Visual Paracorr

[norm]

Hello All,

Are there any negative aspects of using a visual paracorr (eg, Televue type).
Is the wieght an issue and does it diminish the views in any way?

Would you notice anything significant if you used one and then took it off (other than the coma).

I'm thnking any extra glass between the focuser/eyepiece will have some impairment?

Any feedback welcomed!:)

Cheers Norm

[StarLane]

You do loose some light, about 3 percent if I remember correctly?? someone correct me if I'm wrong.

Weight will be an issue if you have a dob and use the Paracorr with a heavy EP.
Other than the removal of coma you also achieve a flatter field, ie, sharp stars from edge to edge.
It also changes your focal length by a factor of around 0.15x.

[Ian Robinson]

I don't know if you've seen this PDF : http://www.cloudynights.com/documents/paracorr.pdf

You can't use it for imaging - that's a very BIG NEGATIVE cf Baader's and the now no longer made Lumicon 48mm Coma Correctors.

I read somewhere that it also is not a TRUE 2" (or 48mm) optic. Another NEGATIVE.

It also lengthens the focal length a bit unlike Baader's (or Lumicon's). I think that is a negative , but you might like that.

Having to buy two separate Paracorrs , one for use visually and the other for imaging, is IN MY VIEW a rip off !!! See the prices of these things (here).

And the tuneable top is a just gimmick designed to coax you to buy TeleView eyepieces.

Have you considered a Baader MFCC.

[ausastronomer]

Hi Norm,

It depends on the scope you are using it in and what eyepieces you plan to use it with. Your sig panel doesn't indicate what scope you plan to use it in, nor your eyepiece set. Care to enlighten me ?

The paracorr in itself isn't that heavy but combined with a big heavy eyepiece, it will cause major balance problems, in a small or medium sized dob. I have a paracorr and only use it in the 18"/F4.5 and only with a couple of my eyepieces. Namely, the 31mm nagler, the 17mm nagler and the 14mm and 20mm pentax XW's. I don't use it with any of my other eyepieces, which are many. The other eyepieces just don't gain enough with it as the FOV is narrower and the visible coma a lot less. In addition, I don't use it with my 10"/F5 dob. There isn't much to gain with it at F5 and it causes balance problems. I use a 27mm panoptic as my low power eyepiece in this scope as opposed to the 31mm nagler in the 18"

My advice would be unless you have a scope over 12" aperture and faster than F5 and own several premium eyepieces that will benefit from the paracorr, save your money. It's better spent on a few good quality eyepieces to better suit your scope.



With the right scope/eyepiece combination it gives an aesthetically nicer view, particularly towards the EOF. Again depending on your scope/eyepiece combo it will show "MORE" stars across the field because brightness of the airy disk is proportional to the spot size which is affected by comatic blur. In other cases you may not see a lot of difference.

To be more helpfull I really need to know what you own and what you might be likely to buy in the future.

Cheers,
John B

[StarLane]

You're right Ian,

I would also consider the Baader MPCC over the Paracorr. Can be used for visual and photographic purposes with no focal length alteration and still achieve a nice flat field.

[ausastronomer]

Ian,

Those comments above tell me that you have never used a Televue tunable top paracorr and are clearly regurgitating internet garbage from someone else that hasn't used one. I own a Televue tunable top paracorr and have also used the Baader MFCC and the older non tunable Televue Paracorr. I can tell you from experience that for visual use the Televue Tunable top model is clearly better than the other two and it isn't even close. The reason being is that different eyepieces require a different adjustment to the focal plane.

In fact the tunable top version works so well you can see the star images tighten up as you tune it and approach the correct setting for that eyepiece, with the settings changing depending on the eyepiece you are using it with. In Norms case its benefit will depend on what scope and eyepieces he owns.

Note my comments relate to visual use only which was the aspect the original poster was enquiring about.

Cheers,
John B

[wavelandscott]

I am new to Paracorr ownership.

I use it only in my f5 12.5 inch Discovery Dobsonian reflector. Although I was not certain that it would be needed at f5 I had the opportunity to get one and I took it thinking at a later date I could sell it if I did not like it...let me tell you I can visually see an improvement and will never part with it.

For the most part I leave it in the focuser all of the time and use it with my Televue and Pentax eyepieces (which use different adjustable settings).

It works a treat and I do now notice a difference with it in and with it out...and I much prefer it in!

The major negative in my opinion is the added weight...combined with Telrad, dew heater etc. it made things a bit top heavy so I have made a sliding weight for the back of my dob to balance it out. It works well.

I don't mind the extra magnification at all...

Yes, I am a convert...

[Alchemy]

I have the visual paracor and it does the job well of improving the edge of field even with quality eyepieces. its not suitable for imaging as you cant get the glass to the right distance for optimal imaging.... for that i bought the Baader mpcc, both items have their uses and are the ones i chose to buy for their respective uses.

[norm]

Thanks All for your replies.

Hi John,

I noticed that Peter Read now makes a compact Truss Dob and if its based on the Obsession (15") I assume its going to be in the F4.2 range. I'm currently tossing up between a Tscope 14" @ f4.7-4.8 (kit form) or possibly the SDM compact - depending on its cost.

I may even bite the bullet on an SDM f15 @f4.8, which I probably could get away without a paracorr. The 2 eyepieces I currently have is a Pentax 10mm Xw and Panoptic 24mm.

Cheers, Norm

[ausastronomer]

Hi Norm,

I have actually used the first protype Peter Read built using his new ultra compact design. It was an 18"/F4.5 and funnily enough we had it side by side with an Obsession Ultra compact 18"/F4.2. Peters' scope had a Galaxy mirror and the Obsession had an OMI mirror. Both optics and scopes were superb.

Peters' ultra compact design copies a lot of ideas from the Obsession Ultracompact, which in itself copies a lot of ideas from John Hudek (Galaxy Optics) original compact design. Peter has also added some of his own design ideas and improvements, which IMO are excellent. Peters' design sacrifices a little bit of compactness in the interests of being a little more solid and rigid. I think Peters' design works the best. In addition to being a little more solid than the Obsession Ultracompact, Peters' scopes are an absolute work of art and a beautifully engineered and assembled product. I also preferred using the F4.5 focal ratio as opposed to the F4.2 focal ratio. Whilst both had Paracorrs the F4.5 scope offers a little bit better performance at EOF. The sacrifice there is that it is a little taller and you need a step ladder more often.

If you are going with a 15" scope, I would be inclined to order it at F4.5 as opposed to F4.2. This would still let you keep your feet on the ground. If going to 16" to 18" aperture you may prefer an F4.2 or F4.3 scope in the interests of keeping the eyepiece height as low as possible.

That aperture and quality of scope certainly warrants spending the money on a paracorr, if it is faster than F4.5. If you end up going with an F4.5 scope or slower, I would get the scope and try my eyepieces in it before ordering the paracorr. I have never felt a need to use the paracorr with my 10mm Pentax XW in the 18"/F4.5. The 24mm Panoptic may benefit from the paracorr at F4.5, but I would try it first.

Cheers,
John B

[GrahamL]

they sure look( I wish I had the spare cash) nice john :)...good luck with your choice Norm.

[ausastronomer]

Norm,

Just to add something which may not be apparent from Peters' website is that he will make the scope at whatever focal length you choose to match the mirror you order. A lot of mirror makers will not deviate from their standard choice of F-ratios but some will. I know Geoff (Starkler) ordered a 15"/F4.8 mirror from Mark Suchting about a year ago to give him exactly the eyepiece height he wanted for his SDM telescope. I know Mark is "trying" to avoid making mirrors at custom focal lengths, but he is also trying to expand his working relationship with Peter. You can always ask him and it would cost extra.

Cheers
John B

[norm]

Thanks John,

That was very interesting news :). I definitely want to avoid using a step ladder or footstool where possible. I'm 6' foot tall, so something like Geoff's 15" f4.8 seems ideal.
I've been getting some prices from Mark Sutching for a 14-15", but might even ask about an 16-18" now. Mark advises he orders blanks twice a yr and wont know the definite prices until May-June.

I'll be giving Peter Read a call in the next few days ans once I know the ~ cost, I'll hopefully have my decision made;).

Clear Skies Norm.

[tnott]

My personal opinion, when considering a big scope, is to either go short and incorporate the coma corrector into the weight etc. of the design or pick a scope that is above F5 if you don't mind ladders. This is what I am doing with my current project based on a Steve Kennedy 22" F3.6 mirror. I have incorporated the weight of the parracorr into my calculations. With my 16" F4.9 scope it doesn't seem to need it.

BTW - With modern multicoatings the light loss from the extra glass would be totally imperceptible to the eye.