Lets talk Paracorr (again)

[scarp15]

Firstly apologies for bringing up this well worn subject to the forum yet again.

I have certain reservations concerning a Paracorr and yet it cannot be ignored. In this forum, there is a tendency towards a yes and a no camp to the merits of this quite expensive lump of metal, dependent on varied and highly debated aspects. However switch to Cloudy Nights and it is firmly a case that if you do not have a Paracorr then you are missing something in terms of a full clean and corrected image within the optical path, even up to F6.

I sit on the fence concerning whether or not to use a coma corrector. My system for observing is a smooth operation. a 2" extension tube that works for all my e.ps and the views I get are fine. I do not use 100 degree e.p's (mostly 82 and 72 degree) but do like to interact amongst varied focal lengths. I had however considered an MPCC MkIII, yet unless I am wrong, it does not appear to be compatible with a filter (I usually screw my filters to the extension tube, this adapter may get in the way). With the Paracorr it is threaded for filters.

My question therefore to anyone that has changed their set up to using a Paracorr; Concerning focal travel, is a Paracorr likely to replace an extension tube - therefore no longer required, fit within the existing one in use, or require a shorter extension tube? 

Any further questions, comments please use this thread

[faulksy]

hi iain, your concerns over the extension tube are close to my heart as well, i have just ordered the sips system, i have looked at all the specs, emailed wayne at feathertouch and no answer. my question was i use a extension tube and i can get focus on all my eps, will the paracorr replace the extension tube, and there has been no answer, so yes im a bit concerned, also if it helps. dan k has just lent a paracorr of a gent on here and dan said he thought his scope was ok but after using this he cant believe the massive differnce

[swamp thing]

I can't comment on focus travel as I use the SIPS system rather than the separate Paracorr.

I would say it certainly cleans up my f/4 Dob giving wonderful views with wide field eyepieces. I recommend them.

[swamp thing]

Can I enquire why do you need these extension tubes?

Seems to be making a simple system overly complicated

[Moonshane]

most if not all OOUK scopes have a requirement of a 2" extension for visual use. this (I presume) allows the additional in focus for imaging if required with nothing more complex than removing the extension.

with my 16" don, I made my own mirror cell and moved this about 2" up the tube so don't need the extension with the paracorr but bear in mind that many syntax crayfords have a lip about 50mm or so down in the drawtuve which prevents full insertion of a paracorr. I got mine machined off.

in my 12" dob I have a Baader steeltrack which does not have a lip and do effectively use the paracorr as a variable extension tube. without the paracorr I use a 50mm extension. note that in most scopes the paracorr brings the focus point inwards by about 18-20mm so if you use a 2" extension now, you could simply remove that and have your paracorr projecting about 30mm from the top of the drawtube - this is what I do. also consider that a shorter extension is often not an issue due to the same kind of lip preventing full insertion.

as for whether or not they are worth it, I believe so but it's not the end of the world and if happy with the views you have then maybe in your case not worthwhile.

I'd urge you to hunt down a type 1 visual paracorr with tunable top. you should bet one for between £150-200 used. it's a shame you are not close to me as you'd be welcome to try mine in your scope. if you are ever down this way and it's clear, bring your scope and give it a go.

[swamp thing]

Thanks Shane for explaining that.

[faulksy]

most if not all OOUK scopes have a requirement of a 2" extension for visual use. this (I presume) allows the additional in focus for imaging if required with nothing more complex than removing the extension.

with my 16" don, I made my own mirror cell and moved this about 2" up the tube so don't need the extension with the paracorr but bear in mind that many syntax crayfords have a lip about 50mm or so down in the drawtuve which prevents full insertion of a paracorr. I got mine machined off.

in my 12" dob I have a Baader steeltrack which does not have a lip and do effectively use the paracorr as a variable extension tube. without the paracorr I use a 50mm extension. note that in most scopes the paracorr brings the focus point inwards by about 18-20mm so if you use a 2" extension now, you could simply remove that and have your paracorr projecting about 30mm from the top of the drawtube - this is what I do. also consider that a shorter extension is often not an issue due to the same kind of lip preventing full insertion.

as for whether or not they are worth it, I believe so but it's not the end of the world and if happy with the views you have then maybe in your case not worthwhile.

I'd urge you to hunt down a type 1 visual paracorr with tunable top. you should bet one for between £150-200 used. it's a shame you are not close to me as you'd be welcome to try mine in your scope. if you are ever down this way and it's clear, bring your scope and give it a go.

quick one shane, my scope with a extension tube i could only just get focus with my 6mm delos, i moved the primary mirror down the tube and it cured it i now have out focus reserve, but thats the opposite to what you did

[scarp15]

Thanks for that Shane, it is a baader steeltrack that is fitted to my VX14 along with a 50mm extension. Interesting comment concerning the lip preventing full insertion in some circumstances. Yes a Paracorr type 1 would be a solution. Certainly will let you know if I get to travel down your way - I am getting very used to traveling with this scope.

[Mike73]

That lip on the extension tube will also stop a Baader MPCC mkIII from slotting in all the way but the Baader does take 2" filters, you just screw them in the end of the CC like you would with a normal EP.

[Moonshane]

quick one shane, my scope with a extension tube i could only just get focus with my 6mm delos, i moved the primary mirror down the tube and it cured it i now have out focus reserve, but thats the opposite to what you did

it's not a problem just a matter of moving the mirror whichever way it needs to go. my cell made the mirror sit in a different position to the oouk cell so I moved the mirror up. in your case you needed to move the mirror down. no biggie. skinning cats and all that 

[John]

quick one shane, my scope with a extension tube i could only just get focus with my 6mm delos, i moved the primary mirror down the tube and it cured it i now have out focus reserve, but thats the opposite to what you did

Many Tele Vue eyepieces tend to need more outward focuser movement than many other brands. If you get any more Ethos (to add to your 13mm) you will find their focus points vary widely. I believe all the Delos use the same focal point apart from the 17.3mm which needs more inwards focuser travel. If you mix Baader Genuine Orthos with Tele Vues it can be fun because the Baader GO's need more inwards focuser movement than many other types of eyepiece - so much so that newtonians with low profile focusers can have trouble accommodating the full range of movement needed.

I use a 35mm extension tube with my OO dob and that seems to cope with everything I've tried in it, so far.

On topic, I can't really be of help because I've never used a Paracorr or any type of coma corrector I'm afriad.

[Dude_with_the_tube]

Can't comment on your focusers as I use moonlite with paracorr 1 and with moonlite it is quite simple to adjust so you don't need any extension tubes.moonlite has base plates what can be added or removed on your desire .that's what I did with mine.i just kept adding or removing them until I found the sweet spot where all EPs get into focus with out the help of ext.tubes but with paracorr.However,based on this,you can surely manufacture additional base plates for yr focusers to help raising it off the scopes base if necessary?

[Don Pensack]

It depends on your scope, and its ability to hold weight in the focuser, the apparent field of your eyepieces, the darkness of your sky, your tolerance for coma, the type of targets you usually pursue, etc.

To wit:

--at f/4, you'll definitely see the need for coma correction.  At f/6, probably not.  F/5 is on the cusp.

--if the scope has an issue with extra weight in the focuser, then adding a pound with the Paracorr is going to be an issue.

--the wider your apparent fields, the more you'll see the need for coma correction.  Take two 20mm eyepieces, one of 50 degrees, and one of 100 degrees.  The field is twice as wide in the 100 degree eyepiece, and coma in the stars will also be twice as wide at the edge of the field, so you'll see more coma in the 100 degree eyepiece.  Now, substitute a 10mm 100 degree eyepiece.  The true field is 1/2 as wide, so the coma in the star images is also 1/2 as wide.  But, the magnification is doubled, so the appearance of the comatic star images at the edge will appear the same as in the 20mm.  Hence, the visibility of coma depends on the apparent field size of the eyepieces.

--if you view in bright skies, the stars will appear to have less coma.  The outer parts of the comatic image are fainter than the inner point, so comatic stars will all appear smaller in brighter skies.  If, on the other hand, you have truly dark skies.......

--how much can you tolerate non-round star images?  If your eyepieces had a lot of astigmatism at the edge, could you even notice coma?  Or does it not matter at all if the star images at the edge aren't small points because you don't look there anyway?  if you have a low tolerance for aberrations of any kind, then a coma corrector can help.

--if you pursue large star clusters and want pinpoint star images, then a coma corrector can be valuable.  If you typically view planets or small objects, held in the center of the field and not allowed to drift across the field, then a coma corrector may be unnecessary.

So there are many reasons (I just mentioned a few) why one might want a coma corrector, and perhaps others why one wouldn't.  I can tell you, though, one of the greatest advantages to one is that in pursuing the best correction for each eyepiece, a coma corrector will parfocalize all your eyepieces.  The total amount of movement in my focuser, now, is about 1mm.  The parfocalization is accomplished with the tunable top on the Paracorr (and ES HRCC), while it is done with spacers on the MPCC and parfocalizing rings and barrel extenders on the GSO.

Firstly apologies for bringing up this well worn subject to the forum yet again.

I have certain reservations concerning a Paracorr and yet it cannot be ignored. In this forum, there is a tendency towards a yes and a no camp to the merits of this quite expensive lump of metal, dependent on varied and highly debated aspects. However switch to Cloudy Nights and it is firmly a case that if you do not have a Paracorr then you are missing something in terms of a full clean and corrected image within the optical path, even up to F6.

I sit on the fence concerning whether or not to use a coma corrector. My system for observing is a smooth operation. a 2" extension tube that works for all my e.ps and the views I get are fine. I do not use 100 degree e.p's (mostly 82 and 72 degree) but do like to interact amongst varied focal lengths. I had however considered an MPCC MkIII, yet unless I am wrong, it does not appear to be compatible with a filter (I usually screw my filters to the extension tube, this adapter may get in the way). With the Paracorr it is threaded for filters.

My question therefore to anyone that has changed their set up to using a Paracorr; Concerning focal travel, is a Paracorr likely to replace an extension tube - therefore no longer required, fit within the existing one in use, or require a shorter extension tube? 

Any further questions, comments please use this thread

[Scooot]

I still need to use the extension tube with the paracorr.

[scarp15]

Just out of interest what size extension tube do you use, did you change size from before you started to use a paracorr? I currently use a 50mm with my VX14, would a paracorr possibly require shorter, say 35mm perhaps?

[Scooot]

It's the one that came with the scope, just measured it, 40mm. The Paracorr adds about 50mm but I think the the coma corrector itself alters the focal length required. In other words if I added 50mm to my focuser without the corrector I'd probably have a problem focussing. So I think you're fine to just slot the Paracorr in without altering your set up.

[scarp15]

That's interesting, thanks for that Richard.

[Moonshane]

that's an excellent post Don.

[YKSE]

Great explanation Don!

Now just want to learn more about paracorr:

Assuming that we have a f4.5 Newt and paracorr, also assuming that coma is too much in 82 degree EPs, but totally acceptable in 72 degree EPs, do you see any advantage in buying a 17mm Nagler over 17.3mm Delos or 12mm Naglers over 12mm Delos? Nagers with Paracorr will virtually yield the same TFOV as Delos without paracorr with 15% higher magnification, as I understand the calculations in Televue homepage.

[Scooot]

My scopes f4.8.

I can't say I see much coma in the 17.3 Delos. However I've read the corrector improves the view across the whole FOV not just the edges. The improvement across the middle say, probably doesn't warrant the extra expense. More of a problem is that the paracorr narrows the FOV a bit. I leave my paracorr in all the time for convenience, but when I'm using my 17.3 looking at the double cluster for eg, I'm not sure I'd prefer the wider FOV with a little coma or the paracorr view. So I could change to the nagler but otherwise I wouldn't have to.

[Scooot]

Great explanation Don!

Now just want to learn more about paracorr:

Assuming that we have a f4.5 Newt and paracorr, also assuming that coma is too much in 82 degree EPs, but totally acceptable in 72 degree EPs, do you see any advantage in buying a 17mm Nagler over 17.3mm Delos or 12mm Naglers over 12mm Delos? Nagers with Paracorr will virtually yield the same TFOV as Delos without paracorr with 15% higher magnification, as I understand the calculations in Televue homepage.

Just realised I missed your point. As mentioned I don't see much coma in my Delos but the corrector is meant to sharpen the views across the whole FOV. I'd rather have a 17.3 without the corrector than the 17mm nagler with one. And definitely the Delos comparison with the 12mm.

[Moonshane]

whether you buy a paracorr or not depends whether or not you wish to tighten the star shapes by a lesser or greater degree (depending on focal ratio) across the whole field. if you buy one used they are less than the price of a new Delos or Nagler and will positively affect the whole eyepiece case. for me it's a no brainer.

[Mike73]

Hope linking to another forum isn't breaking any SGL T+C's but I think this thread of Don's about the effect of coma at high magnifications was really interesting and relevant to this thread.

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/5662033/page/0/view/collapsed/sb/5/o/all/fpart/1/vc/1