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jetstream

Paracorr II @ f4.1

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The Televue Paracorr II finally arrived after almost going to the wrong address- this is life here in rural Ontario.

I'm most likely going to say what the dob mob already knows- this thing really works. It gave a super clean, textbook split of the Double Double with the 21E- edge to edge.  This set up gave what might be the best view of the Veil yet, actually Pickerings Wisp was a mass of twisted nebulosity right around that "bubble" I mentioned in other reports.

I had been told that the reduction of coma should in theory give me better contrast on nebula and the advice givers were extremely experienced optics and scope makers- if this one obs session is any indication it does.

The 30ES 82 was set up using Pensacks advice- I set up the 21E on "A" and focused, then the 30mm ES was focused using the Paracorr's adjustment on "F" roughly. This method worked well, however the 30ES did not offer the contrast the 21E did- by a long margin despite the greater illumination.  Large features or small on the Veil the 21E won handily.

The 10BCO is parfocal with the 21E and used "A" (using TV's Paracorr adapter).  NGC 7331 was observed with its bright core showing well, not all the fleas were observed but the pending moon and some very light high cloud moving in may have affected the view. Stephans Quintet needs very dark transparent skies to show well even in the 24", under the best conditions these galaxies show brightly but get down around 21 SQM mag and or avg to poor trans and the view is killed, observable yes, great view no.

The 10BCO/Paracorr showed M57's central star with ease- a very good sign.

In summary the Paracorr II in the 24" f4.1 dob worked really well. It seems to up the contrast on nebula in particular and doesn't seem to impact galaxy views but more time on this is needed. The 21E came into its own with this device, I was rocked back by the view on the Veil. BTW I have a 17E, 17 Nikon HW to compare to with no Paracorr...

The sky was 21.5 mag with avg trans that worsened with time.

Edited by jetstream
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Sounds great, Gerry! :)

Any chance to test the Docter and Zeiss Zoom too? I've not read a comment on how those two eyepieces perform with the paracorr2. 

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Match made in heaven, 21mm E and the paracorr. Don't be afraid of going away from the markings for various eyepieces, I actually feel the 31mm Nagler is better a little away from where they say.

Alan

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52 minutes ago, alan potts said:

Match made in heaven, 21mm E and the paracorr. Don't be afraid of going away from the markings for various eyepieces, I actually feel the 31mm Nagler is better a little away from where they say.

Alan

Thanks Alan, I will try this- what a difference the Paracorr makes.

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58 minutes ago, Piero said:

Sounds great, Gerry! :)

Any chance to test the Docter and Zeiss Zoom too? I've not read a comment on how those two eyepieces perform with the paracorr2. 

Thanks Piero,I will when the weather co operates - last night I had a tiny window to try it out. I wanted to see how it affected the 10BCO first but the Docter was next up but so was the moon. I hope this straightens out the Zeiss zoom, it doesn't like f4....

I can tell already the Paracorr is a winner Piero, I want to test it on those small faint averted galaxies to see any impact.

You buying one?

Edited by jetstream

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13 minutes ago, jetstream said:

Thanks Alan, I will try this- what a difference the Paracorr makes.

I have always said why pay TeleVue money and then put up with coma.

Alan

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Funnily, the line from @alan potts is a dilemma I am having in another thread.
Is the Paracorr 1 still good chaps?
Not enough funds for a 2 at this time.

Edited by Alan White
typos of course

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4 minutes ago, Alan White said:

Funnily, the line from @alan potts is a dilemma I am having in another thread.
Is the Paracorr 1 still god chaps?
Not enough funds for a 2 at this time.

Alan,

I did a comparison review in the equipment section about 3 years ago as I had both at the same time. In brief the PC 2 is better but there is not a lot in it, PC2 was better with the Ethos rang as when the PC 1 was marketed the Ethos didn't exist, still very good I would say. The trouble is I think there was actually 3 paracorrs from TV and I had the very first, never been able to substantiate that though.

Alan

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When you know...you know..have fun mate 

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2 hours ago, jetstream said:

Thanks Piero,I will when the weather co operates - last night I had a tiny window to try it out. I wanted to see how it affected the 10BCO first but the Docter was next up but so was the moon. I hope this straightens out the Zeiss zoom, it doesn't like f4....

I can tell already the Paracorr is a winner Piero, I want to test it on those small faint averted galaxies to see any impact.

You buying one?

I haven't decided yet. 

Before this, I want to reduce the currents in the tube and understand the interplay between fan and light shroud.

I am not a great fan of 100 Deg eyepieces (which is a good thing when coma concerns), but I can still see coma with my other wide angle EPs. At f5.9 this is not too much on star fields, but it is a bit annoying when observing the Moon e.g.  with the docter. I am pretty sure planets are also affected although I tend not to observe them near the edge. 

Your new paracorr2 will also give you rewarding views with your 15", given the small native diffraction limited area.

I think you made a very good decision in getting one. You should also get considerably better views of fields of galaxies (e.g. virgo).

Edited by Piero
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How is coma impacting your view of the Moon Piero ?

 

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1 hour ago, John said:

How is coma impacting your view of the Moon Piero ?

In terms of loss in detail off-axis (from about 50 deg AFOV). To my eye it is rather soft when compared to the crisp axial view. 

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Is that Coma or
is that the Parabola of the mirror having differing focal distance centre to edge?
or is that Coma?

Sorry for such an odd question, just curious.

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16 minutes ago, Alan White said:

Is that Coma or
is that the Parabola of the mirror having differing focal distance centre to edge?
or is that Coma?

Sorry for such an odd question, just curious.

This link explains it simply, spherical aberration " Characterization

  • Image Appears Blurred, Rays from Edge Focus at Different Point than Rays from Center
  • Occurs with all Spherical Optics
  • On-Axis and Off-Axis Aberration
  • Third Order: W040 = ρ4"

image.png.2876ee3d04732967642414359bb7d522.png

 

Coma," Characterization

  • Occurs When Magnification Changes with Respect to Location on the Image
  • Two Types: Tangential (Vertical, Y Direction) and Sagittal (Horizontal, X Direction)
  • Off-Axis Only
  • Third Order: W131 = Hρ3;cos(θ)"

image.png.8fe9f21a101cda79a54f67d400df8d55.png

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10 hours ago, Piero said:

Before this, I want to reduce the currents in the tube and understand the interplay between fan and light shroud.

In my brief experience with truss dobs it seems to me they have unique likes. At times I leave my shroud pulled back at the top of the mirror box to allow any heat to rise up there. I cool with the fan-pre seacan that is- and then turn it off to observe, I find tiny blurring caused by the fan.

The white seacan keeps the 15" and 24" within 4 deg F of ambient, which is great and with no early morning dew. When its really cold I keep the shroud all the way on to combat body heat wrecking the views, this can make a big difference as does the stance taken to observe.

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7 hours ago, jetstream said:

In my brief experience with truss dobs it seems to me they have unique likes. At times I leave my shroud pulled back at the top of the mirror box to allow any heat to rise up there. I cool with the fan-pre seacan that is- and then turn it off to observe, I find tiny blurring caused by the fan.

The white seacan keeps the 15" and 24" within 4 deg F of ambient, which is great and with no early morning dew. When its really cold I keep the shroud all the way on to combat body heat wrecking the views, this can make a big difference as does the stance taken to observe.

Thanks for sharing this, Gerry. I agree with you about the fact that truss dobsonians can be quite demanding.

In my case, I saw a noticeable improvement since the telescope lives in the shed. This is a well built one. Temperature inside is about 1 C higher than ambient.

In the summer, the telescope dislikes the light shroud, whereas it loves the fan. The effect is rather obvious. As we are approaching the colder seasons, body heat does affect the views, so I started using the light shroud. This works, but does not like the fan on! Therefore nowadays the fan is on when I mount the telescope, but switched off when I mount the light shroud before starting observing. 

I will try your approach with the light shroud next time I observe.

---

Regarding the coma corrector with medium fast newtonians, my position is as follows:

1) it is beneficial in the same way a good collimator can make a difference.

2) if UWA eyepieces are used, coma can damage the views. If a coma corrector is not used, it makes me wonder why one would get premium eyepieces and telescope optics, without wanting to fix an intrinsic issue that certainly (due to physical laws, not personal opinions) transforms that "premium" to "average". Cost is the only factor I can think of. Still, if one spends 3K pounds in eyepieces, 500 pounds in a CC is not a big deal to me, particularly considering that the views through all the eyepieces will improve.

3) the larger the aperture the more coma becomes an issue. Again not my opinion but basic physics, due to the increase in brightness of the airy disk.

4) I think coma is less noticeable under light polluted skies, simply because the comatic smear mixes with the brightness of the sky. Said this, it is still visible on planets and the moon off axis. On planets coma would be less critical if the mount is auto driven.

5) I think that other issues need to be sorted before tackling coma. For example, careful analysis of the mirror cell and secondary mirror attachment to avoid astigmatism, collimation to avoid miscollimation-dependent coma and potential astigmatism when the miscollimation is large, structure robustness (mirror cell, truss poles, secondary mirror holder and spider, focuser), minimisation of temperature difference between environment and optics, tube current minimisation, decent eyepieces that don't have noticeable astigmatism and chromatic aberration off axis. Some of these factors affect the views very negatively (and regarding some I speak by experience..). Finally, once all the above is within acceptable tolerance, meaning that they are not perfect but they don't affect the views significantly, it is the time to assert coma. Compared to the previous points, this is easy to deal with as coma correctors are available in the market.

---

Regarding myself, yes, I am not excluding the purchase of a coma corrector, because:

1) I like dobsonians and don't see myself without one in the future

2) I like refractor-like views

3) if I ever had to change dobsonian in the future, the replacement will be faster than f5.94. In fact, below 12", I would only get a 10" Chinese dob which is faster, above 12" it must be faster, and if I move into night vision a faster telescope works better. In all cases a coma corrector will be useful. 

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2 hours ago, Piero said:

Still, if one spends 3K pounds in eyepieces, 500 pounds in a CC is not a big deal to me, particularly considering that the views through all the eyepieces will improve.

In my case there were a couple of issues for the wait- cost yes, with a but...I wanted to be re assured that the Paracorr would work very well, its too much money (for me) to buy and have not work.

Second, my 15" at f4.8 gives such good lunar/planetary views I didn't think it was needed, however I think an improvement will be made with the CC. I'm curious about 2 things- on axis sharpness and threshold objects with the CC.

Oddly enough I use far less eyepieces with the 24"?:dontknow: some might be on the chopping block, with a Delos or 2 coming (back) in. I just cant wait to try the 21E/Paracorr II on the Swan neb.... the Nikon HW might get chopped, we'll see.

Down the road I would investigate sub f4 dobs Piero, in the 16"-18" range and the corresponding need for truly engineered mirror cell design. Lockwood is one of the only makers really into this, but a couple of others can do it ie Terry O, not sure about EU. NV will add a different perspective on objects and offer much flexibility to observing.

Me, I'll slog away with a few pieces of polished glass and a dark sky lol!

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John Nichol makes outstanding mirrors below F4...

Gerry, the bigger the dob the less ep's you use..most of my observing sessions are with the 21 and 8 mm ethos...Mapstar has plenty of nights where the 13 ethos is the only ep he uses in his 22".

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On 19/10/2019 at 14:32, jetstream said:

Second, my 15" at f4.8 gives such good lunar/planetary views I didn't think it was needed, however I think an improvement will be made with the CC. I'm curious about 2 things- on axis sharpness and threshold objects with the CC.

I originally thought I would use my paracorr for dso's and not use it for lunar but having tried with and without in different circumstances now I always observe with it in. The moon is super sharp from edge to edge.

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Plus 1 for the Paracorr Type 2, makes a massive difference, I use one in my 20 inch f4 and the difference is astounding.

 

Enjoy,

 

Mark

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On 20/10/2019 at 12:36, estwing said:

John Nichol makes outstanding mirrors below F4...

Gerry, the bigger the dob the less ep's you use..most of my observing sessions are with the 21 and 8 mm ethos...Mapstar has plenty of nights where the 13 ethos is the only ep he uses in his 22".

Had a poor sky last night so I got around to fine tuning the 24" and tried different EP's with the Paracorr. The 21E/17E really work with the PC- TV knows what they are doing.

I can see why you guys only use a small few eyepieces now- whats your thoughts of the Delos in the bigger scopes? I'm thinking of a deep eyepiece or 2 for easy use with the Paracorr.

My Nikon HW so far wont work with the PC which is too bad, it is a superb eyepiece. The Docter is most likely out too, but I'll try TV's infocus adapter before getting the 13E.

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