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Old 10-10-2021, 01:01 PM
knightrider
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Coma Corrector Setup Q for Visual Use

Hi Team,

I'm planning on getting a CC for my f4.7 10" SW dob for visual use. Looking at the GSO one for budgets sake, but I believe the setup concept is the same regardless of brand.

I need to clarify my assumptions before I buy one. I've read through half of the CloudyNights thread on the setup for it. And the basic setup procedure is to use a spacer between the CC element and the focus point of whatever EP you use, which is apparently 75mm..ish for the GSO unit.

My plan was to ditch the packaged extension tube. Use my existing 50mm in height 2"/1.25" EP adapter + 25mm spacer + CC Element. Which will net me 75mm to the top shoulder of the EP adapter. I wanted set the 75mm from CC to the top of the EP adapter and use the focuser to achieve focus like normal.

However, the discussion seems to be talking a lot about then parfocalising all the EP's, which is something I wasn't planning on doing.

SO.... Does that mean, proper setup is to have the CC static in the focuser drawtube, then setting up each individual EP to it's focus point in relation to the CC? Essentially locking off the focuser and not even using it at all for focus adjustments?

I look forward to your thoughts
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Old 10-10-2021, 03:28 PM
sunslayr (David)
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Well if I'm understanding it correctly each ep will have a slightly different focus point when seated in the coma corrector so they will have different amounts of coma correction whether that be over or under. That's why spacers probably wont help you because unlike a camera there is no rigid backfocus shoulder you can measure from. So in order to get each ep set optimally you would get one ep at the right distance that gives good correction then once it's also in focus you can put in any other ep in and once that is in focus it will also have good correction. That's why people parfocalize eps, it means all you have to do is adjust focus, no need to set distance.
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Old 10-10-2021, 07:58 PM
knightrider
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I think I'm confusing the reason of parfocalisation of EP's with actual focusing rather than fine tuning the distance of it from the CC.

That's making more sense now Dave.

Optimising the distance of the EP to the CC would determine the effectiveness of the CC. And you would use the focuser as per normal to achieve focus.

So, in practice, setting the "basic" distance/mm recommendation to the top shoulder of the EP adapter should get 90% effectiveness, give or take some depending on the EP.
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Old 10-10-2021, 08:22 PM
sunslayr (David)
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Yeah should be pretty close, just depends how far away the field stop is from the coma corrector when it's sitting in the adapter. The GSO CC is one of the more forgiving in terms of distance, ideal for eps.
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Old 11-10-2021, 07:47 AM
astro744
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This is why the Paracorr uses a tuneable top. It’s worth it!. Keep an eye out for a used Paracorr type 1 with tuneable top. There where two versions that I know of, one with green writing and single eyepiece set screw and one with white writing with double eyepiece set screw. (They had different part numbers). You need the Tele Vue 2”-1.25” adaptor supplied with it for correct spacing when switching from 2” to 1.25” if you follow the recommended set marks for Tele Vue eyepieces. You can of course determine correct position of any eyepiece easily as per Tele Vue instructions.

The tuneable top has a separate set screw that unlocks the helical rotation of it with 5 tick marks on the side from low to high position. You can lock at any position including in between tick marks.

You may pay the same for a used older Paracorr as a new other branded coma corrector but it’s worth it. Note the original Paracorr did not have the tuneable top so beware when buying. The tuneable top was available later as an option to retro fit onto earlier models but I’m not sure you can still buy it.
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