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TeleVue Paracorr Type 2 for imaging on 200PDS


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OK, so hardly "new" ground but I thought I'd write up my experience with the Paracorr.

This relates to the Type 2 Visual-Imaging Paracorr with tunable top, bought from FLO; I'm using it with a ZWO imaging train and Skywatcher 200PDS in a Baader Steeltrack focuser.

Quickly after getting it I realised I was missing a key part to adapt the front surface to T2 threads - the supplied threads under the tunable top are an "odd" size and won't fit anything without adaptation. Unfortunately FLO don't stock this but Teleskop Express do, so that was sorted quickly enough.

Build quality is the usual TeleVue excellence and it's packaged robustly, so no complaints there. The fact that the top lens surface protrudes beyond the top flange is a definite annoyance, but otherwise the construction is sensible.

With the T2 thread adapter installed, it's very hard to actually measure or reason about accurately the focal plane offset and thus flange-to-flange distances required because while there is a great mechanical drawing with optical focal points on the Televue site there's no drawing showing how the T2 adapter fits into this. Consequently there was a fair bit of trial and error on my part to get everything shimmed out properly which could've been avoided.

The T2 thread adapter does have a "locking" grub screw which is just a friction lock but does a nice job of minimising risk of rotation in situ, which is good to see.

This replaced a Baader MPCC Mk3 with Baader VariLock offset spacer, both of which were removed; the spacer was not required as I added an OAG and fixed spacer as part of this adaptation which achieved the required bulk offset.

The Paracorr has to sit much further out owing to its significant length, but not so far that I could bring it to focus with a standard extension tube installed, meaning a good 1-2" of the unit was sat unsupported outside of the drawtube.

Once set up and installed I immediately saw a quite drastic improvement in the reduction of coma but was still seeing some tilt in CCDinspector and flats. Today I replaced the standard Baader focuser locking ring with a Baader Clicklock adapter I had bought for visual use and this appears to have resolved the tilt issue; at the same time I also replaced the fixed spacer with a tilt adjustment unit just in case further adjustment was required, but this doesn't appear to be the case. Adding the Clicklock adapter probably resolved the tilt, and by providing extra length to the drawtube it actually allows the Paracorr to be pushed fully into the focuser and still achieve focus with about 5mm of travel remaining for focusing. A set of Baader metal shims were used to adjust distances precisely, using a Mitutoyo caliper to measure the distance from flange to flange with an educated guess on the T2 flange offset made.

The "money shot" for me is this comparison of two images taken with the Paracorr and with the MPCC in CCDinspector. FHWMEccentricity in PixInsight showed similar results. The MPCC didn't actually resolve the coma in the corners even at f/5, and on close inspection actually introduced errors in the middle of the field. The Paracorr is incredibly flat. This is only an APS-C sensor at f/5 but fair to say I'm quite impressed with the results.

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All in all, worth the cost? If you've got the money then it's the correct CC to get for sure. The Baader is pretty solid for how cheap it is and there are new low-cost CCs on the market these days which are probably worth exploring but as a solid premium option it definitely delivers.

 

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  • 2 weeks later...

I use a MPC-III on my Newt,  I never realised the curvature was that  bad.  From what you have shown that is shocking compared to the Paracorr.

I will have to keep my eyes out for a second hand one.

 

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  • 4 weeks later...

I'm having a bit of a tilt issue myself at the moment with the Baader steeltrack and Teleview Paracorr combination. Looks like I might need the clicklock. 

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Just now, mbalkham said:

I'm having a bit of a tilt issue myself at the moment with the Baader steeltrack and Teleview Paracorr combination. Looks like I might need the clicklock. 

It does help, but I've since aded a tilt adjuster to the train. Haven't adjusted tilt with it yet, but at some point I'll get around to it!

I'll also add here - in case it's handy - that TeleVue helpfully confirmed on Twitter that with the T2 adapter fitted, there is exactly 55mm from the top surface of the T2 adapter to the focal plane.

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I have a tilt adjustment on the ZWO ASI 071MC Pro so hoping that would work if I need more adjustment. My optical chain is Paracorr>ZWO OAG>ZWO filter drawer>Camera which I think gives me the correct back focus. I'm toying with removing the OAG since I'm running 10mins unguided these days.

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Is this anything like your setup? I'm happy with back focus. Some potential improvements:

  1. I wonder if a paracorr to M48 rather than t2 (M42) might help open up that end of things (reduce vignette?).
  2. Have also been looking for a way to go to screw connections right through but can't think of a good way to connect the steeltrack to the paracorr and then the rest of the chain. Hoping clicklock is good enough (and better than the standard clamp). Would be nice to have it screwed right through so open to ideas.
  3. Might take out OAG in favour of a spacer. I don't like the 3 thumbscrews at this point as it could introduce tilt.

Thoughts?

20210301_211114.jpg

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Posted (edited)

My optical train goes: Baader Clicklock 2" focuser, Paracorr, T2 adapter, ZWO tilt adapter, ZWO OAG, ZWO Mini EFW, straight onto the camera - flange distance is different on the 183MM. I can't remember the spacers/threads off the top of my head but I do have a shim or two on the back of the EFW.

Going Paracorr to M48 rather than to T2 would likely help on vignetting, no doubt there - more relevant for the 071MC which has a slightly bigger sensor than the 183MM.

The OAG could also go further away from the sensor's light path, which is another consideration to avoid vignetting. I do want to stick to OAG as I've found it much more robust against differential flexure than the guidescope route even with guidescope rings machined as a friction fit from solid blocks of alu!

I'd ideally like to end up with a rotator in the mix somewhere, but I don't think I can achieve that without replacing the focuser end with a rotator directly and I'm not sure what on the market (if anything) would support that; there's not enough backfocus on the back of the Paracorr to support it even without an OAG.

Edited by discardedastro
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