlue has been saying that seeing is between 0.3" and 0.55" a lot lately. Their chart has been green across the board except for the jet stream, which has been yellow and orange (when it is usually blood red.) In the past when I've seen stars like this, my guiding (at least with the 600mm setup with the external guidescope) has dropped to 0.3" RMS. With the OAG on the RC, my guiding the last few nights has been 0.7-0.9" RMS. Most of these clear nights have been totally windless (also very rare for me). That pretty much just leaves the mount. I rebalanced it, tried the weight on a string to keep it east heavy, tried different amounts of weight, tried different amounts of shifting the weights down the CW shaft, tried different amounts of DEC imbalance. I've checked and tuned the mount's external bolts, nuts, screws and the bearing preload on both axes. Everything is solid and tight (but not too tight). The mount just doesn't seem to be tracking that well. Ironically, my 600mm setup with all the heavy ADM dovetails and rings, is actually heavier than the RC! And it tracks a lot better.

 

So, some of the softness is probably the tracking. Almost 1" RMS with an 0.482"/px image scale is going to soften things up. I honestly don't know if poorer tracking is going to affect SGP AF or not...but the AF has been terrible. I tried manually testing and setting my L vs. NB filter offsets a couple nights ago, and that improved things a little, at least earlier in the evening...then I went to bed. The later subs from about an hour after that were much softer. So the cooling of the scope was getting away from SGP. I then manually tested and tweaked the steps/deg, which SGP originally calculated at -42, then after trying again was updated to -48, and I finally ended up on -55. I think -55 is still a bit low, and I might try -60. 

 

The interesting thing is, when I focus manually with SharpCap with a full video feed, I can really tell when the star is focused. I can start to see the airy disk, even with a moderate amount of scintillation. With SGP, even when it gives me a nearly perfect V graph and seems to think it's really nailed AF with it's HFD measurements, I can tell just by looking at those subs that they are not as well focused as I can get with SharpCap. I've tweaked the focus steps per AF iteration, down to as little a 5 steps (which is usually not enough to get a V graph), and it still cannot seem to give me the kind of tighter stars I was getting with manual focus in SharpCap. The other problem is when I switch from L to NB, the offsets are clearly wrong, and the stars end up even softer. I've fiddled and fiddled with the offsets, using the AF module as well as manually. I have not yet figured out the right offsets. I am beginning to wonder if the offsets may change over time as well. These are AstroDon filters, 3mm thick. I wonder if their thickness is enough that the difference between 22C and 9C could result in a change in the focus offset? There do seem to be times when the offsets work better than others...usually earlier in the evening they seem to be more closely matched, but by the end of my sequence things are pretty far off and the defocus is obvious. 

 

Anyway. I've basically given up on the RC. I'm not an RC kind of guy. It would be interesting to get higher resolution images of planetaries and some smaller nebula, would be nice to do solar system imaging, but this thing has been nothing but a hassle. Collimation never holds, I have to tweak it every night and it's never quite perfect unless I fiddle for an hour. My mount's tracking (even guided with OAG) doesn't seem to be sufficient. It never stops cooling and the rate of cooldown seems to be fairly significant (between 90F days and 48F nights), so ironing out the AF details has been a pain. It's just...heh, I hate this thing. :p I'm not made for RCs.

 

If I get another high res scope, it'll be an EdgeHD. At least the collimation will be simpler. At this point, I'd welcome mirror flop. O_o And I think the Edge would better cater to my overall goals...the smaller secondary and higher F-ratio is better for higher contrast solar system imaging. It can be reduced to f/7.1, which would be great for other things, and it could be converted to hyperstar for fast wide field. I think, if I get back into high res stuff, an SCT and I will get along much better. 

 

Hopefully someone else will be able to make much better use of this RC, the moonlite and the TSRCFlat2 than I have been able to. 



#17 Jon Rista

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Posted 17 June 2016 - 06:53 PM

Do you mean for my AF?



#18 glend

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Posted 17 June 2016 - 07:06 PM


"...... I've basically given up on the RC. I'm not an RC kind of guy. ..... I'm not made for RCs."

 

I sold my RC08 recently, and the TAK Collimation scope, I reached the same conclusion. On the other hand I love my Mak-Newt.



#19 bmhjr

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Posted 17 June 2016 - 07:32 PM

There was also a lot of smoke and haze last night. Did that have any affect?

#20 syscore

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Posted 17 June 2016 - 07:44 PM

And as far as your focus goes, it is a little soft, but welcome to longer focal lengths and higher resolution. You'll get use to it.:)



#21 Jon Rista

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Posted 17 June 2016 - 08:35 PM

 

Do you mean for my AF?

 

AF = auto focus?

 

No, I mean the reason you are struggling with the dim detail in M57 is that it is very dim and the noise implicit in your LP at your location is hiding it. Your result looks just like this one...

 

http://www.cloudynig...ject/?p=7277232

 

And that is simply due to noise. Since we know that the camera isn't adding the noise that is hiding the detail, then it must be your LP. I realize that you are using an NB filter, but that still lets in LP, and even though it is a small amount of LP, what you are trying to image (the dim details in M57) is very dim.

 

I don't think focus is enough to explain your results.

 

jmo

 

 

You think I am getting skyfog limited subs with a 3nm Ha filter in a mere 300 seconds? Seriously? O_o

 

Considering that people have said it takes 5400 seconds or more to get skyfog limited subs with a QSI6120 and similar Sony ICX cameras, which has similar read noise, and similar pixel size, and higher gain than I've been using, I have a very hard time believing that... Even if they were using the 20x rule, I would be amazed if I could even get my read noise swamped by enough LP in 300s such that the LP mattered enough more than the read noise and dark current to be the primary limiting factor.

 

I think you and Tolga both used 10-11" scopes, with 8-12nm filters, and your data was binned 2x2. Keep that in mind as well. I was not binned (as it turns out, sadly enough, binning is indeed quite useless on the ASI1600 outside of for frame rate increase, and costs you another two bits), using a 3nm filter on an 8" scope, with 8.2-8.5 pixel (4" or larger) FWHMs. I know I am capable of getting down to around 1.3-1.5" FWHMs when my skies are like that. I think between the smaller aperture, significantly narrower bandpass without binning, and significant blurring, that probably covers the differences. My smaller aperture and considerably different bandpass was why I preferred to compare data from you and Tolga, who have rather closely matched systems. The difference in non-obstructed area between an 11" SCT and an 8" RC is about 25,730mm^2, which is more than my total unobstructed area in and of itself (which is only about 17,190mm^2). (Another reason I want an EdgeHD! Even the 8" would have more unobstructed area, and paired with the reducer would make for a faster system. :p)

 

I would be pretty surprised if I had enough LP to swamp read noise & dark current/glows and cost me that much in terms of faint detail. If it really was true, then I'd say that was pretty awesome, to be able to get skyfog limited unbinned NB subs in 5 minutes. (Honestly, would be amazing.) But, I don't think that is the case. 


Edited by Jon Rista, 17 June 2016 - 08:49 PM.


#22 Jon Rista

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Posted 17 June 2016 - 08:39 PM

And as far as your focus goes, it is a little soft, but welcome to longer focal lengths and higher resolution. You'll get use to it. :)

 

It is very soft. Even with your data binned, my stars were several times larger than yours. O_o As I said, I was getting much sharper results with SharpCap and manually focusing on a star with a video feed. I think the diffraction limited resolution of the scope is 0.57"? The image scale is .48"/px, and I've been having seeing so good lately that in SharpCap I can see very stable airy patterns of the stars that are bright enough. So, I am fairly sure that if I can focus properly, I'd be operating close to the limit of the scope, at least on nights like those.

 

I just can't seem to get sharper results and have everything automated at the same time. Which is rather depressing. :\



#23 Jon Rista

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Posted 17 June 2016 - 08:46 PM

There was also a lot of smoke and haze last night. Did that have any affect?

 

Was there? Huh...you know, I fiddled around with focus for about two hours, and it just wasn't working so I ultimately bailed and packed it in. I don't actually think I even looked at the skies much. I guess I do kind of remember thinking it still seemed fairly bright with too-few stars when I went out to set up.... I didn't think about it too much though (moon was out, I guess I figured that was it).  :shrug: I wonder if that complicated things even further last night.   :sigh:

 

Is there more smoke tonight? Even though it is clear, if it's going to be fruitless...I could really use a break. 

 

(EDIT: Guys, I guess I thought this was my SGP thread. I didn't realize this was all in the ASI1600 camera performance thread. Should probably get back to topic.)


Edited by Jon Rista, 17 June 2016 - 08:52 PM.


#24 bmhjr

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Posted 17 June 2016 - 08:54 PM

Smoke was coming up yesterday from Ft Carson as well as New Mexico. It was better today.  But I can still see some haze.


Edited by bmhjr, 17 June 2016 - 08:56 PM.


#25 Jon Rista

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Posted 17 June 2016 - 09:40 PM

Here are some FWHM measures from some subs. One of M101 from SharpCap (best sub I could find from that night, most of the data has been backed up onto BluRay disk and deleted), the other from M57 from SGP. I chose the best I could find from my M57 batch from two nights ago:

 

SGP_FWHM.jpg

 

SharpCap_FWHM.jpg

 

So, in one case I've got 3.7" FWHM and in the other I've got 0.46" FWHM. You guys seriously don't think that would affect signal strength?

 

I agree that noise is going to obscure faint details. Absolutely no disagreement there. I just don't think that noise is the primary reason my signal is faint. I think the reason the signal is riding the noise floor is it's been smeared around, so there is less signal in more pixels, rather than more signal in fewer pixels. 

 

However the rim of the outer halo should be about 8 pixels across, and instead they are about 25 pixels across (Assuming I could get back to ~1.3" FWHM rather than 4" FWHM). Concentrate all the signal from those 25 pixels into 8 pixels. ;) Instead of ~240 ADU per pixel, I'd have ~750 ADU per pixel. Or instead of 14e- per pixel it would be about 43e- per pixel. I mean, why does the SNR on a guide star jump from 4:1 to 30:1 or 50:1 or more when you go from a few steps off focus to dead on focus? It's signal gets concentrated onto fewer pixels. 

 

As for f-ratio. It doesn't really matter all that much, although in both cases you guys had both larger apertures (more total signal) and faster f-ratios (quicker saturation per pixel per unit time). Aperture matters. Every pixel-sized point in the sky that falls within your field has it's light concentrated onto a pixel on the sensor, and the quantity of that light is determined by the aperture. The total aperture gathers light for each and every point. Your two scopes were gathering far more light per pixel. By more than a factor of two in the case of the 11" SCT. And you were binned 2x2, which at least doubles SNR again. (I really gotta get a darn EdgeHD! :p)


Edited by Jon Rista, 17 June 2016 - 09:50 PM.



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