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buzz

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Posts posted by buzz

  1. Well, after quite a few frustrating sessions trying to use the GoldFocus mask readout as an alternative assessor in an out-of-focus star test, I'm pleased to announce I was able to get the GoldFocus mask to work very well indeed.

    Jeff worked on an updated instruction manual for dual mirror adjustments. It was not something you would have figured out by random intuition but looking at the procedure, it does make sense. In an hour I was able to collimate both mirrors on a 10"RC with high accuracy.  That was also helped with a night of good seeing and changing my locking grub-screws to pointy tipped ones. The original ones were cork-screwing the main mirror by a few microns (laterally) and causing the air to go blue.

    The great thing is that it achieved this using numerical analysis and one iteration between secondary and primary movements. I could have gone for a second iteration but clouds stopped play. Out of focus stars were even symmetrical  donuts but I'm sure that a second iteration would have nailed the last few percent.

  2. Mike - It is sort of like three Bahtinov masks looking at the variation of focus by angle. When all the light focuses at one point, the three focus positions agree, i.e. collimated. For any coma, astigmatism or other abberation, the focus is smeared and the readings for the different orientations of the mask differ. It is virtually impossible to distinguish between small primary or secondary errors with one reading, just like star testing.  The focuser part is really useful, since it can directly tell you (after calibration) just how far to move the focuser with one reading. The collimation is intriguing but more elusive at the moment, hence the post.

  3. Yes Chris - I have one of those too - part of a big evaluation of all the major techniques for collimating a RCT. It is running to 25 pages already. I made a discovery early on with my Altair Astro Truss RC and that was to do all primary adjustments with the OTA vertical. If you don't the mirror sometimes shifts when you loosen an adjuster.

    Currently a simple HG laser to line up the focuser/secondary and the hall of mirrors on the primary get me very close. I found an excellent reference by Deep Sky Instruments on a particular form of star testing that is very sensitive too. According to the website, the focus mask technique should be able to improve on that, especially in medium seeing conditions. The science is compelling but so far, ahead of my practice.

     Collimation Procedure

  4. I'm trying to collimate my 10" Truss RCT from Altair using the GoldFocus mask.  I'm not having much success and suspect I am doing something wrong. I can get 'good' collimation with classical outside focus star testing but when I use the GoldFocus mask, to try and improve upon it, for some reason the adjustments make things worse.

    Originally I was using the mask in the same way as the DeepSky Instruments use star testing - adjusting the primary mirror to set a central star and the secondary to balance the appearance of peripheral stars. After getting some advice, I switched over, with little effect.

    I would be interesting to see if any SGL members have used this clever device to set both mirrors, and how they went about it.

    regards

    Chris

  5. Hi Steve - I could do the same but I have an all-screw coupling policy!  I got Astrophad to make me a feather touch to SCT adaptor.  I just bought a Baader Neo filter - hopefully it has the same glass thickness as the RGB filters and I won't need to shift focus after a filter change.

    What I'm thinking of is getting another one of these adaptors made up but putting in a 2" filter thread on the hole you can see there.

    post-16414-0-25494100-1387218357_thumb.j

  6. I did some more experimentation - just using the IDAS in the filter wheel, I need to move focus positions between that and the RGB filters (by about 0.2mm) . My OAG is then affected too by the focus. I think I can just find a mid position on the OAG so that it is focus enough for guiding in both focus positions.

    Ideally I would like to put the IDAS upstream and keep the original L filter in the filter wheel. To do that I'm going to have to make myself an adaptor, probably within the throat of the filter wheel. I think the extra glass in the optical path is still going to affect the path length, so I may have to think about field flatness adjustment once more.

    Perhaps I should just ask Baader to come up with a substitute to their L filter that is par-focal!

    Chris

  7. I had some more thoughts on the IDAS filter. I started doing what I think the others have done, taking 300s exposures through an L filter and the IDAS. The background level was much lower on the IDAS shot and the stars were a little dimmer, as expected. The finished image noise level was not amazingly different though.

    I then realised that I was missing a trick that I uncovered when I was researching exposure.  Signal to noise improves with longer exposures as it does with stacked multiple exposures - I think you have to compare exposures between the two at equivalent SNR.  Using the exposure calculators that take a ratio of sky noise to read noise, I realised that I was doing the IDAS image a disservice. I could expose for double the time and keep the same SNR. I think that is when I will see the real benefit - since the better depth to the image will not require as much stretching.

    • Like 1
  8. The filter suppliers often have Light Pollution Suppression filters in their range but they never group it with their LRGB sets. I realise that they are really for use with Bayer array'd cameras but I was thinking that if they improve image contrast, wouldn't it make sense to use them for the luminance channel too in LRGB imaging?

    Apart from a probably focus adjustment for the filter thickness, is there a flaw in my musing?

    Has anyone tried it and found it not to work very well?

    regards

    Chris

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