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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 |
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Hi Jérémie, It's common to add weights to the front of the telescope, I do the same thing with my SCT. There are dovetail counterweights, which is simply and dovetail clamp (losmandy for example) and a counterweight, you just slide back and forth to find the exact balance. Some companies even do them with side mounted counterweights, if you're problem is in that axis Clear Skies - Michel |
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Jérémie: 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. |
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Jérémie: Hi Jérémie, can you please explain what do you mean "change over night"? Since if both axes are balanced for One position they are balanced for every other position too and there is no adjustment required. Or do you change setup? |
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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 |
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Michel Makhlouta: Fantastic ! I have to look at that then ! Many thanks ! |
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Jérémie: 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. |
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Bradley Watson: Thanks ! I’ll check that. Problem solved thanks to you all again ! |
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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 |
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I hate to place dead weight on top of the saddle unless absolutely necessary. I also thought about it. The OAG isn’t very heavy (and the guide camera is a « mini » from zwo). Most of the weight is on the cooled camera and filter wheel (that I consider less and less as useful, as it is not precise enough to take flats after having turned it many times...). But it has the advantage to put useful weight more on the front of the RC. I switched from guidescope to OAG thinking it was more precise on the guiding. Wouldn’t I lose something ? Maybe not considering that above 1600mm FL, seeing is the limiting factor and that what you gain in terms of guiding error can’t be seen due to seeing being overwhelming. Was that your thought in abandoning OAG ? |
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Jérémie: 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 |
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I have an 8" RC and also find the scope camera-heavy. I use a dovetail counterweight adapter that you can hopefully see in the attached image. It serves 3 purposes. First, obviously, it balances the camera-heavy side of the scope. Secondly, it acts as a stop in case the scope decides to slide off the plate. And third, because I can position the weights anywhere along the threads, I can position them closer to the center of the mount so the counterweights on the counterweight bar are also closer to the center of the mount. Ron  |
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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   |
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Very interesting @John Hayes , it reminds me some courses many years ago :-) Indeed it totally makes sense, that’s basic engineering but I completely missed that in my reflections. I was focused only on balance in all axis, but not on the other aspects of the problem (inertia, damping, resonance frequencies) I will try to optimize my setup keeping that in mind. Thanks for the heads up ! |
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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 :-( |
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With the picture |
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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. |
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Hi @Rod Kennedy thanks for your feedback and tips. Is your guidescope f/5 (ig 60mm / 300mm FL ?). Is that sufficient for guiding at full FL on the RC8 (1624mm) ? Maybe f/6 ? Concerning the deconvolution : I already deconvolved the image using a PSF made from the smallest star (dynamic PSF), and made around 75 iterations. How did you achieve to get the biggest star round like that ? That’s magical ! Maybe I wrongly use the deconvolution tool in PI ? Thanks again ! |
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(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:
Examples of Main Scope/camera configurations:
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:
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:
And it is not just stars; when done properly it pulls out much hidden detail; as in my recent RC8 Sombrero Image Rod |
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Jérémie: Jeremie, So first off, that’s not completely bad news. With a perfect mount that’s perfectly polar aligned, the DEC motor should never run. So let’s think about why it might be running it the first place and then let’s look at the mount to see if there might be a mechanical issue. First, DEC guiding will run when PA isn’t very good, when you have cross-coupling between RA and DEC signals, or when you have the DEC sensitivity turned up way too high. The required PA accuracy depends on your exposure length and your guiding point and when there’s enough of an error, you’ll see circular guide errors centered on the guide point. I don’t see that in your data so that’s probably not the problem. Cross coupling between the guide signals from the two axis can happen for a lot of reasons. First, make sure that you calibrate your guider. Be sure to watch how the stars move during calibration and you’ll be able to spot backlash, which can introduce big errors. I personally prefer to carefully align my guide camera so that the X-Y axis of the sensor is registered with the RA and DEC axis. In principle, you should be able to orient the sensor in ANY direction and the guide software should sort it out; but, aligning the camera reduces chances of small cross-coupling errors. You can do a simple test to see what’s going on by simply turning off DEC guiding to see how well the system runs. If your mount is well aligned, you should see the system track for somewhere between 1-5 minutes with virtually no error in DEC. If you see that happen, that tells you that you can turn down your DEC response to a very low level. BTW, you can look for cross-coupling in your guide signals by comparing the response in both axis. If both axis are consistently doing the same thing over time, you’ve got a cross-coupling problem. If they appear to be independently random, you are good to go. Mechanical errors in the mount can lead to your problem as well. One of the big issues is mechanical backlash in the drive system. You can easily feel backlash if you turn off the mount and carefully wiggle the scope about the DEC axis to feel for play. You can also measure backlash by taking an unguided image and moving the DEC axis back and forth in a saw tooth pattern to see how the star trails in the image. Some scopes have a means to adjust that play to allow minimizing the issue. Otherwise, you have to add a weight to the scope to bias the drive system to touch the gears only on one side. You might also look at your software to see if there’s an option for backlash compensation. That might help with pointing but that parameter might not do anything to improve guiding accuracy. Reducing backlash and getting your feedback parameters set properly should go a long way toward improving your guide performance. Problems like yours are usually not due to a single cause so it takes a little patience to track down and minimize the issues that contribute to the problem. In general, having a problem with the DEC axis is usually easier to resolve than a problem with the RA axis. John |
