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How would you proceed?

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#1 pgs/sdg

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Posted 22 September 2019 - 02:18 PM

Here's how I'm imaging right now: 

EQ8 (Orion HDX110)

AT65 scope (420mm)

ASI174 guide cam on Celestron OAG

T5i imaging camera

 

I switch off between the AT65 and the 8" Edge. 

My stars look good - but this is @ 420mm FL.

I want to improve my guiding. 

 

Here's a guide log from last night with the above equipment set up

:

Attached File  PHD2_GuideLog_2019-09-21_204653.txt   187.51KB   6 downloads

 

Here are a few things that I THINK I know from running calibrations and running the guiding assistant:

I do have some DEC backlash - when I run the GA in some quadrants - it is negligible - other quadrants - not so good - in some places - pretty bad. 

 

Here is the result from the last time I ran the guiding assistant:

Screenshot (21).png

 

And here's my last calibration:

Screenshot (28).png

 

As I mentioned - sometimes the backlash test yields much better results (nice matching lines) - sometimes worse results than the one posted above.

 

I have not yet run PEC on the mount - but plan to do so soon using the EQMOD procedure. 

 

My question is - what would you do next?

Go into the mount and make adjustments to the DEC axis to reduce the backlash? I understand how to do it - but would rather not make mechanical adjustments to the DEC axis unless I just have to. 

 

Run PEC on the mount and see if that helps?

 

I usually try to perfectly balance. (The EQ8s are not easy - but if you work at it - you can get it balanced)

Should I try a little east heavy and a little camera heavy?

 

Or just try to improve the guiding by making changes to the PHD2 settings? 

 

 

I plan to go back to imaging with my 8" Edge HD and I believe I am going to need to get my guiding to perform better in order to get good images at that FL. When I have the 8" Edge on the mount - my guiding generally has been about the same - not much better or much worse. Sometimes as low as .6 or .7"  but often 1.2" and higher. Seems to waffle around a lot. 

I am wondering if you all think running PEC on the mount and trying to adjust the DEC backlash might improve overall performance.

Can't hurt? 

 

Thanks for any advice or recommendations you might offer...

 

Paul 

 

 

 

 

 

 



#2 bobzeq25

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Posted 22 September 2019 - 03:33 PM

One possible strategy.  Deliberately misalign polar, just a bit (3 arc minutes?).  Then, guide DEC in only one direction.  it's much like "East heavy" for RA.



#3 pgs/sdg

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Posted 25 September 2019 - 11:16 AM

One possible strategy.  Deliberately misalign polar, just a bit (3 arc minutes?).  Then, guide DEC in only one direction.  it's much like "East heavy" for RA.

 

Okay…that's interesting. Had not thought of that and it sounds a bit counterintuitive (with so much emphasis on getting a good PA). But

I guess I see the strategy. So, ease off the PA a little and then do the usual:

Scope on west side - guide south

Scope on east side - guide north

Right? 

 

Just looking for some possible alternatives before unscrewing things and going into the mount, where I might run the risk of over tightening. And as I mentioned - in some parts of the sky - when I measure the backlash - its not a problem.

The procedure for increasing or reducing the DEC backlash in the EQ8 is pretty straightforward from the EQ8 manual - I just don't know anyone who has actually done it. I may try the Skywatcher EQ8 forum  - surely someone there has done the procedure.

I guess part of my indecision is that my stars are nice and round in my subs, but that's at a relatively easy focal length of 420mm.

When I put my 8" Edge back on the mount...I expect things are going to be needing a bit more tightening up. 



#4 einarin

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Posted 25 September 2019 - 11:25 AM

Focal reducer makes life easier...



#5 pgs/sdg

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Posted 25 September 2019 - 05:43 PM

Focal reducer makes life easier...

Yes. Totally agree. When I switch back to the 8" Edge from the AT65 - I'm going to give it another try at f/7. 

Initially, several weeks ago, when I tried setting up the Cel. .7x FR on my 8" Edge - I ran into problems (not the setup described above) . The problem was not with achieving the BF distance - I was able to work that out using spacers & adapters that I had on hand. But I never could seem to get a good clear full illumination of the guide cam sensor. Which surprised me a bit because the ASI174 has a big sensor. Others here on CN have reported challenges when trying to set up an image train with the Cel .7x FR, Cel OAG, and a DSLR.. and if anyone has been able to figure out a way to do it - I'm listening!

I set it up this way:

8" Edge > FR > blue fireball adapter > adapter > Cel OAG w/ASI174 > adapter > T5i

The camera side opening of the FR seemed like it was too narrow in diameter to allow full illumination of the prism. And I had the prism ALL the way down in the light path. 

I will give it another go and see if I can get it to work when I switch scopes out. 

Right now I just want to see what I can do to reduce my backlash and get better guiding performance. 

 

.



#6 rgsalinger

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Posted 26 September 2019 - 09:35 AM

The problem is that the prism is too small for the chip. Sad but true. I would not worry about it, though unless you actually have trouble finding guide stars. 

Rgrds-Ross


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#7 spokeshave

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Posted 26 September 2019 - 10:13 AM

You can try adjusting the backlash, but the EQ8 is notorious for its DEC backlash. I suspect that the reason you are seeing varying amounts of backlash is because your DEC axis is too well-balanced. On a mount with fairly high backlash, if the DEC axis is too well-balanced, the OTA can "float" in the backlash band - sometimes floating closer to one side of the band and other times floating closer to the other side. How much movement is needed to take up the backlash will depend on where the OTA is floating in the backlash band. You can try introducing a small imbalance in DEC. This will cause the OTA to remain at one side of the backlash band (until a flip, where it will go to the other side). This will make backlash compensation in PHD2 more effective. 

 

I only recommend guiding in one direction in DEC as a last resort, particularly if you are not continuously attending the mount. The PHD2 backlash compensation process is pretty good for modest backlash that is well-behaved. Yours is not terrible, and you can make it well-behaved by introducing a small imbalance.

 

tim


Edited by spokeshave, 26 September 2019 - 10:14 AM.

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#8 pgs/sdg

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Posted 26 September 2019 - 12:38 PM

The problem is that the prism is too small for the chip. Sad but true. I would not worry about it, though unless you actually have trouble finding guide stars. 

Rgrds-Ross

I think you are probably right. As I recall..when I had the 8" Edge on the mount a few weeks ago, I had no problems at all finding guidestars @ f/10.  Then I added the .7x FR, reworked the BF, but found I still had some problems. Maybe the problem was the BIG sensor/small prism combination. I will try again and maybe I can get it to work well enough to get guidestars.

 

 

You can try adjusting the backlash, but the EQ8 is notorious for its DEC backlash. I suspect that the reason you are seeing varying amounts of backlash is because your DEC axis is too well-balanced. On a mount with fairly high backlash, if the DEC axis is too well-balanced, the OTA can "float" in the backlash band - sometimes floating closer to one side of the band and other times floating closer to the other side. How much movement is needed to take up the backlash will depend on where the OTA is floating in the backlash band. You can try introducing a small imbalance in DEC. This will cause the OTA to remain at one side of the backlash band (until a flip, where it will go to the other side). This will make backlash compensation in PHD2 more effective. 

 

I only recommend guiding in one direction in DEC as a last resort, particularly if you are not continuously attending the mount. The PHD2 backlash compensation process is pretty good for modest backlash that is well-behaved. Yours is not terrible, and you can make it well-behaved by introducing a small imbalance.

 

tim

That's encouraging.

Right now I have no bias in the balance. It took a while tongue2.gif  but its just about perfectly balanced.

Maybe the way forward at this point then is to rebalance a little east heavy and a little camera heavy. Or at least a little camera heavy for sure. What you describe.."the floating in the backlash" kind of seems to coincide with what I think I see going on in the dec axis? (Maybe) I am really kind of still learning how to interpret the guide log data. I think I will adjust the balance a little cam heavy, try some guiding, run the GA and see how it does with a little DEC imbalance. I (almost always) have the DEC guiding set on "Auto"

 

I have a separate (but perhaps related) question...

One would obviously assume that doing PEC, if done correctly, is going to improve overall guiding performance.

Does running PEC improve DEC guiding performance or does it improve RA performance only?

 

I have been putting off doing PPEC on the EQ8 and I really need to do it...particularly if it might help with DEC guiding.



#9 Stelios

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Posted 26 September 2019 - 01:04 PM

First of all one question: I notice that your equipment profile says AT65>CelOAG>ASI174>T5i -- does this have the correct focal length for the Edge? (If it does, it has the wrong focal length for the AT65).

 

*Slight* camera heavy is a no-brainer. 

East Heavy assumes you will be monitoring and changing as needed with the target. It's a laborious thing, but it does help.  

PEC will only benefit RA guiding. Periodic Error refers to RA. There's no driving in DEC and thus no period.

You should consider using PhD2's PPEC algorithm. Costs nothing and should benefit whether you use PEC or not.

 

Finally, you should run Guide Assistant every time, as the suggested MinMo's are seeing-dependent and will change. When seeing is great, MinMo's should be low (and guide exposures shorter), when seeing is poor, MinMo's should be higher (and guide exposures longer). Remember--you can never guide better than the seeing.


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#10 pgs/sdg

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Posted 26 September 2019 - 02:26 PM

First of all one question: I notice that your equipment profile says AT65>CelOAG>ASI174>T5i -- does this have the correct focal length for the Edge? (If it does, it has the wrong focal length for the AT65).

Yes. That did confuse things. My screen shots (with the AT65>CelOAG>ASI174>T5i train) were what I was originally talking about and I kind of conflated two separate issues by introducing the questions about the 8" Edge.My bad. 

 

*Slight* camera heavy is a no-brainer. 

East Heavy assumes you will be monitoring and changing as needed with the target. It's a laborious thing, but it does help.  

PEC will only benefit RA guiding. Periodic Error refers to RA. There's no driving in DEC and thus no period.

That's what I thought - but I thought I would ask... 

 

You should consider using PhD2's PPEC algorithm. Costs nothing and should benefit whether you use PEC or not.

 

When I do run PEC - I was going to run the PPEC routine in EQASCOM. 

 

Finally, you should run Guide Assistant every time, as the suggested MinMo's are seeing-dependent and will change. When seeing is great, MinMo's should be low (and guide exposures shorter), when seeing is poor, MinMo's should be higher (and guide exposures longer). Remember--you can never guide better than the seeing.

waytogo.gif

BTW - Here's one randomly picked sub from my last time out - M31... 120"  ISO800   - how would you rate the stars?

This is with the AT65>OAGw/ASI174> T5i set up. 

They look pretty good to me - and they were all about like this. But many of you have a much better "eye" for gauging image quality than I do.

Maybe I am obsessing a little bit over guide performance.  But I know when I go back to f/10 - things are going to need to be tighter. 

The only place the stars seemed a little wonky to me was in the lower left corner. 

 

 

L_M31_EOS_Rebel_T5i_120s_ISO800_0012_2019-09-21.JPG

 

Thanks for your replies and insights... I know I can get this all working better....

 

Paul


Edited by pgs/sdg, 26 September 2019 - 02:32 PM.


#11 rgsalinger

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Posted 27 September 2019 - 12:35 AM

No one has a better eye or at least I certainly don't. What happens is that sooner or later you acquire software that lets you measure the eccentricity and FWHM of your results. Just eyeballing things is fine as far as it goes but it's really hard to tell if changes really make a difference if you aren't measuring their effect. 

Rgrds-Ross


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#12 the Elf

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Posted 27 September 2019 - 07:22 AM

The image is too small for a rating. I fed it into PI anyway. Scale is 6.75 arcsec per pixel, FWHM is 1.5 pix in the center up to 1.8 at the left border. Eccentricity is 0.64 in the center and lower at the borders. That's not surprising because slightly out of focus stars (as a result of a non flat field) appear rounder but larger. With my shorter lenses I have 1.2 to 1.5 pixels which is the optical resolution, for longer focal lenghts it is the seeing, 2.0 arcsec being the best I had so far. If you post a Raw image in full size I can try again. So far I'd say it is in the ballpark but not award winning.


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#13 pgs/sdg

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Posted 27 September 2019 - 12:09 PM

I am still getting the hang of using the 2 focusing tools in APT. One is a focus aid with running measurements of HFD and FWHM. The other is a Bahtinov aid. I usually end up using the Baht. method ..probably only because I'm so used to using it. 

 

I added 30 more 2 minute subs last night. I have 20 or so from a prev. night. I may have to toss a few. Will take my flats and biases later today and then have a go at processing. Haven't shot M31 before so I'll see how it goes. Will post it here and maybe some of you can comment/critique.

 

It looks like we have several clear nights coming up here northern Arkansas...so I need to line up some more targets. Double cluster, Pleiades... this is really fun!

 

Thanks for all the comments and advice!



#14 the Elf

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Posted 28 September 2019 - 01:23 AM

You are using the same scope as I do, the AT65 = TS Imaging Star 65. Here is what I got on an EQ6-R. Note the over all exposure time: almost 5 hours.

http://www.elf-of-lo...romeda2018.html

If you want to bring the blue color out you need either dark skies or several hours of exposure time. My quad is perfectly flat. I wonder why you have bigger stars in the corners.

The pleiades are much brighter. This is taken with the same same setup and an unmodded T3i, 1h22min exposure time. According to clearoutside I am in a Bortle 4 area like you are. You should get similar results.

http://www.elf-of-lo...eiades2018.html


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#15 pgs/sdg

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Posted 28 September 2019 - 04:55 PM

You are using the same scope as I do, the AT65 = TS Imaging Star 65. Here is what I got on an EQ6-R. Note the over all exposure time: almost 5 hours.

http://www.elf-of-lo...romeda2018.html

If you want to bring the blue color out you need either dark skies or several hours of exposure time. My quad is perfectly flat. I wonder why you have bigger stars in the corners.

The pleiades are much brighter. This is taken with the same same setup and an unmodded T3i, 1h22min exposure time. According to clearoutside I am in a Bortle 4 area like you are. You should get similar results.

http://www.elf-of-lo...eiades2018.html

That's a really nice M31!

Here's a first shot at processing what I got. This was two nights. When I get my POD set up (so that I can completely remove the dome) - it will enable me to shoot much longer and image a lot of targets at maximum altitude.

Ended up with 1 hour 44 minutes... a far cry from 5 hours! I may add some more.. but wanted to process what I had. Impatient I guess! After throwing out 2 frames with jet trails - ended up with 52x2min exp. ISO 800.

Dithered every other frame in RA only. Still had quite a bit of noise. I'm still not real confident about my dither settings in PHD2 &  APT - not sure if its dithered enough or too much. When I tried to dither in DEC as well as RA - on some of the dithers - the DEC guiding went berserk. Probably because of backlash, I guess. So I switched to just RA dither. 

But, I think a dedicated cooled astrocam is somewhere in my future...

 

My original post in this thread was more to do with guiding performance - but actually - the stars in this image look ok to me. Maybe I should be satisfied for now with my guiding performance and just start taking lots of shots of different targets, more time, and more learning of processing. 

Where I am at the lowest point on the learning curve is with processing..much more learning to be done there. But this was fun. 

 

M31-A.jpg

 

Thanks for your examples of your work and your feedback.  



#16 the Elf

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Posted 29 September 2019 - 04:57 AM

Here is another example what to expect from an unmodded DSLR:

https://www.cloudyni...h-unmodded-t3i/



#17 pgs/sdg

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Posted 29 September 2019 - 01:43 PM

Those are both great shots! 

 

I have wanted to give Ha a try. I understand there are clip in Ha filters for the Canon EOS cameras.. but I have not investigated it much and Ha is an area I'm not very familiar with, but I do love the look of the Ha images. 

I find the differences in nm passband a little confusing... 5nm, 7nm, 12nm... etc. 

 

The one I saw here:    https://www.highpoin...rm=ASF-HA12-EOS

 

would seem like it would work - its 12nm - so I don't know if there are others - or if the 12nm would be fine - or one of the other nm filters like a 5nm or a 7nm would work better.  

Since its a clip-in for EOS cameras - I assume it would work fine with my T5i without having to do anything to the camera. 



#18 the Elf

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Posted 29 September 2019 - 10:34 PM

Two things to consider. Let's start with the nanometers. The nebulae emmit at exactly the well known wavelenght with a bandwidth of a fraction of a nm, so basically all filters are wider than the objects. No matter which filter you use, even if you use none, the nebula will appear with the same intensity. So what is the filter doing then? It is keeping the disturbing star light out of the image! The full spectrum is some 400nm wide. If you use a 20nm Ha filter 380nm of the stars are reflected back to where they came from and the stars are dimmer and so is the sky background. If you use a 3nm the stars are even dimmer. As you know the smaller filters are more expensive. I recommend 7 or 8 nm because you probably like to see the brightest stars in life mode for alignment.

The other thing is a DSLR or any OSC camera is far from optimal for it. The DSLR is little sensitive to Ha. Removing the IR filter helps to increase it. Still the bayer pattern consists of 1 red, 2 green and 1 blue pixel in a 2x2 square. With the Ha filter attached only the red one can see anything, the other 3 are dark or thermal noise only. The red filter on the pixel aborbs a lot of the light which is useless, the light is red already. That is why very few people remove the bayer pattern. I got a camera like this, modded by Brent Oliver in the US. The vast majority uses a mono astro camera for the job. Depending on how deep you want to dive in a Ha clip filter is only a poor version of Ha imaging. You will be limited to bright objects and the resolution is 1/2 in each direction but you will have a higher contrast than before. I recommend you get a cheap mono astro camera and the filter. As you can see from my images, the color shots from the unmodded DSLR and the Ha shots from a mono can be combined. This will be my next video, I guess. (Lot's of rain over here, video time.)

The clip filter is ok, but it only works with a telescope or an EF-type lens. An EF-S type lens cannot be used, it needs that bit of extra space where the filter sits.




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