HOW TO : Balancing a RC scope on the mount ?

Jérémie:
Hi All,

I have a RC8 - pseudocollimated thanks to the help of the community - and I think I should really improve my guiding.

Among the things to improve that I have spotted is the balancing of my system : with the RC8 and the camera/filterwheel/OAG/ASIAIR, all the weight is on the back of the scope (primary mirror + imaging train).

Therefore, when the target has high Dec (+90), I have no problem in adjusting the weight on the RA shaft of the mount for whatever RA.

But for mid-Dec object (let's say the Cat's Eye Nebula at +66 in Dec), balance will change depending on time...

How to do you proceed to balance this kind of reflectors (where the weight is mainly on the primary mirror + imaging train) ?

Do you put additional weight on the front of the telescope ? (how do you fix it ?)

Do you just change the adjustment of the weight on the RA shaft during the night ? (with risks to lose PA if you shake the setup too much)

Any pictures are welcome to see how it works practically !

Thanks !!

PS : my mount is a CEM70

I have a cem70 also. This is no way to nicely balance the RC8 on it without counter weight or get some tube rings which I did with the 8 and mount the dovetail 5-6 inches past the back of the tube. 

the 10” is perfect out of the box btw.

Hi Jermie,

I had the same problem with my RC6, i solved it using a non traditional method, a boating weight. However its better to use a clamped weight as it's then easy to move along the dovetail as you change the image train. See link to weight for your scope
Baader 1 kg counterweight and V clamp combination (astroshop.eu)


I dont have a filter wheel only a draw but a 1kg weight should do the trick.

Hope this helps
CS 
Brad

Jérémie:
Hi All,

I have a RC8 - pseudocollimated thanks to the help of the community - and I think I should really improve my guiding.

Among the things to improve that I have spotted is the balancing of my system : with the RC8 and the camera/filterwheel/OAG/ASIAIR, all the weight is on the back of the scope (primary mirror + imaging train).

Therefore, when the target has high Dec (+90), I have no problem in adjusting the weight on the RA shaft of the mount for whatever RA.

But for mid-Dec object (let's say the Cat's Eye Nebula at +66 in Dec), balance will change depending on time...

How to do you proceed to balance this kind of reflectors (where the weight is mainly on the primary mirror + imaging train) ?

Do you put additional weight on the front of the telescope ? (how do you fix it ?)

Do you just change the adjustment of the weight on the RA shaft during the night ? (with risks to lose PA if you shake the setup too much)

Any pictures are welcome to see how it works practically !

Thanks !!

PS : my mount is a CEM70

I have a cem70 also. This is no way to nicely balance the RC8 on it without counter weight or get some tube rings which I did with the 8 and mount the dovetail 5-6 inches past the back of the tube. 

the 10” is perfect out of the box btw.

I would have loved to get the 10’’ (or more), but it had to fit under my bed (not joking) for storage... :-)
Yeah... living in Paris is all about compromises.

Hi Jérémie,

I had a similar problem with my SCT. A lot of equipment at the and and nothing at the front, so how balance in DEC?
My solution: get rid of the OAG. My effective focal length is max. 1550mm and I chose to place a guide scope on top of the SCT. With that I removed the OAG weight and place additional weight on the other side of the scope.

I hate to place dead weight on top of the saddle unless absolutely necessary.

Just as an alternative idea.

Cheers,
Björn

Jérémie:
Was that your thought in abandoning OAG ?

The reasons were manyfold to give the guide scope a try:
- With the DSLR's APS-C chip, it's rather easy to cover parts of the chip with the prism. Something I didn't like, even though flat frames can compensate for that. 
- With a guide scope it's much easier to find a suitable set of guide stars. With the OAG, the FOV of the guide camera is horribly small in my opinion. 
- When I turned to the mono camera and I had to use a filter wheel, the spacing w.r.t. to the reducer became a problem. There was too much stuff between reducer and chip that the working distance couldn't be met. 

However, there are some arguments for an OAG which shouldn't be neglected:
- depending on the angular resolution of your guide scope-based system, the guiding might be inaccurate to that extend that guiding errors will be visible in the image. And OAG gives a higher resolution of course. Nonetheless, it also depends what your mount can achieve.
- Flexture can be a problem for long exposures where the guiding is perfect but you still have star trails as the main OTA points to a different direction.

Hence, switching to a guide scope must be assessed and testet to make sure that it's working. Up to now, it seems that the possible issues (inaccurate guiding, and flexture) are under control in my system. It's not said this is automatically the case on another system.

Björn

First off, I'm not completely sure why having the scope heavy on the rear end is a problem.  Is your dovetail saddle so short that you can't slide the entire OTA assembly forward enough to come to balance?  A photograph would really help when you ask a question like this.

Allow me to try to clarify a couple of things that are very important about balancing and mounting a telescope in order to achieve maximum performance.  First, in order to achieve perfect balance over the entire sky,  the OTA assembly (telescope+camera package+whatever else you have attached to the OTA) needs to be carefully balanced about all THREE axis--RA, DEC and through the centerline of the OTA.  The last axis is the one that most folks miss.  Having the OTA balanced about its centerline means that it can be rotated freely without changing the overall balance of the whole system.  IF the CG of the OTA is offset from the center line, you can cheat by putting the CG precisely on a line formed by the DEC axis and still get the system to balance in all orientations, but that can be a challenge to find just the right orientation.  When the OTA is properly balanced, you can point the OTA at any point in the sky and it will stay there.  That is the ultimate test for perfect balance.

The penalty for not balancing the OTA on the mount is varying load on the motors as the scope moves and the potential for inconsistent guiding results depending on where the scope is pointed.  Sometimes you might want to bias the balance a little to keep the drive gears engaged on on side of the teeth but that's not a reason to start with a poorly balanced system in the first place.  Introducing a bias is better done with a small separate weight specifically chosen for a particular orientation.

The suggestion to hang weights on the the front of the scope can help to solve the problem and (in some cases) may be necessary; but in general, that's a bad idea.  Adding weight to the OTA introduces two undesirable side effects.  First it lowers the mechanical resonant frequency of the system.  A lower resonant frequency increases the chance of vibration when the system is mechanically excited with a wind gust or other small disturbance.  You always want to raise the mechanical resonant frequency by keeping the weight of the OTA as low as possible and by using heavy enough counter weights to keep their arm as short as possible.  The other reason that you don't want to be adding weight to the OTA relates to the rotational moment of inertia (RMI) of the system.  The RMI is what limits the acceleration of the mount in response to torque from the motors.  Lowering RMI ultimately contributes to improved guiding performance, lowers wear on drive components, and reduces wear and tear on the motors.  Of course, beefy mounts can power through a poorly balanced rig with higher than necessary RMI but keeping everything as light as possible is generally a good thing if you want to maximize system performance.

Here are a couple of pictures that illustrate the things that I've mentioned.  You can hear more about this stuff in my presentation on long focal length imaging at AIC 2022!  Be there or be square...

John

I just received today a 1kg weight I can put below the RC, on the Losmandy dovetail, to counterbalance the weight of my imaging train, as I can't slide the whole system sufficiently "up".

I will share here if there are any improvement in guiding thanks to a better balance, but it may takes time as clouds and rain are back….

Last night I had a test at 1624mm (no 0.67x reducer this time), with the ASI183MM Pro camera, and guiding was therefore REALLY important.

The RA guiding was correct but Dec was awful, almost twice of three times as bad as RA in terms or error : 0,3''-0,5'' in RA vs. 0,9'' or more in Dec, with very cyclic patterns in Dec. My stars are elongated / bad eccentricity due to that. When I stopped guiding and let the iOptron run alone, I increased my RA error, but my stars were at least rounder (biggers, but rounders….

I tried PEC, but I think it records only corrections in RA (iOptron CEM70). And I didn't noticed changes.

I post here the first results of the night, on M106, that is right above my head at the start of the night, from Paris. The RC8 was shooting straight up the sky, which should have stabilized it in Dec (weight below)… but it seems not :-(

Salut Jérémie

For me, a (cheap) 60mm guide scope balances my RC8 scope, which is rigid (fixed primary), so I don't use an OAG.  (Years ago when I used a SC for Deep Space I needed an OAG; mirror flop and all.)

You can see the guide scope is pushed well forward to balance.  



You can use a modest amount of deconvolution to reduce star eggs such as in your picture; I did a quick/rough example on your image (normally used on a stacked image not a subframe).



You are right that the seeing limits the resolution and all design choices.  

Rod

PS Brilliant info from John.

(Since I'm at your antipod (Australia) our comments are well out of sync.

First question (long answer I'm sorry)

The guidescope has focal length 240mm and therefore f/4.The camera is an ASI290MM mini (2.9µm pixels) and so the guidescope/camera runs at 2.49 arcseconds/pixel.

To apply corrections the developer of PhD2 states that the worst case is you can detect star movements of 1/5 pixel.  The guider sensitivity is therefore 0.5 arcseconds/pixel. So it operates at the lower end of the limits provided by seeing (0.67 - 2 arcseconds/pixel).  The ASIAir guiding would be much the same.

So to answer your question: yes this guidescope/camera is sufficient.

In fact it is sufficient for any focal length.  Your main imaging focal length will be limited by seeing (stars will be dancing around).  You are probably oversampling but that is not a problem.

To summarize:

• Make sure your guidescope/camera combination is around 2.5 arcseconds/pixel (or lower)
• Make sure your main scape/camera combination is around 0.67 arcseconds/pixel (or lower)

Examples of Main Scope/camera configurations:

• I run another RC10 at 1420mm (reducer down from 2000mm) and with a ASI294MM 2x2 (4.63µm pixels) so at 0.67 arcseconds/pixel (ideal).
• I'm also collecting data at 1x1 (2.315µm pixels) so at 0.335 arcseconds/pixel.  I really don't see an advantage (yet) and the image files are much bigger.
• My RC8 at 1220mm (reducer down from 1624mm) with a ASI1600MM 1x1 (3.8µm pixels) works at 0.64 arcseconds/pixel (ideal).
• I also have a ASI183MM; if paired with full FL RC8 then it'll be 0.30 arcseconds/pixel (oversampling by factor of 2).  I'm getting quite ok data running at 0.335 arcseconds/pixel. You can bin 2x2 but do it in postprocessing.

If you need a guidecamera then ASI290MM mini (2.9µm pixels) is an exceptional choice (I have 2).  Lots of guidestars anywhere you point when operating at f/4 and f/4.8 for me.  It would be happy with f/6 guidescope no problem.

Deconvolution

No need to be a magician. I'm not sure what, say PixInsight, is attempting when it starts with a measured PSF.  If you look at an imperfect image of a star there are 3 convolutions:

1. from the telescope aperture "diffraction limit";
2. from the atmospheric turbulence "seeing" and
3. from drift or tracking blurring.

Ultimately in deconvolution you can only fix the last one and I would do it by hand.

I always play with the Aspect Ratio and Rotation in Parametric PSF; in fact all 4 parameters; but you can follow this simpler version on YouTube
Fix egg shaped stars in PixInsight

So in PI Deconvolution(s) either:

• adjust Aspect Ratio and Rotation in Parametric PSF; as well as what you are doing or
• you can fix the egg stars using the Motion Blur PSF after the more basic Parametric PSF.

And it is not just stars; when done properly it pulls out much hidden detail; as in my recent  RC8 Sombrero Image

Rod