Evaluation please

[Paul Porters]

As a beginner I'm still getting acquainted with all the gear and procedures.

I have already experienced, and learned from Bruce that I don't have the best (easiest) setup to start with (working on that).

I'm running an Ioptron CEM25P with a Bresser MC152 telescope. Guiding with a Orion 60/240+Bresser Deepsky Camera (Touptek IMX290)

The specified focal length of the Bresser is 1900mm, but according to astrometry.net it is 2481mm -> Is it normal to have such a large deviation?

After some startup troubles, I now have the rig running and I'm starting to try and do some imaging over the next weeks.

This night I did a test run on the guiding and took some images to be evaluated to determine if I'm on the right path.

Here's the guiding log: https://openphdguiding.org/logs/dl/PHD2_logs_7ZYO.zip   (the large excursions, that's me fiddling on the camera (Nikon D3200) to adjust exposure).

And here is the full size 6 minute exposure: https://www.dropbox.com/s/zbpax0s8klqbph9/DSC_0102.jpg?dl=0  -> Are those stars round enough (I really don't have a clue)?

If anyone can spare the time and have a look, that would be appreciated very much.

Just let me know any tips, suggestions and/or comments!

Paul

[Bryan]

Paul

I will leave evaluation of your logs to the real experts here.

With regards to your focal length, any scope that adjusts focus by moving the primary mirror will change focal length, e.g. SCT or a Maksutov-Cassegrain like yours.

This thread discusses the topic of Mak-Cass optics.  I suspect there are many others.

https://groups.io/g/tec-scopes/topic/2842748

This article shows that the change can be quite dramatic.

http://www.brayebrookobservatory.org/BrayObsWebSite/BOOKS/EFLMAKCASS.pdf

Bryan

[Bryan]

Paul

Although unrelated to PHD2, can you tell us what camera and ISO you used for the sample image?   Also, was this captured recently, i.e. during fairly bright moon? 

Bryan

On Friday, June 21, 2019 at 7:14:13 PM UTC-6, Paul Porters wrote:

[bw_msgboard]

Hi Paul.  If we ignore the big excursions – you really can’t be touching stuff when you’re guiding – your overall guiding RMS was about 1.3 arc-sec with similar results for both Dec and RA.  I think you’re running with overly large minimum moves, which is probably limiting the guiding performance.  This happened because your earlier GA run included a large excursion that was really an outlier.  Since you’re just getting started, it would be best to do a fresh calibration each night, then run the GA to sample the seeing conditions.  You should watch the guiding graph while this is being done to be sure there aren’t weird large excursions.  I think a repeat run will produce lower recommendations for min-moves, probably less than 0.5 px.

 

We can’t really tell much from a compressed jpg image in terms of star shapes.  Since you’re using a DSLR, you probably can’t produce FITs files, but an uncompressed TIFF version of a full-frame image will let us use some analysis tools to measure the star elongation. My expectation is that the stars are round enough but perhaps larger than they need to be.

 

Cheers,

Bruce

 

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[Paul Porters]

@Bryan:  Thanks for the info on the FL subject, I already suspected that it might have to do with focussing, But the Bresser doesn't have a moving mirror, it has fixed mirrors and a 2.5" focuser:  https://www.bresser.de/en/Astronomy/Telescopes/BRESSER-Messier-MC-152-Hexafoc-Optical-Tube-Assembly.html.

Does it also apply for this particular setup, I mean, the distance to the sensor is variable after all?

I shot the image last night on a clear night (90% Moon) with a Nikon D3200 at ISO3200 / 360s. Normally I snap RAW, but forgot to turn the camera back on that format ;-)

@ Bruce:

You're absolutely right, I did 2 GA runs yesterday, and the first one had much smaller values. 

The culprit on the second one wasn't me by the way. My little dog Suzie decided it was time to come and join me, and her line snagged on the tripod ;-p (she's forgiven).

I will take some RAW images next time I'm outside with the scope.

Thanks!

Paul

[bw_msgboard]

Hi Paul, see below.

 

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From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Paul Porters

Sent: Saturday, June 22, 2019 12:54 AM
To: Open PHD Guiding

Subject: [open-phd-guiding] Re: Evaluation please

@Bryan:  Thanks for the info on the FL subject, I already suspected that it might have to do with focussing, But the Bresser doesn't have a moving mirror, it has fixed mirrors and a 2.5" focuser:  https://www.bresser.de/en/Astronomy/Telescopes/BRESSER-Messier-MC-152-Hexafoc-Optical-Tube-Assembly.html.

Does it also apply for this particular setup, I mean, the distance to the sensor is variable after all?

 

As I recall, you have a giant star-diagonal in the main imaging train, not something I’ve ever seen before.  That increases the back-focus by a huge amount and thus changes the observed optical properties of the scope.  I really think you need to get that out of there.

 

I shot the image last night on a clear night (90% Moon) with a Nikon D3200 at ISO3200 / 360s. Normally I snap RAW, but forgot to turn the camera back on that format ;-)

 

@ Bruce:

 

You're absolutely right, I did 2 GA runs yesterday, and the first one had much smaller values. 

The culprit on the second one wasn't me by the way. My little dog Suzie decided it was time to come and join me, and her line snagged on the tripod ;-p (she's forgiven).

 

These GA runs are intended to be fairly precise.  When something goes wrong during the data collection phase, you should just stop the run and certainly not apply any of the recommendations.  Otherwise it’s likely to be garbage-in-garbage-out.

 

Bruce

 

I will take some RAW images next time I'm outside with the scope.

 

Thanks!

Paul

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[Paul Porters]

Yeah, you're absolutely right there, garbage in = garbage out, that's a truth on almost every subject. Indeed, I should have discarded that one. All part of the learning curve I guess.

The diagonal is only in place for visual observation, when imaging, there's only the DSLR straight into the focuser (with adapter of course).

It looks like tonight might be a clear night again, so hope to do some more testing and will post the results.

Paul

[Paul Porters]

Okay,  out in the yard again on (I think) a night with good circumstances.

I did the PA, One star alignment, 

a couple of platesolve & syncs in Sharpcap (I assume it narrowd down the alignment, is that correct?), 

did a fresh calibration and a GA run, 

then I just let the mount guide until my roof came in the way (Meridian & Equator crossing is on the other side of my house).

Here are the logs:  https://openphdguiding.org/logs/dl/PHD2_logs_Ejsi.zip

Are the very large Dec corrections the backlash compensation, or did I mess something up again?

Thanks for having a look.

Paul

Here's a little joke as compensation ;-)

[Paul Porters]

So... I 'took out the garbage' and started from scratch with a new profile.

Here are the results of that one:  https://openphdguiding.org/logs/dl/PHD2_logs_6vrl.zip

To my untrained eye it looks as if the guiding is not as tight as the previous runs. 

Well, dew hasn't fallen yet, so let's tinker on I guess...

Paul

[bw_msgboard]

Ok, I see.  Even so, that looks like a considerable distance from the scope backplane to the camera sensor.  The optical design may be like SCTs wherein the effective focal length/ focal ratio depends on the distance from the secondary to the focus position.  With mass-produced SCTs, the quoted focal length is usually based on a nominal focus position, some value chosen by the factory.  Of course, most of these SCTs have movable primary mirrors but yours may not.  Bottom line, you can believe the plate solve results over the advertising. J

 

Bruce

 

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From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Paul Porters
Sent: Saturday, June 22, 2019 8:32 AM
To: Open PHD Guiding
Subject: [open-phd-guiding] Re: Evaluation please

 

Yeah, you're absolutely right there, garbage in = garbage out, that's a truth on almost every subject. Indeed, I should have discarded that one. All part of the learning curve I guess.

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