Back Focus Issue?

[PhotoGav]

Update to pixel size - I just looked on the Starlight Xpress website in the Lodestar X2 manual, which quotes pixel size as 8.6 x 8.3?m... Which do I enter?!

It also clearly states that the prefered mode is 2x2 binning, so I will definitely be trying that tonight (Sky Gods permitting!).

[geoflewis]

13 minutes ago, PhotoGav said:

I also see that I am not binning the guide camera. Would that be a good idea and improve the situation I wonder?

Hi Gav,

I always bin 2x2 with my X2 (the same as I did with the original Lodestar). NB the Starlite Xpress X2 handbook (page 4) recommends binning when guiding with the X2 - "The preferred guiding mode is 2x2 binned, as this gives both fast downloads and high sensitivity without any ‘interlacing’ issues."

See the below link for the full handbook.

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=2ahUKEwitkZP4rpDgAhXuXRUIHSk6CosQFjABegQICBAC&url=https%3A%2F%2Fwww.sxccd.com%2Fhandbooks%2FHandbook%20for%20the%20Lodestar%20X2.pdf&usg=AOvVaw115LwmEFnVPnw8Xj0GOup_

Cheers, Geof

[geoflewis]

4 minutes ago, PhotoGav said:

Update to pixel size - I just looked on the Starlight Xpress website in the Lodestar X2 manual, which quotes pixel size as 8.6 x 8.3?m... Which do I enter?!

It also clearly states that the prefered mode is 2x2 binning, so I will definitely be trying that tonight (Sky Gods permitting!).

You beat me to it...... ?

[PhotoGav]

I can now enjoy the whole day safe in the knowledge that all my astrophotography woes will be solved tonight when I simply select 2x2 binning in PHD2!!!!

[geoflewis]

Just now, PhotoGav said:

I can now enjoy the whole day safe in the knowledge that all my astrophotography woes will be solved tonight when I simply select 2x2 binning in PHD2!!!!

If only......??

[tomato]

Gav, I think you deserve a round of applause for allowing the great and the good to examine your images in this fashion, I would be mortified if people started looking in my corners with this level of scrutiny!

And 0.2 RMS! I have managed to see 0.4 on my put up and take down Mesu rig  but that was willing it with all my heart and soul to get there on a particularly good night. It normally runs at 0.6-0.7.

I have so much to look forward to when I get an observatory.

[PhotoGav]

10 minutes ago, tomato said:

Gav, I think you deserve a round of applause for allowing the great and the good to examine your images in this fashion, I would be mortified if people started looking in my corners with this level of scrutiny!

And 0.2 RMS! I have managed to see 0.4 on my put up and take down Mesu rig  but that was willing it with all my heart and soul to get there on a particularly good night. It normally runs at 0.6-0.7.

I have so much to look forward to when I get an observatory.

Hah! It's the only way to improve - bare my corners and enjoy the wealth of knowledge and help on here. I'm glad to see that the thread seems to be of interest and use to others too. I am about to post subs from last night's session for forensic examination...

This idea that Mesu 200 mounts guide under 0.2" RMS seems to need some kind of small print to go with it. I am very interested to read it when available!!

As for an observatory - it really is the single best upgrade anybody can make, you will love it. Be warned - they do seem to make very comfortable homes for small fluffy Gremlins though!

[PhotoGav]

Here are some subs from last night's session for your forensic analysis. I have dark subtracted these and done a 15% DPP in APP. Happy with them all, except for the Blue, which shows significant coma. Is this the SCT equivalent of chromatic aberration on view? Is there anything that can be done to correct it? Should it even be present with the 'amazing EdgeHD system'? Is my scope a duff?!!?

RED (300 seconds)

 

GREEN (300 seconds)

 

BLUE (300 seconds)

 

HYDROGEN ALPHA (1800 seconds)

 

OXYGEN III (1800 seconds)

[steppenwolf]

2 hours ago, PhotoGav said:

Our setups are very different in the software department. Makes me wonder! I still have the pointing model rattling around in my brain as a potential point of improvement? Despite ruling it out once already!

The sky model will not help with guiding using the encoders that you have on your mount (and my mount for that matter) - their resolution is only 2.16 minutes/tick which is way outside your guiding.

[PhotoGav]

Just now, steppenwolf said:

The sky model will not help with guiding using the encoders that you have on your mount (and my mount for that matter) - their resolution is only 2.16 minutes/tick which is way outside your guiding.

Good, thank you. I will rule it out again and leave it alone, firmly ruled out!

[vlaiv]

Gav, indeed - everything seems to be in order for best possible performance. Do try 2x2, and don't worry about exact figure 8.4 or 8.5 will be fine - there is small difference in terms of RMS to have significant impact (it really does not matter if guiding of 0.3" is reported as 0.33" for example - as long as you get consistent results).

With binning x2 you will have about 17um pixel size, and with 1400mm FL that is 2.5"/pixel guide resolution - should be plenty to give accurate readings up to 0.3-0.4" RMS.

Only other thing that I can now think of that would impact guiding in that manner is local heat of sorts. This could be down to heat management in obsy, but more likely it could be due to imaging direction. Do you image in direction of houses? It's winter time and people are heating their homes. Heat from chimneys and in general can create local turbulence and cause guide stars to jump wildly.

Only way to fight this is to go longer in guide exposure. Usually people go with 2s exposure, and consider 5 or 6s to be very long. With smooth mount such as Mesu I believe one can comfortably go with as much as 10s without impact on precision of guiding. This has certain drawback though - dither lasts longer (waiting for guiding to settle).

[PhotoGav]

Vlad, progress has definitely been made. It's curious that the blue sub is poor compared to the others - hopefully it won't be too obvious in the final results (though now I know it is there, it will stick out like a sore thumb to me!). I will try the 2x2 guide camera binning and am fascinated to see how this affects things. I will also be trying longer guidecam exposures. I am up to 3s already, but will extend that and see how it changes things.

As for heat sources... I am in the middle of a small housing estate, so am looking out over a number of houses, the closest being within 30 metres. They might well be having an impact. Not much I can do about that, beyond the guidecam exposure increase as you suggest.

The good thing is that the system seems to be tuned up significantly compared to the start of this thread. So thank you to everyone who has contributed so far. Further reports to follow...

[wxsatuser]

12 minutes ago, PhotoGav said:

Here are some subs from last night's session for your forensic analysis. I have dark subtracted these and done a 15% DPP in APP. Happy with them all, except for the Blue, which shows significant coma. Is this the SCT equivalent of chromatic aberration on view? Is there anything that can be done to correct it? Should it even be present with the 'amazing EdgeHD system'? Is my scope a duff?!!?

What appears to be coma top left OIII.

[vlaiv]

I would say that those subs look lovely.

Issue with blue can be two fold.

Either from scope, or from reducer. Both use refractive elements and there is dependence on wavelength to get good correction.

SCTs are sensitive to blue, even EdgeHD, and have a bit of spherical in that part of spectrum.

Look at spot diagram for EdgeHD 800 (found in document linked by Steve and discussed earlier):

Blue is simply not diffraction limited on this scope (above is for 486nm, but depending on UV/IR cut used - you will be working with wavelengths down to 400nm).

Add reducer in the mix and you can get even more distortion for blue channel (we need combined spot diagram for whole system to see what would star shapes look like).

To my eyes green sub looks the best - and it should according to above spot diagram.

[PhotoGav]

2 minutes ago, wxsatuser said:

What appears to be coma top left OIII.

Yup, there too, but then OIII is in the blue part of the spectrum, so will suffer from the same issues.

Vlad - thanks for posting that part of the document. I seem to be proving that they are right with their bench tests! At least the system is performing as expected. I think that in the past the guiding with the HEQ5 was relatively erratic so that I had nice round stars, but big fat stars that hid a multitude of sins! As with every kit upgrade, it reveals a new weakest link in the system. The EdgeHD 800 has just stepped up to that position, I think!

[PhotoGav]

5 minutes ago, vlaiv said:

To my eyes green sub looks the best - and it should according to above spot diagram.

After I had fiddled with collimation at the start of the evening, I did the green subs first. When the first sub rolled in and I looked at the corners I nearly fell off my seat - it looked as perfect as perfect can be! Then I moved to the blue filter and reality returned...!!

[PhotoGav]

I couldn't resist making a quick combination of the five subs... No flats involved, as is easy to see! Good news is that star shapes are within the PhotoGav boundaries of acceptability. Now all I need to do is up the total integration time from 1 hour 15 minutes to somewhere in the region of twenty to thirty hours. What are the chances of that in England at the moment?!

[PhotoGav]

Question - do I need to re-do my guidecam darks because I change from 1x1 binning to 2x2 binning?

[geoflewis]

1 minute ago, PhotoGav said:

Question - do I need to re-do my guidecam darks because I change from 1x1 binning to 2x2 binning?

Yes, you should redo darks and calibration. What software do you use for guiding? Ideally you should create a new profile for the camera binned 2x2.

[PhotoGav]

13 minutes ago, geoflewis said:

Yes, you should redo darks and calibration. What software do you use for guiding? Ideally you should create a new profile for the camera binned 2x2.

Good call Geof, thank you, I will make a new profile. I was going to redo calibration for sure, but had a brain freeze re. darks!

[steppenwolf]

3 hours ago, PhotoGav said:

At least the system is performing as expected.

I think that is very much the case.

57 minutes ago, PhotoGav said:

do I need to re-do my guidecam darks because I change from 1x1 binning to 2x2 binning?

For what it is worth, I have never used guidecam Darks with my Lodestar although I have the feature available in MaxIm DL if I choose to do so.

[PhotoGav]

10 minutes ago, steppenwolf said:

For what it is worth, I have never used guidecam Darks with my Lodestar although I have the feature available in MaxIm DL if I choose to do so.

Is this your magic ingredient for perfect guiding?!

I have just made a Bad Pixel Map in PHD2. I’ve always used darks in the past, so it will be interesting to see if the BPM gives good clean results. I know I shouldn’t be changing multiple variables all at once, but.... no, no mitigating circumstances at all, if it all goes wrong it’s my fault, again!

Shooting flats for the project at the moment, but with any luck the lovely clear sky out there now will darken beautifully and persist for long enough to let me test everything out. 

[martin_h]

On 24/01/2019 at 10:31, PhotoGav said:

I bin my loadstar  2x2 and use a BPM with a guide pulse of 3 seconds.