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Hello fellow RASA8 users, I have a problem with guiding my RASA8 and would like to kindly ask for your support as I am not able to solve my problem by myself. The problem that I have is that I get elongated stars with 5 minutes subs. At 2 minutes subs - I think that everything is ok. I am using an iOptron CEM 70 with William Optics UniGuide 50mm Scope and an Asi120MM mini. The UniGuide scope is mounted on top of the RASA 8 on a second dovetail rail which I installed. The guidescope is mounted through an ADM clamp which holds it firmly in place. The ASI120mm is firmly locked in place with the rotolock of the scope. No cables are freely hanging from the cameras and equipment. With all of the above I tried to lock my equipment as firmly as possible in order to eliminate any differential flexure. Actually I made the upper dovetail / ADM clamp upgrade yesterday hoping to solve the problem with elongated stars. Unfortunately it didn't help  I checked my frames from yesterday. Everything is ok with stars till the moment when the imaged object rises above 75 degrees (comes close to the meridian). Then I get elongated stars on 5 minutes subs. Things improve when the object descends below 75 degrees on the other side of the meridian. I read that the best solution for dealing with this problem on an SCT is to use off-axis guiding. However this is no good with the RASA. Considering the above I want to ask you to share your experience: - whether you have the same problem; - whether you have found a solution to it and if "yes" then what is the solution  The solution that I am thinking right now is to switch to 2 minute subs when I approach the 75 degrees mark and then switch back to 5 minutes subs after that. Ah yes - I am using 5 minute subs in order to image some quite dim objects like the SH2-91 veil nebula. Thank you for your support in advance! CS, Ivan |
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Hi Ivan There are so many factors that could be at fault here. I would do/check the following: * Guide scope with longer focal length and bigger apperture. Be careful not to overexpose guide signal as PHD then has a hard time finding the center of the guide star. * Careful setup of PHD2 and have attention on not over correcting, remember you can't guide on the seeing. This often means that agressiveness should be lover and min move values higher than what you think. Sometimes you just have to accept bad nights with bad seeing/guiding. It took me quite a long time to learn to read the guiding curves and adjust accordingly. But being able to read the curves are essential. * More careful polar alignment, how do you do that? I also have a CEM70 and I use NINA for polar alignment, which works just fantastic. Try to expose without guding or guide with PHD agressiveness settings set to very low or zero and use this as a platform for tuning your setup and polar alignment. You should be able to expose for at least a minute without guiding and have rreasonable round stars in fair seeing. * Wind and vibrations can cause elongated stars, especialy wind gusts can ruin guiding. Do you have stable surface for you tripod? Hope this helps / Steen |
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Thank you for your suggestions Steen. The focal length of my guide scope is 200 mm vs 400 on the main scope. I use 3s for guiding. The guide star is not overexposed - I have a nice sharp curve on the top. I think the min move is set to 0.1 but I will have to check. I do the polar alignment with SharpCap through my guide scope. I think I have set the aggressiveness of the guiding to 70%. What value would you suggest? I use predictive pec for RA and for Dec - the standard one (I don't remember the name). The surface for the tripod is as stable as possible - on the 6th floor in the block of flats that i live in. Otherwise I have a windshield for my setup. CS, Ivan |
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What does your guiding graph / RMS look like? I have a GEM45. I use predictive PEC for RA and Hysterisis for DEC. I usually hit 0.5” avg on RA and 0.4” on DEC. I had trouble doing 600s subs, but 300s works well. My eccentricity hovers around 0.55, which gives me almost perfectly round. Also I use 1.5s exposure for guiding. |
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Hello rgenier, My guiding values are similar to yours. And eccentricity is close to yours on average. CS, Ivan |
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| Since the issue happens above 75 degrees - perhaps your balance? |
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Hi Ivan! One of the reasons why I am not using the RASA for quite some time anymore was your problem. Ok, not the only one, but that bothered me the most. Every time the object rises above a certain degree, "guiding" seemed to get much worse. So I looked at the guiding graph and did not see a change in RMS at all during that time. Did you see the same? This lead me to the conclusion that there must be some flexure somewhere. Since my setup is very similar as yours I did not think that this had something to do with the guiding scope. I think the answer came when I was outside when the scope went above ... yeah ... 75 degrees sounds about right, I heard a low click. The mirror was moving! So I listened more and yes, one could actually hear it once in a while. To be sure that you have the same problem watch the guiding graph when this happens. If you do not see any changes you can be sure that it is not your guiding. From what you answered to Steen I do not believe that it is. The parameters in PHD are not as important than a lot of people think. And your FL ratio between main and guide scope is more than good, for example, I use the 50mm (same guiding scope) with my 132FLT at 925mm, and my RMS is 0.2. And 3 seconds is perfect too, I use the same 😊. Do I see some elongation in the stars? Yes, very minimally though at 5 min, even with 10 min not too bad (not perfect though, which is ok since I replace the stars with 1 or 2 min RGB exposures anyway). Let me know what your guiding says during that time, I am curious now! Uwe |
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Hi Uwe, Thank you for your answer. I don't see any change in guiding when this happens. The graph is stable. That's why I think it is some kind of flexure. And this is my question - whether someone has found a fix. Tonight I will try to switch to 2 minute subs in order to avoid the issue near the meridian but it is a little frustrating Usually I am inside when the problem happens. Maybe I should sit next to the scope and see what happens. Below 75 degrees stars are round at 5 minutes subs. Cs, Ivan |
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Since the issue happens above 75 degrees - perhaps your balance? Balance is perfect. I can leave the scope in any position with unlocked motors and the tube is not moving. |
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Hi Ivan Looks like you are using the celestron focus motor. Are you refocusing throughout the night? How often are you refocusing? With the RASA at F2 the critical focus zone is 0.005" or 0.127 mm. This is a very small zone when shifted will cause eccentric stars. I have the celestron motor on my C9.25, but have had good luck with the ZWO EFM on my RASA8. I usually refocused every 0.5-1 hour or 2 degrees Celsius. -Brandon |
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Brandon Tackett: Thank you Brandon. I refocus every 60 minutes. CS, Ivan |
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Look up the recent image by @Chuck's Astrophotography title the E nebula. Note it is of dark nebulae. Using an RASA 8, his exposures were 15 seconds. Take advantage of your RASA and reduce your exposure times. At 15 seconds you won't need guiding, other than to stay relatively centered in your field of view. I too tended to expose for longer time with my RASA 11, but not 5 minutes. It is interesting that Chuck arrived at the same 3 hr integration time that I find to be optimal for many objects when using the RASA. More than one way to skin a cat! Otherwise, the fact that your guiding goes to pot closer to Zenith tells me that your mount is not up to the task or you have a balance issue. I find that my guiding "always" gets better closer to Zenith simply because atmospherics are better. Seeing is almost always better near zenith. But take advantage of your wide field and fast optics and shorten your exposure times. Stop worrying about your guiding. Do ensure your mount is sized properly and is not suffering some errors, mechanical issues or balance issues near zenith. |
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Dear Alan, Dear All, Thank you for your comments. I found a fix and it is similar to what Alan suggested. Thanks to the Holy Nina and its Advanced Sequencer yesterday I set the following plan: - take 5 minute subs; - 35 minutes before the meridian flip - start to take 2 minute subs; - 35 minutes after the meridian flip - again start to take 5 minute subs; This morning I checked the subs - all are with round stars so I am happy CS, Ivan |
