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I'm quite annoyed by how far the focus tube extends into the scope as it is definitely messing with my stars. Has anyone: 1) Found a replacement focus assembly that has a shorter tube? 2) Actually shortened the tube and seen improvements? Thanks in advance, Andy |
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Hi Andy, I replaced the 200PDS focuser with a Baader steeltrack which was a considerable improvement in many ways. At focus the tube doesn't significantly impact the light path (as far as I can recall from last time I used it -- about the same ingress as the clips?). One other thought -- I think (but may be remembering wrong) that it is possible to unscrew thr focuser and set the position of the tube differently? best wishes Tim |
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11.01
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| Given that I had only one response to my question, I've decided to replace the rather reliable stock 10:1 Crayford with a low profile one that will hopefully not intrude so much into the image path and which has caused me so many star issues. I'll update this thread once I have any feedback. |
3.58
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| I have the 200PDS, an Explorer Scientific coma corrector and ZWO ASI 2600MC/MM cameras. For me the focus tube does not extend into the scope tube at all at focus. What I have found is that the focuser has a screw in adaptor that holds the coma corrector in place that is of a larger diameter than the rest of the tube. Hence the coma corrector at the scope end is left with gaps in the focuser tube all around and tilt is a consequence. Even butting the camera flush against the focuser end piece still leaves uneven gaps at the other end. I bought a replacement compression ring adaptor and adapted that so it had 3 restraining screws 120 degrees apart instead of 2 and that did not help either. Like Andy if anyone can recommend a suitable very well built replacement focuser then I would be grateful. The coma corrector needs to be a sliding fit in the focuser tube and preferably screwed in place. I would be interested how others have solved the problems of tilt with this scope. |
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David Moore: Hi David, as mentioned above - replacement with the Baader diamond steeltrack worked for me. It is a much better focuser than the one the PDS200 comes with - it holds my Baader MkIII coma corrector plus filter holder and AS1294 camera firmly with no problems or discernible tilt. Tim |
1.20
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| Yes, with the Octoplus508N. |
1.51
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David Moore: I use a Baader Click Lock with my 200PDS and imaging train. CCDInsepctor and ASTAP report 3% (None) tilt. I was also struggling with getting the camera to sit at a right angle towards the secondary. This purchase seems to have fixed it. |
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11.01
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OK, I've ordered a relatively cheap low profile crayford focusser which has a 24mm drawtube. It also has an extending mechanism within the drawtube just in case I can't reach prime focus with it. My main aim is to see whether having no intrusion (or very little) of the drawtube significantly improves my stars. Currently the stock drawtube extends over 1" into the scope at prime focus with the Skywatcher coma corrector. The nice thing with the focusser I selected is that (in theory) it solves my astrophotography issue of drawtube incursion into the OTA field of view, yet should still allow visual work with its extending drawtube mechanism.  |
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11.01
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OK, new focuser fitted. N.B. It came with no fitting screws, so I had to re-use old ones. Good thing was that holes were in exactly the right place for my Skywatcher 200 PDS. I've also just about managed to fit my ZWO EAF, although the flexible coupling is a tight fit inside the focusser (applied some superlube and hopefully it will wear in with use). It (the EAF) does appear, however, to be coping with the added friction. There are light leaks from the sides, but they were easily fixed with a bit of black insulation tape. The best news of all is that the focusser tube now barely encroaches into the OTA at what I think is prime focus (I still need a clear night to test it). That's an inch less than the stock focusser. If this all works, I'm hoping to have nice symmetrical stars and diffraction spikes as opposed to slightly D-shaped stars and lopsided diffraction spikes. Additionally, the telescoping focus tube should leave me with a scope that's also good to go for visual should I need it. Let's hope it doesn't introduce any tilt as that's been fine for me thus far. |
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Andy Wray: Hi Andy, Hope that it works. I really like my 200PDS and especially now that that it does indeed produce nice symmetrical stars and spikes. Took me a fair while (1.5 years) to reach that happy point though ... the five particular things that I fell foul of (or that were useful) along the way were 1) As delivered the mirror clamps were too tight - took a while to realise that my stars were somewhat triangle shaped 2) Also check tha there is just sufficient support at the mirror sides that it doesn't slide - otherwise the weight shifting can cause alignnment and collimation to slip slightly out of true as the scope moves orientation - but no pressure either that could twist the mirror. 3) A good quality Hotech laser helped greatly to get secondary alignment just right (and also to check for 2) plus corroborate Cheshire collimation. 4) My original focuser with its extension tube flexed under the weight of the camera - and simply couldn't reliably hold the weight (so replace - as you have) 5) The last - a more subtle thing that took the most time to diagnose- my mount was exhibiting jerky movement and excursions n RA on the ~ subsecond scale. Not something that could be picked in PHD2 guiding readout which all looked fine, the effect was to create egg-shaped stars. As you can imagine I spent a lot of time looking for faults in the telescope when it was in fact blameless. You would be unlucky to have as similar problem - but it is quite easy to check for by following star motion during very short frame (e.g. 0.1s) videos during tracking to check that the span of DEC and RA 'wobble' look similar. I expect that you have these things covered but just in case .. Tim |
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11.01
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Tim Hawkes: Yes and no ... I wanted to get the focuser tube thing fixed first before looking at any other issues. I have not managed to remove my primary yet as it seems totally stuck  I didn't have triangular stars when I first got it and used it with a mirrorless canera, so I'm not sure that I have a primary that is locked down too hard, however if this doesn't fix things, then I will have to work out a way of removing the primary and checking on those mirror clamps.
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11.01
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I can now confirm that with this focusser I did need to extend the telescopic focusser tube about 5mm to reach prime focus. That means that I have now totally eliminated any incursion of the focusser tube into the OTA  That's a huge step forward.Also, without trying too hard, I don't seem to have introduced too much tilt. ASTAP reports 4% (None) which is slightly worse than before this focusser, but I'm sure is something I can fix. On the plus side: my stars do appear to be rounder. On the downside: I can now actually see the effect of the mirror clips on my stars (albeit very slight). This is only one night of imaging on a slightly cloudy night, but so far, so good. |
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11.01
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Aaagh! Two steps forward, one step back. Most of my star fuzz has gone, but it has uncovered: 1) I think my backfocus is about 0.5mm to 1mm too far .. I've removed one 0.5mm spacer and will see how that goes. 2) I have introduced a slight tilt issue and will try and address that ... it's really minor, but enough to be annoying. 3) With sharper, clearer stars, I can now see more of my guiding issues. Although I'm down at 0.5 arc secs guiding RMS and average FWHM is around the 2 arc secs mark , the RA axis is at least 50% worse than the DEC axis. More tweaking of my guiding and the RA axis needed. I'm making progress and glad that the diffraction spikes have all but disappeared, but sharp, small, occassionally oval stars are very irritating. [Update] A single 90 sec frame on a very cloudy night after removing the 0.5mm spacer and tightening up the RA axis response is as below. I need a clear night to really tweak this imaging train. That said, it's getting close to what I think I can achieve with my setup. The main issue that remains appears to be a tilt issue which I will work on next.  |
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| As I wrote in another thread here, the Skywatcher 0.9x corrector should be avoided. It has low optical quality, internal reflections, vignetting and it also lowers you f number to 4.5. This significantly increases optical issues compared to f/5. |
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Michele Campini: *** Hi Michele, I'm very interested by this focuser, but unfortunatly there is not a lot of review on it! Could you please confirm that it works well for you without any issue? Thank you*** |
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1.20
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| Yes, i confirm. The Lacerta Octoplus508N is excellent. |
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Michele Campini: *** Thanks a lot! So here we go: let's pull the trigger!  God bless you *** |
2.11
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This is on my to-do list. When I first received the scope I was not happy with the performance. I stripped it completely back to parts and rebuilt it. Once I did this the image quality improved dramatically. I did make the mistake of going with the Baader corrector, once id got the Quattro corrector the image quality was perfect. The focuser still protrudes more than I would like so a replacement is on the cards. I do use the ZWO focuser so I would like to make sure the one I choose will easily fit that. |
