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About davies07

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    Star Forming

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    Cambs, UK
  1. Hi Kookoo. Your findings have confirmed to me the value of the plastic disc method of collimating an RC scope. All of the methods involving lasers, the Tak scope or a Cheshire, define a reference line in space that you try to align the mirrors to. Such a reference line needs to start at a correct position and point in the correct direction. It is extremely difficult to get both of these aspects correct, with the level of accuracy of design and manufacture of the components we are dealing with. The plastic disc defines only a reference point which is the centre of the primary mirror. You then align this position reference with others, such as the centre of the secondary circle, by eye. I think you are so nearly there with collimating your scope. I think you should make a disc and use it to align the secondary. I bet you a glass of ouzo that you will find that the secondary needs just a tweak in the vertical direction. David
  2. Well, my own focal length turned out to be 1660 mm, not 1624 mm. That is where my Ronchi lines are straight. I guess it is pot luck as to what your optics will give you. I wouldn't try to get back to 1624 mm if the scope is not correct at that setting, you'll get bigger, softer star images. By the way I use a TS flattner https://www.teleskop-express.de/shop/product_info.php/info/p4006_TS-Optics-RC-1-0x-Flattener-Bildfeldkorrektor-fuer-Ritchey-Chretien---2--Anschluss.html But if you already have the 2" refractor flattener then it will do the job equally well. You will need a separation of 109 mm between the back of the flattener and the sensor. David
  3. Hi Nicolàs, Are the ovals in the corners equal? Are you using a flattener or reducer? If the ovals favour one edge of the image, I would suspect the secondary being slightly out as I've suggested to Kookoo. If the ovals look equally distributed and you have no flattener, then I suspect you need one and that the scope is collimated. If you are using a flattener, I would ask if the separation from the flattener rear surface to the sensor is correct. I'm using the TS flattener and this distance is quite long at 109 mm. I also use the CCDT67 reducer (TS also sell this). If it is spaced for x .67 reduction, then in my experience that is pushing the optics somewhat. I've spaced mine for x 0.7 reduction and am seeing some slight oval shapes in the corners but I generally lose these when I crop the image. David
  4. By tweak, I mean just to slightly tighten or relax the tension in the secondary screws, not to turn them.
  5. Hi Kookoo, I think this looks pretty good. The star field looks very nice with just some oval stars in the bottom right and left corners which are confirmed by the FWHM and eccentricity plots and might be a bit of astigmatism creeping in. The CCD Inspector plot shows a similar result with collimation 8" out in the vertical. The eccentricity and FWHM readings towards the centre are just lovely. So what is amiss? There seems to be some tilt in the vertical axis. I'm suspecting that this might be due to some slight misalignment of the secondary in the vertical axis. since it is the secondary that governs the distribution of the optical quality over the field of view. I'm wondering, if you used the Tak alignment scope in the focuser, is the focuser itself slightly misaligned with respect to the primary mirror? I wouldn't touch the primary at this stage. I would be tempted to tweak the secondary very, very slightly in the vertical and retest. David
  6. Hello Nicolas, I have been away for a few days and have just picked up your post. Thank you very much for the mention and for the information. What you have made is exactly what I imagined was needed but never got around to making. i must have a go myself. David
  7. If you're trying to line up with a previous image, have you tried loading an existing image and then plate solving it in the framing tab. The plate solver - I'm using ASTAP - will then determine the coordinates you need to continue capturing more frames. If you had saved previous frames from NINA in FITS format, the coordinates are written into the FITS file header and no plate solving of the loaded image is needed. As David mentioned above, you can then go to the Image screen and run a plate solve and auto focus before going to the Sequence and starting it with the plate solve and auto-focus options switched off. As you've probably noticed, there is a set of tools available as sub-tabs in the Image screen but you need to load them from the Tools list at top right. One of these sub-tabs is Imaging and I find it handy to check the framing is correct. Once you've loaded the Imaging sub-tab, open it, set the camera control to 'Looping', set the binning to 4 x 4 or whatever your camera will support and click on the start button. Now go the the Image sub-tab to watch the images come in and check the framing. By the way, I'm currently spending time using auto focus with my various filters to work out the offsets of each of my filters from luminance. I am then putting the offset values into the filter options, so I should be able to get away with focusing with only the luminance filter in future. David
  8. Hi Alan, Under the framing tab, Coordinates topic, there is a button to the right of the word Coordinates, which when pressed will transfer the coordinates of the target from your planetarium software (CdC). You can then move down to 'Replace as a Sequence' and tap 'Slew' to get you into the right vicinity. Turn on 'Centre target' on the Sequence page and NINA will then plate solve and re-centre the image before the first exposure. David
  9. I have to agree with KooKoo_gr. I think you'll be pushing it. Even well balanced, you'll have a lot of inertia on the mount and pressure on the quite small worm gears. The motors won't be able to deliver the torque to move the mount smoothly and in the limit will stall and miss steps. Once upon a time, I had my RC8 on an EQ6 with a 100 mm refractor. Most of the time it seems to work fine but then I started losing guide stars and could see that the whole field of view was stepped a small amount to the east in RA. My RA motor had stalled for a step. Removing the refractor fixed the problem.
  10. I agree. I think the scope is essentially correct. Do I detect a bit of tapering top to bottom; this could be a bit of coma. I would check on a slightly out of focus star image. For the record, I adjusted my secondary mirror inwards by around 0.3 mm. Here is the resultant Ronchi image inside focus: So the tendency to barrel distortion (over correction) has gone and the lines look straight. The extrafocal image does not look so good: There is a suggestion of tapering which I interpret as a bit of coma. Note that this image is closer to the focus point but I can't move the focuser any further out. I shall check this when I get on to a star - the sky clouded as soon as I took that last image. For the moment, I'm calling this done. I'll now use the scope and see how the images look. I did manage a single frame of M13 but with guiding errors due to cloud. It did enable me to check the scope focal length in Pixinsight and it came out as 1658 mm which is very long for this design of scope (should be 1624 mm). I suspect I have a rather edge of spec set of mirrors. The coma I'm seeing on the last image could be due to the very extended image train. I'm using all the extension tubes and the focuser is almost at the end of its travel, out, and I'm sure there must be a bit of flexing which might vary with where in the sky I'm pointing. By the way, I've edited my images in Photoshop to bright the white point down to closer to the image data. This has made the images brighter.
  11. Yes, indeed. I've just adjusted my secondary mirror to push the secondary towards the primary by around 0.3 mm. I will retest once the clouds clear and let you see the results. It would be good if the result shows me to be slightly undercorrected this time. At least I would then know that the correct position is between the previous setting and the current one.
  12. Mmm! Thanks, Merlin66. This is using Es's 25 l/mm Ronchi grating with his artificial star. Es says the grating is very sensitive but give low contrast graphs. I thought I'd update you all on progress. I got out at the weekend and manged to get these graphs off my own RC8 scope using the Gerd Neuman 10 l/mm grating. I've used the setup with my Fuji X-T2 camera, 35mm lens and a 43mm - 52mm step-up ring as shown in the post above. I also added a 50mm extension tube to bring the Ronchi grating to the focal plane of the telescope. I started by slewing to a bright star and centring it with my imaging camera. I then replaced the imaging camera with the Ronchi grating but without the camera. Looking into the grating by eye, I could see the lines and was able to adjust the focus to bring the Ronchi grating to focus. I then added the Fuji camera and lens and took this shot: What you can see here is one grating bar across the centre and we can see some of the imperfections in the mirror surface. Racking the focuser in towards the telescope, the grating lines appear. You get more bars the further from the focal plane the grating moves. So this is an 'intrafocal' graph. And a bit further in, we get: The second picture of each pair is taken with the camera at 90 deg to to the other. I think the lines are almost straight and evenly spaced indicating the scope is operating close to its correct focal length and is more or less collimated. There is a slight suspicion of barrel distortion on the first pair of graphs which would indicate slight over-correction - mirrors too far apart. Racking the focuser to put the grating outside of the focal plane, I get this view: This is the 'extrafocal' view. Now there is a slight indication of pincushion effect, again indicating slight over-correction. So I deem my scope to be nearly there. I'll try to get it to a fully corrected state by unscrewing the secondary mirror centre screw out by, say a quarter turn. That should move the focus out by around 5 mm and I'll test it again. Just a reminder, I've used a plastic disk with a hole in it to collimate the scope as I've described earlier in this thread. I've improved the disk by adding a black cross, intersecting at the central hole. This makes it much easier to see where the reflection of the hole is when you adjust the secondary mirror. All I have done here is to adjust the position of the secondary, recollimate the secondary using the plastic disc and then retest; I haven't touched the primary.
  13. I've made some progress and have got my Gerd Neuman grating working on Es Reid's artificial star test bench. This is my camera setup: One thing to be aware of is that the Ronchi grating needs to be close to the focal plane of your telescope. This is the normal location of the sensor on your imaging camera so the Ronchi test camera has to move further back and you will probably need to use an extension tube. To sight the telscope on your test star you can remove the camera and look directly into the grating, rack the focuser in and out with the grating close to the focal plane. The light from the star will fill the aperture of the scope and you should see the Ronchi lines across the aperture. They are quite low contrast so you need care to spot them. Here is an image from a refractor under test at Es's. I now need a clear night to have another go on my RC.
  14. I’ve been chatting with FLO and we’re all confused. Checking on Gerd Neuman’s website, the photographic version is designed to be use d with a DSLR camera complete with a lens on it. Thus the 52mm male thread. That is designed to fit into a 52mm filter holder on the front of the DSLR lens. If you have a lens with a different diameter you can buy a step up or down ring for a few pounds. But here’s the thing. How does it then work? You’ll have a telescope terminated with a T2 male thread that screws into the Ronchi grating and then have a DSLR plus lens attached to that. How do you focus it? FLO are going to contact Gerd for clarification. So don’t buy anything yet D
  15. I think you can get away with just one adapter if you turn the grating around.
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