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Adam J

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Everything posted by Adam J

  1. Adam J
    Your correct, you will not have had to move the focus tube on a DSLR due to using a coma corrector, well maybe a 1 or 2mm as the badder MPCCMkIII does move it very very slightly. Its with a CCD that the position of the focus tube will move if not using the coma corrector (unless you space it anyway). Its because while the DSLR naturally places the sensor at 55mm from the coma corrector, you cant just screw a CCD into the coma correctors as it will be too close to the sensor and not correct correctly. NOTE: The position of the focus does not change, but that is not the same as the position of the focus tube changing. Different camera back focuses will result in different tube positions to place the sensor at focus. The use of the CC just forces you to modify the camera back focus to match the CC back focus requirement and so when using a CC all cameras result in the same focus tube position, just so long as the CC back focus is correct. See the diagram above.
  2. Adam J
    Here is a diagram mate. Yellow = coma corrector Blue = Draw Tube Green = spacers / t-ring Pink = Camera body. In order: 1) A DSLR with a coma corrector, spacing from the back of the coma corrector to sensor = 55mm, this is made up of the DLSR mirror box 45mm and the T-ring 10mm. 2) A CCD using a coma corrector, spacing from the back of the coma corrector to sensor = 55mm, this is made up of a M48 to M42 spacer. 3) A CCD with no coma corrector, the spacer is not required so the tube is moved out to reach focus...although you could still use an extension to keep the tube in the same position. (Tube now not in light path). So the point is that the camera type has no effect what so ever, even in case 3 you can use an extension. You just need to mark the tube at focus to ensure that you don't take too much off it when cutting. Or as I said before, just move the primary mirror up the tube. This whole problem is why you only use a 35mm draw tube when you use a Moonlight with a 130PDS.
  3. Adam J
    Its because when using the coma corrector you need to place it as a set spacing from the image plane (55mm). As such the camera moves backwards 55mm relative to the front edge of the focus tube (where the cc sits) and so to get focus you need to move the image plane (sensor) in by 55mm. In a DSLR the image plane is 55mm back from the coma corrector due to the depth of the DSLR mirror box so no spacers (apart from the t-ring) are required to gain the required spacing. If you use a CCD there is no mirro box and so you need to make up the spacing using spacers between the camera and the CC. Hence the focus tube position does not change when using a coma corrector irrespective of the camera being used. If you still don't understand I would be happy to provide a diagram.
  4. Adam J
    The issue is more that it does not go across the entire tube diameter and so the angle changes as you move across the image plane.
  5. Adam J
    That still leaves two sections of the focuser in the light path. Also you really don't need to remove that much only 1.5cm that looks like much more. Once you add spacers between the coma corrector and the CCD the focus tube will end up in exactly the same location when using a CCD as it does with a DSLR it has no choice but to. The main issue is when using it for observing...but you can still use an eyepiece extension.
  6. Adam J
    It will not cause an issue, the back focus from the coma corrector remains exactly the same at 55mm so you need spacers between the coma corrector and the camera anyway. The only time it would be an issue is if you did not want to use a coma corrector and even then you could use an extension tube to allow you to reach focus. Removing only a square will not solve his issue he will just be left with additional surfaces at different angles in the light path and more diffraction lines. I know many people who have cut the focuser straight across and had no problem with using a CCD. You just need to add a stop to prevent it from being able to move out of the read bearings at maximum adjustment.
  7. Adam J
    Actually, I think you got it. If I had a penny for every time this issue comes up with the 130pds when coupled with a DSLR...I would have something like 10p....
  8. Adam J
    Here is my first ever astro photo from over two years ago. Ah the memories, but take a look at the brightest star. Looks kinda like what you have? Apologies for my poor focus. I spent about 6 months obsessing over my spiders.... Yet still I had additional spikes! Here is M45, but what you will notice is what you have noticed. See the star ringed in red. My collimation is off and its the closest to the center of the optical field. to the left the extra spikes lean one way to the right the other way. I am going to cut the the big revelation here...its not your spider vanes. Its not tilt. Its not colimation. It is the focusing tube interrupting the light path due to the large amount of back focus required by your DSLR camera. This will only show up on a DSLR because of the back focus required and it also requires that the primary mirror is set as for away from the secondary mirror as is possible on its adjustment screws when skywatcher put it together. You can move the primary up the tube on the adjustment screws (dont go too far) and it will move the focal point out and hence the focuser tube with it. The alternative is to lop about 1.5cm off the focuser tube. Either way this will move it out of the light path. Below is after the fix with a mono modified DSLR I was playing about with at the time. Nothing else changed just the primary adjustment screws moving it further up the tube. Adam
  9. Adam J
    If i could afford one that would be my choice. But drilling that extra hole for a third thumb screw really does help things.
  10. Adam J
    I have one, I dont like it and actually went back to the original adapter, but I did drill and tap an extra hole for another thumbscrew to help prevent tilt. I think that there is a baader click lock clamp available that would be better, it is much more expensive though.
  11. Adam J
    Yes that's the correct piece though I found the one that came with the scope (that one) was very poor and actually caused tilt. It will be ok until you cet a coma corrector but I would not think of it as a long term solution.
  12. Adam J
    Great image but you really cant use flats from one scope with another, I would say you cant even use flats from one session to another especially when the colimation is out.
  13. Adam J
    Changing to a crop frame mode from full frame will not increase your focal length, it will only reduce your field of view, the focal length stays the same. However using it in APS-C mode is probably a good idea anyway as the ZS61 is not going to fully illuminate and give you a flat frame over a full frame sensor. The resolution will stay the same as that is governed by focal length and the pixel size, always think about arcseconds / pixel and take a look at the BBC Sky at night Field of View Calculator. The large pixels of the A7S are probably not the best match for such a short focal length and may make the stars look a little square? But to be honest its still a great camera for astro work.
  14. Adam J
    The ZS61 has better optics than the TS scope i posted so if you are not bothered by the shorter focal length then go for the WO ZS61 I would do. I only noted the other scope due to you wanting over 400mm of focal length. The main difference is that the ZS61 uses FPL-53 glass so the color correction will be much better.
  15. Adam J
    huum thats odd I see it listed as only 2.1kg for the OTA only? I have never been too sure about guiding on a star adventurer to be honest I am not sure how well guiding will work in RA only. My friend does not guide his setup so I cant comment on how well it copes once you add the weight of a guide camera. He uses a 1100D so its fine for 2 min subs with the camera / flatter / OTA. Maybe with one of those 30mm QHY guide scope packages???
  16. Adam J
    I know someone who uses this scope to great effect on the Star Adventurer: http://www.365astronomy.com/TS-INED-APO-70-420mm-Carbon-Fiber-Tube-2-Micro-Crayford-Focuser.html I think that a ZS61 is also a great choice though at a little shorter focal length.
  17. Adam J
    The kit extension is not the shortest available.
  18. Adam J
    It should be possible to use a 2inch extention, that's the normal work around.
  19. Adam J
    security clearance for astronomy...wow what do they think that you are going to use it for?
  20. Adam J
    Does anyone know the percentage central obscuration on the 130PDS? Cheers, Adam
  21. Adam J
    It is effectively used to cancel out the field curvature caused by the light originating from the edges and center of the parabolic primary mirror not coming to a perfect focus when you move away from the primary optical axis. It causes the stars to become elongated away from the center of the image with the effect getting worse as you move further off axis. To cancel it out and achieve a flat field with good stars across the entire image the coma corrector nudges the light incident on it in the opposite direction to the original 'error'. However, this nudge has to take place at the correct distance from the sensor or instead of cancelling out the effect it will either under correct or over correct. As such the coma corrector is designed to operate at separation from the sensor to achieve optimal correction. For a DSLR that is normally easy as with a standard T-ring fitted a Badder multi purpose coma corrector will just screw on and the spacing will be perfect. In the case of a CCD that is more tricky as the distance from the front of the camera to the actual sensor within the camera is not fixed and so people have to purchase the correct spacers to achieve the correct separation as specified by the coma corrector and that will change from one CCD to another. So basically if you have a DSLR dont worry about it and if you have a CCD just ask the shop and someone like FLO will be able to work it out for you and recommend the correct spacers. Fakhri's problems are more to do with his lack of direct access to a good astronomy dealership at his location. (I hope he does not mind me saying so on his behalf).
  22. Adam J
    I don't think that the spacing is correct from the shape of the stars I would guess at it being too much. Are you calculating the spacing to the M48 or the T2 thread on the MPCCMk3?
  23. Adam J
    To be honest the difference between the 130PDS and the 150PDS is one of the smallest jumps in the range with the focal length going from 650mm to a massive (sarcastic) 750mm so in essence there is hardly any difference between the two scopes in terms of focal length and light grasp but 750mm is competitively frustrating in terms of the number of objects that you cant quite frame nicely but that can just fit into the FOV of the 130PDS and then there is the weight difference which if far bigger than you might think for only 1 additional inch of aperture. Yes smaller galaxies will be slightly better but really not that much better..... Get the 130PDS there is a reason there is a 67 page thread dedicated to it and nothing by comparison with the 150PDS.
  24. Adam J
    I made a cool box that worked / works very well. Some others directly cool the chip. Either method will work well if correctly executed. This is my build thread here:- You are actually the last person to have posted in it lol.
  25. Adam J
    Personally I dont like the idea of even smaller pixels without a big improvement in quantom efficiency. The 1200D already has quite small pixels and a 600 / 650 / 700 will not be a significant improvement. I would probably save my money for a cooled camera rather than try to upgrade to a different DSLR. Your signal to noise ratio will improve as the temperatures start to fall.
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