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TeleVue Paracorr Type 2 vs. Type 1

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#1 Peter Natscher

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Posted 05 September 2010 - 08:43 PM

Last night I compared my newly arrived TeleVue Paracorr Type 2 with my older TV Paracorr Type 1 on my 24" Starmaster FX f/3.7 Lockwood tracking dob. My scope was carefully collimated, as usual, and I spent 4+ hours observing deep sky objects from open clusters, planets, planetary and bright nebulae, and globular clusters. The seeing was ~1 arc-sec, RH < 50%, temp remaining at 70 deg. F. all night, and the transparency was superb. The eyepieces used were: 21, 13, 8, 6mm Ethos and a Nagler 3.5mm. The elevation of the observing site is 2,700 ft. on Fremont Peak in California, about one hour's drive from where I live in Monterey by the Pacific Ocean.

By dark, I started my observing comparison between the two Paracorr's by using my older Paracorr Type 1 first as my basis. I inserted a 13mm Ethos eyepiece into the Paracorr providing 200X and a 30' fov and pointed the scope to the globular cluster M15 in Pegasus. M15 was slowly slewed all around the fov in the 13mm eyepiece to see the effect on its many fine stars. The view showed what I was used to: coma-free stars only 3/4 out from the fov center. At the fov edge, a small amount of coma still remained. This performance has always been similar for my other Ethos eyepieces. In contrast, the TV Paracorr Type 2 performed much better with regards to reducing coma. In fact, it eliminated it! With all the Ethos eyepieces and 3.5mm Nagler, there was no coma seen at all. Sharp stars right to the edge of the fov in all eyepieces. I am very happy about this. With regards to object detail performance at the eyepiece fov center, I didn't see any difference between the two Paracorr's. Nor did I see any overall fov contrast differences.

There is one issue I did see: my new Paracorr Type 2 shows extraneous light entering into the view if there is any light still remaining outside in the way of twilight or someone's flashlight (white or red). In the Type 2's design, the longer barrel pushed the field lens (the outer lens) down all the way to the end of the focuser barrel exposing the unbaffled lens to incident light and to any extraneous light outside of the Dob. This shows up while looking in the eyepiece as a full diameter (one side to the other) crescent shaped light reflection hugging the edge of the fov opposite from where the light is entering at the Paracorr's field lens. The Type 2's longer barrel now positions the field lens all the way down to the end of the focuser tube in my Starlight focuser. The field lens is now exposed to light coming in at an angle at the Dob's upper assembly. I switched both Paracorr's back and forth to see this effect and see where the field lens was positioned while standing in front of my Dob's upper assembly and secondary mirror. My older Type 1's barrel and field lens is positioned and hidden (still baffled) farther up inside the focuser barrel. It's baffled against extraneous light. I could easily see the Type 2's field lens having no baffle protection from the focuser tube in its operating position. The extraneous light seen in my eyepiece's fov (in the shape of a 3-day old crescent Moon hugging one side) was only apparent if there was still enough light remaining outside: like someone's flashlight at a distance pointing at my scope or the evening twilight 1-2 hours after sunset. At the time, we were observing Venus one hour after sunset and I was seeing the crescent shaped reflection covering 30% of the view in my Ethos 13mm. I switched back to my Paracorr Type 1 and this reflection wasn't seen. This front-end Paracorr light leak is a big difference between the two Paracorr's. I would appreciate hearing from anyone else who observes this issue with their new Type 2 Paracorr's. Thanks.

#2 gb_astro

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Posted 05 September 2010 - 09:08 PM

And possibly dew formation?

gb.

#3 a__l

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Posted 05 September 2010 - 11:40 PM

To cut pipes truss. If not suffices length to replace focuser with long travel.
Variant to add light baffle. But I would prefer the first variant, including because of dew.

#4 Cyclop_si

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Posted 06 September 2010 - 01:21 AM

As alternetive to longer focuser drawfube, Paracorr 2 can be probably extended with filter threaded tube extenders (like TeleVue 2" skirt or Badder Hyperion 14 mm or 28 mm extenders).

#5 thrawn

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Posted 06 September 2010 - 02:08 AM

Hey that's an intersting report about the new model Paracorr.

Are you thinking about adding baffling somewhere to help that issue?

And a n00b question here: when you say the seeing was 1", does that mean that you could not have black space split a 0.8" double, say? That's pretty poor seeing I guess.

#6 JimMo

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Posted 06 September 2010 - 08:30 AM

Interesting, I hadn't noticed that. I just put the PC2 in the focuser to check and mine IS baffled a little by the 2" draw tube. Even so, I could see it dewing up, especially here in Michigan. A small 12v. blow dryer might have to become part of my kit.

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#7 turtle86

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Posted 06 September 2010 - 10:10 AM

I've always used a 2" Kendrick dew heater strap on my Paracorr, but that was to keep 2" eyepieces warm without having to mess with the strap each time I changed eyepieces. Sounds like the strap will now also help keep dew off my new Type 2 Paracorr.

The threaded barrel extender and blow dryer ideas sound like good ones for those who don't use a Kendrick-type system.

#8 Peter Natscher

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Posted 06 September 2010 - 10:12 AM

The RH was below 50%. Our scopes, vehicles, and all optics were bone dry.

#9 Peter Natscher

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Posted 06 September 2010 - 10:22 AM

I used 1 arc-sec seeing as a general description of magnitude 6 star sizes at 300X early in the evening while comparing the two Paracorrs. I didn't pay any attention to double stars. My 24" mirror was not fully equilibrated yet. The time was one hour after sunset (7:30pm) until 10pm. I'm not sure I want to add anything to the focuser or Paracorr front end since this might protrude into the primary mirror's light path. This stray light issue was only seen before total darkness after twilight. During normal observing at our site (Bortle 4 darkness scale), I couldn't see it the reflection anymore.

#10 Peter Natscher

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Posted 06 September 2010 - 10:27 AM

Yes, but as your photo shows, you can see how exposed the field lens of the Type 2 is to the front end of your scope. It is plainly visible from your angle of view and extraneous light will hit this field lens traveling up through the Paracorr barrel and seen in the eyepiece. I believe that's what happened to me.

#11 turtle86

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Posted 06 September 2010 - 10:48 AM

I'm thinking that a barrel extender might address both the extraneous light issue you raise as well as the potential dew issue raised by another poster in this thread.

Not sure if a barrel extender would work for me as I have an Astrocrumb filter slide...

#12 Peter Natscher

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Posted 06 September 2010 - 11:03 AM

Attached are three photos I took of my Paracorr in the focuser of my 24" Starmaster f/3.7. Focuser is in approximate focus position. The Type 2 has a larger diameter field lens than the Type 1. Its in-focus position and its larger field lens position almost at the end of the focuser allows it to grab more extraneous light than in the Type 1 model. The Starlight focuser's built-in tube baffling is of no use in this arrangement. I might need to resort to my past scope assemble baffle to protect the Type 2's lens exposure.

#13 Peter Natscher

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Posted 06 September 2010 - 11:07 AM

Here are the photos.

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#14 Peter Natscher

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Posted 06 September 2010 - 11:08 AM

Photo 2

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#15 Peter Natscher

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Posted 06 September 2010 - 11:09 AM

Photo 3

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#16 Starman1

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Posted 06 September 2010 - 10:44 PM

Easy to fix the direct light problem in the new Paracorr:
Install a light shield opposite the focuser extending upward from the UTA.
Indeed, on your scope the secondary is so close to the top of the scope a complete light shield (that goes all the way around) would be beneficial for dew protection AND light shielding.
Here is a link to a picture of what I mean:
http://kenmartin.net...ht shield 1.jpg
It definitely helps contrast.
It can be simple to install and remove.

#17 a__l

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Posted 06 September 2010 - 11:13 PM

Very interesting Feather Touch focuser with integrated Paracorr type 2 as standard. Variant to increase the height of a flat base? What offers the manufacturer? Wait a serial dobson with this option ....

#18 Shneor

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Posted 08 September 2010 - 06:31 PM

Hi Pete,
I, too, think a barrel extender is the best solution, especially in the event people inadvertently point lights at your scope. I may have a similar problem with the 9mm ES100° I recently purchased, which has the field lens and field stop butting right up against the filter threads. I did not have a problem with this on first light, Saturday night, but there was only one other observer and Shot Rock was quite dark.
Clears,
Shneor

#19 Starman1

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Posted 09 September 2010 - 12:10 AM

A barrel extender would protrude into the light path, causing diffraction.
A simple light shield does the job without causing any extra diffraction and also serving the additional benefit of reduced dewing issues.

#20 sixela

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Posted 09 September 2010 - 01:42 AM

A barrel extender would protrude into the light path, causing diffraction.
A simple light shield does the job without causing any extra diffraction and also serving the additional benefit of reduced dewing issues.


The problem is that the dimensions of the light shield you need depend on the baffling (i.e. how exposed the lenses are). In theory, put a lens fully exposed at the end of a barrel and the light shield needed to ensure only object light reaches the lens is of infinite size.

So a barrel extender (and moving the focal plane so that the barrel extender does not intrude) may be a sensible part of a total baffling solution (usually also including a light shield or a well sized UTA, which is also a baffle). It all depends on what illumination you want at the edge of the field and how large a central obstruction you will accept...

Of course, you actually have bafling *behind* the lens to ensure that whatever shouldn't form the image gets caught *in* the Paracorr. There is such baffling and flocking, actually, and it's fairly effective at eliminating anything caused by grazing light reaching the lenses (which then means you can get away with a smaller light shield).

#21 turtle86

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Posted 09 September 2010 - 08:36 AM

I just placed my Paracorr Type 2 into the focuser, and found that the field lens is inset only a little bit, pretty much to the extent shown in Jim's photo.

I haven't been able to try out the Paracorr Type 2 in my 18" yet, so I'm a little curious to see how much dew formation and extraneous light might be a problem for me. Right now I'm probably more concerned about the dew than the light--here in Florida the RH is typically around 80% and the Paracorr Type 2 does appear to have excellent internal baffling. I figure that a dew strap on the Paracorr will address the dew issue, and if I do see extraneous light, then I might experiment with a light shield and/or barrel extender. The latter would be the simpler solution, though of course intrusion into the optical path then becomes an issue, as Don points out. Hard to say whether the diffraction would be noticeable, especially if the intrusion were, say, only a half inch or less. Of course, the focal plane can always be moved as Sixela notes. In any case, it does appear that there is more than one solution to the potential problems here. More important, the solutions are simple, the problems minor, and are greatly outweighed by the benefits the Paracorr Type 2 provides, especially for a very fast scope like Peter's.

#22 GeneT

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Posted 09 September 2010 - 03:16 PM

Excellent report! I have been considering upgrading from a Paracorr 1 to 2. You report that the coma reduction is better in the 2, but that you get light reflection in it that you don't get in the 1. Question: your opinion please--does the additional light entering the system counter the better coma correction? Put another way, will you most often use the 1 or 2?

#23 Starman1

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Posted 09 September 2010 - 03:30 PM

The thing that makes me lust for a Paracorr II is the proper correction for the 31 Nagler and 21 Ethos, which combined are 50% of my eyepiece collection. The Paracorr I does not perfectly correct either of these.

#24 Peter Natscher

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Posted 09 September 2010 - 04:50 PM

Once it was dark, two hours after twilight, I noticed no light reflections showing up in my Ethos eyepieces using the Type 2 Paracorr. The Type 2 eliminates the coma that still existed in my Type 1 Paracorr. I would guess that the diffraction limited area in an eyepiece is also increased in the Type 2. I would have to do more planetary and double star observing to test this out. I recommend replacing your Type 1 with the new Type 2.

#25 Peter Natscher

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Posted 09 September 2010 - 04:55 PM

If you're talking about moving the primary mirror forward in relationship to the secondary mirror, then I'm a bit reluctant doing this if you don't know what you're doing. You could cut off some of the primary mirror's light come at the secondary mirror by moving the primary forward too much. This would be like losing aperture.






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