32 replies to this topic

[gumbajoe]

I am posting in the experienced deep sky forum as I though it would be more likely to have members with rotators on their refractor rig. I can't seem to find forum posts on this topic, sorry if this topic has already been covered.

 

I am setting up a new rig consisting of an AP 1100GTO mount, SVX152 refractor with a Moonlight rotator / focuser, Stellarvue field flattener, ZWO OAG-L with a ZWO ASI 174mm, ZWO 7x50 EFW, and a ZWO ASI 6200MM.

 

My Esprit 100ED refractor rig also has an EFW and OAG, so the optical train is not symmetric in weight distribution (relative to the optical axis) as the EFW and OAG-L/guide camera stick out to the side. I don't have an electronic rotator on my Esprit rig, and don't rotate the optical train manually for different targets. I achieve third axis balance by adding a small weight to the top Losmandy plate. As the EWF/OAG stay fixed in position relative to the mount, the third axis stays in balance once set correctly.

 

On my new rig with the Moonlight rotator focuser, everything behind the rotator rotates together (i.e. field flattener, OAG, EFW, camera). I have not used an electronic rotator before.

 

The optical train behind the field flattener on the new rig with the rotator is similarly asymmetric in weight (relative to the optical axis) due to the EFW and OAG-L / guide camera. It seems that if everything behind the rotator changes position when rotating to different angles between targets, any precise 3rd axis balancing done in a different rotation position would no longer be correct in a new rotation position (the EFW/OAG-L would be rotated to a different position, changing the third axis balance point). I do automated imaging and don't plan on going out to the rig to tweak 3rd axis balance between targets captured at different rotation angles.

 

It seems the only solution would be to place an offset counterweight (relative to the optical axis) on the optical train behind the moonlight rotator to counterbalance the weight offset caused by the EFW/OAG-L. This "sum" balance with a counterweight correction should be the same regardless of rotation position, so any additional counterweight placed for 3rd axis counterbalance should remain correct regardless of rotation position. I have never seen this done in practice.

 

How do others with rotators and EFW/OAG deal with 3rd axis balance? Do people with Moonlight rotators just do what would be the "average" third axis balance on the Losmandy / Vixen plate and call it a day? Just ignore it?

 

Thanks!

[aaube]

I was worried about that too.

 

I have an Esprit 100 on top of an Esprit 150.  Both sitting on a Mach2 and both

with F/W, rotator, oag, etc.

 

I balanced as best i could in Dec and Ra but had to average for the 3rd axis.

 

It took some trials and error to get an optimal balance for both sides of the meridian though.

It's been running that way for over a year, with similar results with respect to guiding performances/sub frames quality.

 

I believe that if you can control (as much as possible) the effect the 3rd axis has on your dec balance, you'd be good

to go.  RA seems to tolerate more, even more so in your case with an AP1100.

 

HTH

[gumbajoe]

Thanks for the response aaube. From your experience it sounds like this may be more a theoretical problem than a practical problem, I appreciate the reassurance.

[WadeH237]

Your AP1100 is not sensitive to balance, within reason.

 

Just put your rotator into its home position (wherever that is) and balance it that way.  Your results will not be affected at all if the rotator turns to a different position.

[gumbajoe]

Thanks for the advice WadeH237, nice to have a data point from someone using the same mount!

Edited by gumbajoe, 23 April 2025 - 01:23 PM.

[KGoodwin]

The AP1100 is not sensitive to balance in the third axis for a small offset like this unless you're using mount modeling in APCC Pro/APPM to do unguided imaging.  I found that I did need to counterweight my filter wheel offset to get good results unguided at 0.31"/pixel.  With guiding on the AP1100 it's totally unimportant for the amount of weight offset a filter wheel and OAG cause.

[rgsalinger]

Kyle is spot on about this but consider using the rotator as a balancing weight. I have the moonlite night crawler on my Planewave L350 mount. To perfect the balance I set the zero rotation point at a offset to being orthogonal to the mount axes. Then I offset the camera so that it and the guide camera ARE orthogonal. Works great and it really doesn't compromise my ability to use the rotator. Of course, when I use it, I'm back to being a bit out of balance. I don't see how you can ever have a perfectly balanced system with the Moonlite NiteCrawler as the balance will shift if you rotate it. No idea about other rotators.  

[gumbajoe]

Thanks for the reply KGoodwin: I have not had the rig under the stars yet, but I thought it might be nice to have the option to do unguided imaging. My OAG is on the side opposite the EFW, and the OAG+guide camera weigh less than the EFW. Did you add more weight to the OAG in order to balance? I am unsure how to do this other than wrapping something by heavy around the OAG helical focuser: there are not a lot of places to attach counterweights on the optical train on the side opposite the EFW.

[WadeH237]

The AP1100 is not sensitive to balance in the third axis for a small offset like this unless you're using mount modeling in APCC Pro/APPM to do unguided imaging.  I found that I did need to counterweight my filter wheel offset to get good results unguided at 0.31"/pixel.  With guiding on the AP1100 it's totally unimportant for the amount of weight offset a filter wheel and OAG cause.

One of my AP1100's has the absolute encoders and I only shoot unguided with it.  My wide field scope can't move enough in the saddle to balance in declination, so it's always off balance by more than what's described here.  I use APPM and APCC Pro to create and use a declination arc tracking model and I have no problems.

 

I have used my non-encoder AP1100 with the same scope, but guided with an OAG.  It guides to a fraction of an arc second, even with an imbalance in declination.

 

Seriously.  Don't worry about it.

[KGoodwin]

One of my AP1100's has the absolute encoders and I only shoot unguided with it.  My wide field scope can't move enough in the saddle to balance in declination, so it's always off balance by more than what's described here.  I use APPM and APCC Pro to create and use a declination arc tracking model and I have no problems.

 

I have used my non-encoder AP1100 with the same scope, but guided with an OAG.  It guides to a fraction of an arc second, even with an imbalance in declination.

 

Seriously.  Don't worry about it.

Unless you're trying to go unguided at long focal length, it isn't a concern, I agree.  Every little tiny thing matters when you're trying to get 0.31"/pixel to work unguided for 15 minute subs, though!

 

This is how I balance rotationally: https://rouzastro.co...for-telescopes/

For the Z-axis, I use the 1/4"-20 threaded screw holes on the back of my CDK to mount a small weight on the lighter Z-axis side to balance it.

 

If you have shorter focal length/less demanding plate scale than I mention it's not important, even if you're going unguided.  If I were using my AP130 instead of the CDK on the 1100 I wouldn't worry about it, even for unguided (0.89"/pixel plate scale).

[WadeH237]

Unless you're trying to go unguided at long focal length, it isn't a concern, I agree.  Every little tiny thing matters when you're trying to get 0.31"/pixel to work unguided for 15 minute subs, though!

My hat is off to you.

 

15 minutes unguided at 0.31 arc seconds per pixel is a crazy tall order.

[KGoodwin]

My hat is off to you.

 

15 minutes unguided at 0.31 arc seconds per pixel is a crazy tall order.

It took a while, but average eccentricity now is just under 0.3 and I typically have FWHM that are essentially exactly what you'd expect based on the current seeing and the aperture size, so it's solid.  I didn't necessarily expect success on this when I purchased the mount, so I have an OAG mounted in my imaging train and just viewed going unguided as a personal challenge.  I've tested several times now and unless seeing is excellent the unguided results are superior to the guided ones since there's no guider bouncing it around based on the seeing.

[ngc2218]

It took a while, but average eccentricity now is just under 0.3 and I typically have FWHM that are essentially exactly what you'd expect based on the current seeing and the aperture size, so it's solid.  I didn't necessarily expect success on this when I purchased the mount, so I have an OAG mounted in my imaging train and just viewed going unguided as a personal challenge.  I've tested several times now and unless seeing is excellent the unguided results are superior to the guided ones since there's no guider bouncing it around based on the seeing.

Quick question .. do you measure ecc with CCDinspector or PI?

 

To the OP, I go unguided with a 10M mount, and I had to add small counterweights to the camera in order to offset the FW on the Moravian C3. Once done I was able to balance in 3 axes just fine.

I balaced the camera by placing it on a metal rod with the center of balance on the sensor, and kept adding weight oposite to the FW until it was reasonably balaced. 

 

If you guide, I think the guider will take care of it

Edited by ngc2218, 23 April 2025 - 06:01 PM.

[gumbajoe]

ngc2218, thanks for sharing your solution to this problem.

 

 

To the OP, I go unguided with a 10M mount, and I had to add small counterweights to the camera in order to offset the FW on the Moravian C3. Once done I was able to balance in 3 axes just fine.

I balaced the camera by placing it on a metal rod with the center of balance on the sensor, and kept adding weight oposite to the FW until it was reasonably balaced.

 

I have doubts of my abilities to recreate this solution from items found while wandering the aisles of my local hardware store: exactly how did you attach weights to the camera to achieve balance? Velcro attached to the camera and to the weights? Some kind of band which encircles the camera, under which weights could be placed? 

 

Did you apply the weights to the side or back of the camera?

 

What material did you use as a counterweight? Metal washers always seem to be the default for such a purpose, but they are a bit of an eyesore.

 

Thanks again to all who have responded.

[KGoodwin]

Quick question .. do you measure ecc with CCDinspector or PI?

I’ve done both, as well as checking the values measured by HocusFocus in NINA on the subs (which get recorded in the Target Scheduler database).

[kevinkiller]

It took a while, but average eccentricity now is just under 0.3 and I typically have FWHM that are essentially exactly what you'd expect based on the current seeing and the aperture size, so it's solid.  I didn't necessarily expect success on this when I purchased the mount, so I have an OAG mounted in my imaging train and just viewed going unguided as a personal challenge.  I've tested several times now and unless seeing is excellent the unguided results are superior to the guided ones since there's no guider bouncing it around based on the seeing.

Please describe in as much detail as you stand exactly how you've been able to achieve this result?

I'm looking to run un-guided at 0.3 rms with my TOA-130 on my Mach2 and have been very unsuccessful.

[rgsalinger]

How do you equate CCDI aspect ration results with PI eccentricity results? I consistently get much better apparent eccentricity when using FWHME in PI when compared to CCDI's Aspect Ratio calculation. 

 

With my CDK14 on an L350 mount, we end up at just under .4 most nights with seeing at around 1.8", and that's guided. Unguided just doesn't work with our system.

 

Where do you image from? 

 

I don't really think that a consistent value under .3 is possible unless the skies are very very good. 

Edited by rgsalinger, 23 April 2025 - 09:40 PM.

[psandelle]

Although I go unguided with my Tak E-160 on my 10Micron and everything seems fine, I am going to experiment with a Moravian centerline filter wheel when the new one comes out, and remeasure eccentricities then to see if I can get better. The FW should balance itself that way. I, too, am not good with handbuilt kludges, having failed arts & crafts in kindergarten. 

 

Paul

[Broz22]

I use the same RouzAstro camera balance as KGoodwin with my CDK14, QHY-L filter wheel, Sagitta OAG, and QHY600m camera on a Planewave L-350. It works well throughout the rotation angles. And, in conjunction with the Rouz dovetail balance kit, allows me to easily rebalance through various image train configurations without the need to redo the mount motor tuning. These 2 pieces of gear really made things a lot easier and faster for me.

John

[KGoodwin]

Please describe in as much detail as you stand exactly how you've been able to achieve this result?

I'm looking to run un-guided at 0.3 rms with my TOA-130 on my Mach2 and have been very unsuccessful.

I started the effort by ensuring the complete rigidity of the entire system, upgrading the CDK Hedrick focuser to a Gemini focuser/rotator, using AP 2.7" extensions to it to ensure the imaging train stayed rigid (they're both large in diameter and thick-walled), and rotationally counterweighting my imaging train precisely using Rouz Astro's system.  I installed the top dovetail to both enhance the tube rigidity and allow me to mount my power and USB hubs on the scope centerline.  I routed a USB and power cable through the mount and mounted the scope.  I ensured all cables were well routed and secured such that the rotator can rotate freely without any cable snags and the cables cannot snag on any other part of the mount.  I then z-axis counterweighted the scope using small 1/4"-20 threaded weights and the spare mounting holes on the back of the CDK.  I balanced the scope precisely in both RA and Dec by disengaging the worm from the worm wheel (I don't think this is possible with the Mach2 as it is on the 1100) and with the weight as close to the top of the counterweight bar as possible by using extra weights.  I configured APCC Pro to utilize my weather station for temperature/pressure/humidity.  I built a fairly dense all-sky model using APPM, enabled it in APCC Pro, and turned on dec arc tracking mode.

 

Probably the most critical part is the part that isn't mentioned here: make sure the scope and the mounting rings/dovetails are extremely rigid.  Everything is in the observatory where it is mostly sheltered from the wind by the walls (and my location has very little wind at night, anyhow).

 

I'm happy to answer any specific questions you have.

[KGoodwin]

How do you equate CCDI aspect ration results with PI eccentricity results? I consistently get much better apparent eccentricity when using FWHME in PI when compared to CCDI's Aspect Ratio calculation. 

 

With my CDK14 on an L350 mount, we end up at just under .4 most nights with seeing at around 1.8", and that's guided. Unguided just doesn't work with our system.

 

Where do you image from? 

 

I don't really think that a consistent value under .3 is possible unless the skies are very very good. 

Here's the method of converting from the aspect ratio in CCDInspector to the eccentricity given by PI or NINA (HocusFocus) which use the same calculation:

 

https://pixinsight.c...ty_Eccentricity

 

"Eccentricity is a measure of star profile distortion. Given an elliptical star profile with major axis diameter a and minor axis diameter b where a is greater than or equal to b,
the star profile eccentricity equals (1 - b2 / a2)0.5, the star profile aspect ratio equals b / a and the star profile flatness equals a / b - 1. A distortion with an eccentricity
less than about 0.42 is not perceptible to most people."

 

I image from Deerlick Astronomy Village in Georgia, which is about an hour west-northwest of Augusta.

 

I assure you that my skies are very frequently not good at all.  Our seeing ranges from very rarely 1" to 4.5" or so, which a typical value in the summer of a bit under 2" and in the winter of a bit over 2".  You can choose to think whatever you want about whether it's possible or not, but I'm doing it.  You also say that unguided "doesn't work with your system" which could have the same root cause as not achieving lower eccentricities.  Eccentricity just under .4 is good enough, regardless.  Typically my eccentricity is a tiny bit better when seeing is bad since it blurs the FWHM to be larger and thus covers up any eccentricity.

[rgsalinger]

No offense was meant, just guilty of an awkward turn of phrase.

 

The link you posted, though, gets a 404.

 

 

Your response is interesting. I have been wondering for a long time how the seeing/FWHM in long focal length image affects (or does it at all) the eccentricity. I'd be curious to see the eccentricity of some images showing under 2" in PI. Maybe I have better seeing out in the desert than you do. If I see my CDK14 images creep over 2.5" (3-5 minute) guided exposures on an L350 mount, I either stop imaging (if awake) or just discard them in the morning. I run each night's data through Subframe Selector to do the culling.

 

Here's an unculled example of a target that only gets to 60 degrees of altitude, so the overall run wasn't really long enough for a great picture. Still, the data is pretty good. It's a tad under sampled at .61"/pixel if you think htat matters.  

 

 

Looks to me as if there is a distinct inverse correlation in this run between FWHM and Eccentricity. 

 

Care to share a run? 

 

 

[KGoodwin]

No offense was meant, just guilty of an awkward turn of phrase.

 

The link you posted, though, gets a 404.

 

 

Your response is interesting. I have been wondering for a long time how the seeing/FWHM in long focal length image affects (or does it at all) the eccentricity. I'd be curious to see the eccentricity of some images showing under 2" in PI. Maybe I have better seeing out in the desert than you do. If I see my CDK14 images creep over 2.5" (3-5 minute) guided exposures on an L350 mount, I either stop imaging (if awake) or just discard them in the morning. I run each night's data through Subframe Selector to do the culling.

 

Here's an unculled example of a target that only gets to 60 degrees of altitude, so the overall run wasn't really long enough for a great picture. Still, the data is pretty good. It's a tad under sampled at .61"/pixel if you think htat matters.  

 

FWHM_ECCNTR Draco Trio 2025-04-24 154832.jpg

 

Looks to me as if there is a distinct inverse correlation in this run between FWHM and Eccentricity. 

 

Care to share a run? 

Yes, there's definitely an inverse correlation, and mine looks just like yours in that respect.  I suspect you probably do have better seeing than I do a lot of the time, but I also cull at 2.5" (or lower if I'm going for a high detail target like a small PN).  Because I use Target Scheduler and have a lot of targets in flight at once I never get a long time in a single run on a target, so I have to combine multiple targets to see the correlation the way you present it over a night, but it's the same exact situation.  When I guided the slope of the curves was greater (so things got worse in FWHM and better in eccentricity faster when seeing got bad.  Unguided it's not as steep because the seeing doesn't affect the tracking at all, but when the seeing is excellent the results are exactly the same guiding or not guiding.  When the seeing is not as good unguided is a little bit better and when the seeing is bad unguided is a lot better, but it doesn't matter because I throw them out anyhow.  I let everything run automated all the time, so I never stop it, and frequently the seeing varies a lot over the course of the night and even in different areas of the sky.  I just sort it out later when I cull.  Since the tracking is excellent I'm really only culling if something happens outside the observatory like seeing is poor, a low flying airplane obliterates a sub, something like that.

 

Unfortunately it seems PI took down their doc page for SubframeSelector after I copied that link, but I quoted the relevant portion anyhow and the full docs are available inside PI if you're interested.  I can't seem to find where they moved it to online, sorry.

[ngc2218]

ngc2218, thanks for sharing your solution to this problem.

 

I have doubts of my abilities to recreate this solution from items found while wandering the aisles of my local hardware store: exactly how did you attach weights to the camera to achieve balance? Velcro attached to the camera and to the weights? Some kind of band which encircles the camera, under which weights could be placed? 

 

Did you apply the weights to the side or back of the camera?

 

What material did you use as a counterweight? Metal washers always seem to be the default for such a purpose, but they are a bit of an eyesore.

 

Thanks again to all who have responded.

Like this (post 167) https://www.cloudyni...5-camera/page-7

[ngc2218]

How do you equate CCDI aspect ration results with PI eccentricity results? I consistently get much better apparent eccentricity when using FWHME in PI when compared to CCDI's Aspect Ratio calculation. 

 

With my CDK14 on an L350 mount, we end up at just under .4 most nights with seeing at around 1.8", and that's guided. Unguided just doesn't work with our system.

 

Where do you image from? 

 

I don't really think that a consistent value under .3 is possible unless the skies are very very good. 

What focuser do you use? It took me a while to sort this unguided business out. Just yesterday I put some wide shimms (0.5 mm aluminum plates) under the top rings (removed the felt) so they grab the scope better (wider surface area). I suspected some movement after I upgraded to a Gemini focuser, and seeing that in some positions the errors in the model are significantly highier. 

The results were good. I usually get 4-6" RMS model (90 points), yesterday I got 2.3" RMS. 

After that I had a bit under an hour of clear skies and the aspect ratio in CCDinspector was pretty constant with very few hiccups. 

Browsing the 10M forum, it seems the mount takes a while to settle, so next night I will test putting some 10-20 sec delay after a dither.

 

Exposures: 2 min

FL: 1333.5

PixelSize: 3.76 

10M 2000 HPS 

CFF 8 inch frac

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Edited by ngc2218, 25 April 2025 - 04:27 AM.