11.01
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Tonight was probably my best night setting up my backyard imaging session. It went like this: * Took out the tripod, pointed it close to north and used a spirit level to get it flat * Added the mount and tightened it down tight * Added counterweights * Tube rings and OTA added and tightened down * Balanced the whole thing in 3 axes * Autofocussed on polar region * Used Sharpcap to polar align and got excellent result within about 10 mins * Slewed to meridian in SSW using Stellarium and APT where I have clear sky and did PHD2 calibration * Used APT to do a platesolve * Used APT to goto M106; it missed it by a mile but that was fine * Platesolved again and did a goto M106 with APT ... it ended up pretty centered in the view * Grabbed my previous night's images of M106 and platesolved that and used Goto++ in APT to point at exactly (well within 24 pixels) the same point as my previous session * Started my capture sequence All-in-all: It's still painful having to take my scope out every few nights, but Sharpcap polar alignment, platesolving and autofocusing have made the whole thing so much simpler; learning how to use those three things will make anyone's experience a lot better. |
7.75
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Thank you for sharing your experience and congrats on encountering a clear night. I'm still a bit puzzled about about this "balancing in three axis" statement which popped up in another thread already. In my humble understanding with an equatorial mount we have a Dec axis and an RA axis. We achieve balance in the Dec axis by placing the centre of mass of our OTA (with whatever we may have attached to it) right on the Dec axis. Balance in RA is achieved by positioning the appropriate counterweight on the counterweight bar in a way, that the centre of mass of OTA + attached stuff + counterweight + counterweight bar is centred on the RA axis. Now I understand that it makes perfect sense to ensure that the centre of mass of OTA + attached stuff + counterweight + counterweight bar + mount + whatever may be attached to the mount is right in the centre of the triangle spanned by the tripod, in order to avoid your setup to trip over. Is this what you mean by "third axis", or is there something else to care about for balance? I'm still undecided about auto focus. I fully understand its importance for long focal lengths where proper manual focus becomes increasingly difficult. I understand too, that it's convenient to have.On the other hand I'm currently operating with focal lengths up to max 400mm and didn't yet have the impression that it's really a necessity. What focal length / images scale do you believe an auto focuser is a must have? Clear skies Wolfgang |
10.16
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I'm still a bit puzzled about about this "balancing in three axis" statement which popped up in another thread already. As far as I understand, the third axis is the longitudinal axis of the telescope. If you have a weight hanging on one side of the telescope (e.g. an autofocuser or a camera on a Newtonian), it may be useful to balance this with a counterweight on the other side. Otherwise you will get an imbalance in your setup due to the rotation, which will disturb your tracking accuracy. TTo your second question: Whether you need an autofocuser or not, only your pictures can tell you. If the stars are nice and small and round, it is not necessary. If you have thick stars or stars with colorful rings, you are constantly or temporarily out of focus. Then an autofocuser can be the rescue. When I take wide-field shots with a photo lens, I focus by magnification on the display and by hand. At medium FL, I use a semi-automatic focuser that can turn the ring finer than I can by hand. After that, the setting stays for the rest of the night. This works well up to 200mm (I don't have longer FL with lenses). My next FL is 560 on the telescope with reducer. There is but as well as an autofocuser on it, so I use that. |
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7.22
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I'm still undecided about auto focus. I fully understand its importance for long focal lengths where proper manual focus becomes increasingly difficult. I understand too, that it's convenient to have.On the other hand I'm currently operating with focal lengths up to max 400mm and didn't yet have the impression that it's really a necessity. What focal length / images scale do you believe an auto focuser is a must have? Acually, isn't the focal length that matters but the focal ratio. That is, in terms of depth of focus. Pixel scale is the other matter of concern. So, whether you need an auto/motorised-focuser (apart from the obvious mental crutch the gizmo is, frees you from having to do it yourself and having to blame oneself when you get it wrong) or not depends on these two parameters. My very long focal length telescopes, for example, are a breeze to focus (and they stay that way) because they are f/10 and over. Even the f/6s are a doodle. It is when I use the 300mm f/2.8 lens that it really gets tricky. F/4 is doable but sensitive to night to night changes. Even having the dew-heater on makes a difference over the hours. Funnily enough the only piece of kit that would benefit from an auto-focuser hasn't got one 'cause there aren't any to be had. One day I might design and make one though. |
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7.22
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Olaf Fritsche: The longitudinal axis of the telescope is normal to both RA and Dec axes and as long eveything is symmetric about both of them there is no inbalance, assuming you did balance both axes by shifting the tube+craddle fully dressed up beforehand for the Dec balancing and by adjusting the RA counterweight. The only other degree of freedom there is the rotation of the tube around the craddle axis (assuming you have one, obviously) which is what folk refer to as the third axis. Which isn't really an rotational axis at all, from the point of view of the mount. |
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11.01
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andrea tasselli: I probably should not have called it a 3rd axis. I find that with my 8" reflector (where all the camera, filter wheel, autofocuser and coma corrector all stick out at one end of the tube in different directions) it is quite important to rotate the OTA in it's cradle to minimise any imbalance. To be honest, I think I will just mark that rotation point with some tape and then just always align to that. |
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7.75
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Andy Wray:andrea tasselli: Thank you for the clarification, makes a lot of sense now. Clear skies Wolfgang |
