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Hello all, I recently upgraded the camera for my scope. Currently I have a RASA8 together with ZWO ASI2600 MC-Pro and a Starizona RASA8 Filter Holder. Last night was the first test of my current setup. I see some curvature on the edges and wanted to ask for your opinion. Is this normal in you setups as well and if not then I kindly ask for suggestions on how to improve. The RASA8 manual shows that the APS-C sized sensor is outside of the optimal performance of the RASA but is doesn't mention what nature of degraded performance to expect in the corners. I used CCD Inspector to measure my frames and it shows tilt between 1% and 10%. If I measure the total curvature of all frames then CCD Inspector reports tilt of 1%. Have you ever tried to collimate your RASA8 with the ASI2600? This camera is huge and it totally blocks the collimation screws. I guess the camera has to be taken off in order to make the collimation adjustments?  I am sharing an integrated image of my imaging session and kindly ask for your opinion: integration2.jpg |
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Hello Sektor, Sorry, i don't have the same camera. your image integration2.jpg shows round stars to the center right to the edge (right corner shows slight egg shape stars past the 80% zone) and also shows normal vignetting. According to you, CCD inspector shows acceptable tilt and off-axis abberation. Why do you think, your system requires adjustments? it appears to me, good to go! Beautiful target dead center... Just my opinion and I am no expert. clear skies R |
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I have not managed to get perfect stars in the corners with an APS-C chip and your image is about as good as I managed to get them. They can be fixed in processing to an acceptable level (for me at least). With regard to collimation I had to do it on one of my two RASA 8 while the other was fine from the factory. I did it by attaching a guide camera (ASI290MM in my case) so I could reach the screws (the RASA comes with a c-mount adapter for that type of camera). Worked fine. But I would only attempt to do it if defocused stars are not symmetrical (so don't fix something that is ok). It looks like you have a bit more vignetting than I have - I use 2" filters, maybe you use something smaller? Cheers, Göran |
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| I have the bigger brother of your RASA. If my stars looked like yours (and I only have M4/3“, albeit smaller pixels) I would not dare touching anything on the scope. The slight aberrations at the corners are, for me, more than acceptable on such a fast scope and I would fear messing with things could make them worse. |
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Hello Sektor, interestingly I face the same issue at the moment and already did some investigating on that issue - more or less systematically because the issues I experienced were of random nature. In the past I operated my RASA8 with an ASI1600mm Pro and always had pinpoint stars in all corners with an average FWHM of 2.2x Last Year I purchased the Starizona (RASA8) Filter Holder and the Artesky Filter Holder - both were back-ordered for a long time at the dealership and suddenly were shipped at the same time... Anyways, since the weather didn't cooperate all winter, I was only able to test with may ASI071MC Pro (17,5mm back-focus). The pictures looked terrible - very bad seeing, star elongation and vignetting all over. With the introduction of the new ASI2600MM Pro in March, I upgraded my equipment with this exact camera and additionally it's older color sibling, the ASI2600MC Pro. Right from the start I noticed elongation with both cameras on the RASA but none of it with a 70mm Refractor (with full FOV) So I ruled out camera tilt. Here are some other lessons learned, since I started to work on the issue more systematically: 1. wiggle-room The APS-C sensor size more or less covers the whole FOV of the RASA8 with some minor vignetting to be expected due to the RASA specification page. But since elongation sometimes is very bad in my pictures, this FOV-limitation might play a role, but certainly is not the only factor because it is not consistent. The retaining ring that holds the camera/Filter Holder leaves some wiggle-room for the Filter holder to shift a few mm in all directions. Unfortunately the ring does not force the Filter Holder to sit centered but allows for a slightly off-centered assembly. This certainly affects the focus (in my opinion) - so I try to "manoeuvre" the FilterHolder centered when tightening the retaining ring. I don't know if any off-center placement plays a factor here, but it certainly makes sense. Celestron/Starizona/Artesky should provide a retaining ring that ensures a centered Filter Holder... 2. back-focus when using filters The RASA8 has very tight back-focus, 29mm from the lens group and 25mm from the original camera adapter. Note that the protective window (clear and light pollution) with their 2mm thickness contribute around 1mm to the back-focus. (F divided by 3) If for example you remove the protective window, this leaves 26mm back-focus from the camera adaptor. Both, the Starizona and the Artesky Filterholder are designed to leave room for a camera with 17.5mm back-focus. The Starizona additionally requires to remove the protective window, otherwise it would not fit - The Artesky brand does not come with such information. So there already is room for error when for instance - using the Starizona without filter and without protective window. The back-focus would be about 1mm off at 28mm/24mm. - using the Artesky with filter and with protective Window. The back-focus would be about 1mm off at 30mm/26mm. I'm using Baader f/2 Filter - based on the manufacturers specification page, their glass is 2mm thick, which should perfectly compensate for a removed protective window. The Baader f/2 filter come with and additional .5mm plastic ring (compared to regular LRGB, which don't)- I never paid much attention to this ring - but now since I am investigating this, I wonder if that .5mm ring has some deeper meaning for the RASA8. Maybe the filter glass is not exactly 2mm thick. I have attached a picture I shot last night to show my vignetting on a single 180second sub from the ASI2600MM. Now that I look at this example, I also notice some elongation in the middle but still worse star shapes in each corner. sadr_elongated.jpg (interestingly) I also noticed that "Sadr" is not engulfed in this "Rasa"-halo. That' another issue I kept working on for months now. I wonder If my back-focus experiments have something to do with it. <-- one hot tip here: If using the shiny ASI camera in (bight) light-conditions, it is possible that the red aluminum reflects some light to bounce around, even when using a large dew shield. I used black velour to cover the reflective camera sides. This did some good but never eliminated the bad "Rasa"-Halo. 3. additional Equipment attached to the tube I'm running my Rasa with two additionally attached scopes mounted to the rear cell, using the mount holes. These scopes are tiny and light weight - the Celestron ASPA - 50mm guide scope The rear cell is very stiff, so I did never think about bending issues here - and still don't. But a couple days ago I experimented with a setup to use both ASI2600 at the same time and target. To do so I attached a 70mm refractor to the Rasa, using an additional Losmandy dove plate on top of the tube. The refractor, it's heavy 4-lens setup, with focus motor, the camera and some adaptors (filter wheel, rotator) runs at about 5 kg. Yesterday i ran this setup on NGC 7000. The mount slews the Rasa into a 90degree angle which leaves the refractor "hanging" aside the tube - pulling with it's whole weight. Again I did not pay attention that this might cause problems, since alle equipment always seems very solid. But in this case, the pictures where "off" on one whole side - the stars on the right side were showing a donut hole already. When I brought the right side back into focus, the left side was off. I initially tweaked the setup according to 1. and 2. and lost most of this very clear and precious night over it - that' so rare at my place, just thinking about it... I have to wipe away a tear. Only after I disassembled the whole setup and removed the refractor on top of the tube, the next sub I took looked much better. That means, the weight of the additional accessory on top of the Rasa affected the light path while bending the tube (I guess). Maybe you have attached a zoo of stuff to your tube and it might play a role. ---- OK, I hope this makes sense to you and helps to identify the issue. I'd appreciate any input into this issue myself, but I guess in the end it's the FOV of the telescope and it's limits to run an APS-C camera sensor Oh, two additional quick thoughts about the vignetting/elongation. - If dew shield is not attached properly and being slightly off or oval shaped, this might affect the light path - currently at my place, the summer nebulas are visible from around 2am just atop some trees. Similar to a misplaced dew shield, the trees - although not visibly affecting the picture - might obscure parts of the view - something I'll look into as soon as I get the chance... Take care and clear skies, Stephan |
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i think your results look fine. ( i have the same setup on my rasa8). i wouldnt touch collimation. i think only your right-bottom corner is problematic, but i would just rip the stars out with starnet, make them bit more round with tools, and put them back in  collimating with the 2600 on is impossible. and with a small cam you wouldnt see any of the corner problems i guess... if you do want to fix a corner, you could try to adjust the tilt plate on the 2600 a bit (or even put a bit of aluminum foil in that corner) and screw it back on carefully. on my rasa11 with 6200 i got a Octopi.space tilt adapter to fix tilt and such, but the chip itself is just not nice and flat. but on the 8" there's not enough backspace, esp with the 17.5mm of the 2600. I'm thinking about getting a asi294mm which just has 6.5mm like the 1600/183. my feeling is that ccd inspector misrepresents vignetting (and the effect on starshapes) as tilt or collimation sometimes. -arjan |
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Tilt can be measured using ASTAP CCD inspector. |
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Thank you all for your great replies. Knowking that more or less the conditions with my setup are normal I will not make any adjustments to colimation. I have also observed the findings of Stephan. The wiggle room is present. I also try to center my camera while installing it. I am imaging from the city so for me a filter is a must. Yeaterday I got best results with the Starizona Filter Holder without any other spacers. My filter holder came with a 1mm plastic spacer preinstalled. With it tilt increased to about 20% and without it it is below 10%. I also noticed that edge elongation also depends on the position of the star on which I am focusing. Yesterday I focused on a star which was not in the center and this worsened things on the edges. I have only a small guider scope on top of my Rasa so no additional weight. And I am using a dew shield which may be blocking some light. Best regards, Ivan |
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| Interesting. I have the feeling I see some shifts in how corner star look from night to night. I have sturdy dovetail bars on the top of both my RASA8s, so that should help stabilizing the scopes. I have no astrodarkness up here until late August but then I plan to have a double RASA 8 rig up and running, with a SW72ED guide scope on top of one and a Samyang 135 (or possibly a Canon 200) piggybacking on the other. I may be in for trouble, but who is not in this hobby...... |
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Hey all. I have started to collect subs for a mosaic of the Sadr-Region over the last couple weeks - so the scope orientation and weight distribution is more or less identical between sessions. I dissemble the setup and re-attach the camera every night before imaging. When comparing subs from each session I figured that within the session all subs suffer from elongation in the same areas - usually near corners. What's interesting is that subs from different sessions but from the same mosaic piece differ as well. And I was also able to shoot two "perfect" subs in an unfortunately very short session <--- which means, it's possible to do flat APS-C pics with the Rasa 8. So , since the only real change in setup between each session ist the (re-)attachment of the camera, I'm more and more convinced that the star elongation is caused by bad positioning of the camera adapter onto the lens group. As I mentioned in my first post, it is possible to shift the camera adapter quite a bit in each direction when attaching it with the Retaining Ring. This allows the camera sensor to be in a shifted position when the camera is fixed. The Retaining Ring does not force the camera adapter into a centered position - in my view this is clearly flawed. Last night I thought about 3d-printing some sort of spacer ring and in the process I found some older discussions about this issue. So it seems to be a common RASA(8) issue. Examples - https://www.cloudynights.com/topic/670798-rasa-8-camera-ring-mod/page-2 - https://www.cloudynights.com/topic/681876-rasa-8-spacer-ring-3d-file-share/page-5 That' going to be my plan for the next step as well - to figure out a solution to ensure that the camera is (securely) centered when attached to the scope. Cheers, Stephan |
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| I think you are right about the problem with sideways camera movement. I have two RASA8 now. Celestron has changed the retainer ring from the first to the second one so they apparently reacted to the issue. In the new one it is tapered so it centers the camera when tightened. With the first one I tried to fix the problem with sideways camera movement by putting in a circular piece of 1 mm steel wire behind the threads so the camera could not move sideways. Seems to work. |
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Dear Goran, If it is not a problem can you please share a photo of the design of the retainer ring from the newer RASA8 scope - the tapered one. A photo of how the 1mm steel wire worked will be much appreciated as well. Thank you in advance! Best regards, Ivan |
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I do not have an image of the new tapered version and I rather not remove it from the scope right now. But in essence Celestron has filled out the space behind the threads in the retaining ring. I do not think it needs to be tapered. You find an image of the wire-ring on this thread: https://stargazerslounge.com/topic/368295-closeup-of-the-tadpoles-from-a-rasa-8-wide-field/?tab=comments#comment-4003131 It is a wire I bought in a garden store and has a thin green plastic cover. That made it a bit too thick so I removed the cover leaving a steel wire with 1.0 mm diameter. That worked better. It has to be perfectly curved and made in a circle that makes it press outwards so no part of the wire comes between the front plate of the scope and the adapter (which will cause tilt). When properly positioned it will fill up that space behind the treads that allows the adapter to slide sideways. As you see I use an Artesky RASA8 adapter so I can screw in a 2" filter but it should also work with the original adapter. Cheers, Göran |
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Thank you! Don't disassemble anything I wrote to Celestron in order to see what will they recommend. In addition I will order the adapters above to be 3d printed to check them as well - Thank you Stephan. Best regards, Ivan |
