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Piero

Star atlas + DSO handbook

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Posted (edited)
4 hours ago, jetstream said:

Many thanks again for the adapter advice Piero, the Baader Pushfix is permanently mounted to the Docter and the TV infocus adapter to the 17.3 Delos.  The Paracorr is also increasing DSO contrast but I need more time to be sure whats happening. As you say it makes the newts much more forgiving.

Sorry to sidetrack your thread Piero, I am ordering another atlas when I get a chance, Uranometria is good but a bit boggling in its format.

One more thing- Mr Nagler said his eyepieces were "sharp to the edge"...when using the Paracorr in newts. I'm puzzled how there are reports sharp to the edge without one in fast(ish) newts.

How do you star hop with the newt Piero? you have a finderscope or use a widefield-30mm UFF?

 

No problem. :)

Yes, the paracorr2 makes the stars tighter, within the Rayleigh limit, across the field of view. 

Note that the paracorr2 doesn't make the Newtonian collimation more forgiving, but the opposite. The tolerances for the focuser axial alignment become tighter:

0.03 * D     : no CC
0.005 * D   : Paracorr1 (6x)
0.0007 * D : Paracorr2 (42x)

where D is the telescope aperture.

 

 

Which atlas have you ordered if you don't mind me to ask? With a 24" under dark skies as yours, I suspect that even Uranometria is "limited". Probably, the best guides to use are those in Alvin's faintfuzzies.com for 20"-30" dobsonians.

 

 

Regarding the "Sharp to the edge without coma corrector".... well, to be short, I don't believe that... Even the concept of "coma free" can be quite misleading. The only coma free region is on axis in a perfectly collimated Newtonian. The "coma free region" only means that although coma is present, this is within the Rayleigh limit of 0.82 Sthrel. If the optics are better enough, coma is certainly visible in this area too.

Some time ago I was testing the effect of coma in my F6 dobsonian. To make it more evident, I thought about mimicking the coma by misalignment. So what I did was to collimate the telescope as well as I could. Then I pointed a bright star and defocused it so that the diffraction rings were visible. Then I left this drift towards the edge. Interestingly, it was possible to see the signature of coma. This was noticeable as soon as the star was no longer on axis and the first diffraction ring was affected from about as little as 30 degrees off axis from the centre. When the star is focused, this is less obvious of course, but doesn't mean that it is not there. This was in a F6 dobsonian free from astigmatism (you know my mirror cell...) and minimal spherical aberration. Repeating a test like this on a F5 or below, for sure, doesn't get any better.. so yeah, a coma corrector is beneficial in my humble opinion.

 

 

Star hopping. I have a telrad and an Antares VS60 RACI finder (great Canadian finder :) ). I use the telrad for pointing, although I often manage to broadly point with the RACI finder directly. Then star hopping is done using the finder and the eyepiece. I don't really use the 30mm APM UFF for star hopping. This eyepiece is used for large low power targets or when I like to scan an area of the sky with a larger field of view. Normally, the docter is in the focuser and so star hopping is done with that eyepiece (145x) plus VS60. This combo works quite well for me. I've also done star hopping with the Zeiss zoom (150-200x) plus VS60. Again, no problem. I like the VS60 a lot and prefer it to 8x50 finders. The reason is that I'm quite accustomed to the light gathering of the TV60, which is also a 60mm clear aperture. One day, I might try something like a 16mm 82 deg and install a cross axis. Doing so I could get more magnification facilitating the detection of DSOs in the finder directly.

 

Edited by Piero

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12 hours ago, Piero said:

Which atlas have you ordered if you don't mind me to ask? With a 24" under dark skies as yours, I suspect that even Uranometria is "limited". Probably, the best guides to use are those in Alvin's faintfuzzies.com for 20"-30" dobsonians.

 

None ordered yet and yes the 24" challenges Uranometria and I do use Alvins excellent resources, gotta love those Hicksons and Arps.

With regards to the PCII and Delos sharpness- I think but don't know that the PCII barlow (telecentric?) has a positive effect on the Delos and if so the PCII "adds" sharpness and reduces coma.

With respect to "adding" sharpness, most likely the wrong term but my 16mm T5 Nagler also benefits from barlowing- with the VIP. The eyepiece is sharper with the VIP and was tested with the TSA120 as well as the newts. I have other eyepieces that benefit in this way too.

Does the PCII increase the coma free area?

Man, I shoved your thread sideways Piero, again sorry.

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9 minutes ago, jetstream said:

None ordered yet and yes the 24" challenges Uranometria and I do use Alvins excellent resources, gotta love those Hicksons and Arps.

With regards to the PCII and Delos sharpness- I think but don't know that the PCII barlow (telecentric?) has a positive effect on the Delos and if so the PCII "adds" sharpness and reduces coma.

With respect to "adding" sharpness, most likely the wrong term but my 16mm T5 Nagler also benefits from barlowing- with the VIP. The eyepiece is sharper with the VIP and was tested with the TSA120 as well as the newts. I have other eyepieces that benefit in this way too.

Does the PCII increase the coma free area?

Man, I shoved your thread sideways Piero, again sorry.

:thumbsup: for Alvin's observing guides. 

 

> I am not sure whether the paracorr2 sharpens the Delos, meaning that it makes them "sharper on axis". However, because the whole field does not have coma, the whole field will be sharper. The fact that the Delos are more similar to the Docter with the paracorr2 , to me is more related that on axis the Docter looses a little bit of transmission, actually becoming closer to the Delos. Said this, on-axis is 1 point, off axis is an area. Therefore, in general the benefit of a coma corrector is considerable. I think you chose wisely when you decided to get one with your telescopes. I am considering one myself, despite the fact that I have a F6 dobson. This because I do see coma. Although not distracting, I like the views delivered by refractors, and the only way to get these views with a newtonian is to add a coma corrector.

> The PCII makes the whole field (up to 42mm field stop if I remember correctly) coma free.

 

> Regarding the 16mm 82 deg, my though is about using it with the VS60 finderscope to get a 14x60 finderscope, with 5.7 deg AFOV and 4.23mm exit pupil. At that power, many DSOs are visible in the finder directly, hence facilitating star hopping. At F3.78, the eyepiece must be free of off-axis astigmatism though, so a Nagler would be the best choice, although expensive for a dedicated use like this. Will see :)

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Posted (edited)
On 07/03/2020 at 13:53, jetstream said:

With regards to the PCII and Delos sharpness- I think but don't know that the PCII barlow (telecentric?) has a positive effect on the Delos and if so the PCII "adds" sharpness and reduces coma.

Here's some data regarding the TV paracorr2:  

( Source: http://www.televue.com/images/TV3_Images/Images_in_articles/Paracorr_2_chart.jpg )

=> That's a massive improvement.. 

The plot to the right is what a lot Newtonian owners prefer to ignore. As you can see, there isn't really a "coma-free region" without a coma corrector.

In my understanding, when the curve is above the Airy disc radius, the optics behave < 0.82 Strehl. 

Paracorr_2_chart.jpg 

Edited by Piero
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2 hours ago, Piero said:

=> That's a massive improvement.. 

The plot to the right is what a lot Newtonian owners prefer to ignore. As you can see, there isn't really a "coma-free region" without a coma corrector

Excellent research Piero!

I havn't seen this before and  because my 15" gives such sharp on axis views without the PCII I am excited to observe the moon and planets under excellent seeing using it!

This might also explain why the DSO contrast is improved noticeably-thoughts?

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Considering "on-axis" like the central dot in the field of view, the views through the PC2 should be almost identical to the views without PC2, minus a tiny loss in light transmission due to the extra glass. Outside that dot, the view through PC2 should be better, because the light is not smeared due to coma. This affects all targets, so yeah, DSO, moon, and planets are included.

To my understanding, the plot to the right essentially shows that without PC2, the view degrades below a Strehl of 0.82, quite quickly as one moves away from the central on axis dot of the field of view. Beyond that point the telescope is behaving sub-optimally, as described in Suiter's book with the MTF tool.

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