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Starizona Nexus 0.75x reducer with pvs-14

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#26 Gavster

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Posted 05 August 2022 - 01:19 AM

Gavin,

Interesting!  Both reducer/correctors were designed specifically with AP in mind.  What's the clear aperture of the Nexus?  I read on their site that it produces an image circle of 28mm, suitable for an APS-C sensor; the TS advertises an image circle of 30mm, suitable for the same APS-C sensor.  I saw nothing about clear aperture on either site.  Both have M48 threads for the camera connection and M48 threads for filter application. 

 

Have you used the Nexus in afocal? 

 

About 20 months ago, I screwed a TV 55 to my TS reducer... didn't like the results, although I don't remember exactly why.  Never tried afocal with the reducer again and soon after, sold the 55.  

Ray

Ray,

Here’s a pic I took of the top of the TS and Nexus coma corrector/reducer. You can see the TS has a lip that gives vignetting when 2 inch eyepieces are used afocally. The Nexus doesn’t have this lip.

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#27 sixela

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Posted 05 August 2022 - 04:28 AM

Hi sixela,

 

Thanks for the further clarification.

As I'm using an integrated Paracorr, SIPS, I need to find out if the set position of the Paracorr, although correct for my Televue eyepieces, is also optimal for the Antares and if not what additional or reduced spacing is required.

Most NVDs can't be used in prime focus mode with the SIPS placed optimally. Mine (an OMNI-M) has the photocathode unusually close to the telescope end, and still needs 7.5mm more in-travel than a 21mm Ethos, and that's on a 'plain' P2 where part of the power button can be put closer to the focuser that the tunable top's shoulder. On a SIPS scope the power button interferes with the FT focuser, so that costs you 5mm more.

 

Detuning a Paracorr 12.5mm doesn't do wonders for the spherical aberration and the coma correction, although at 7.5mm off optimal you're still OK (you're not using very high powers which tends to hide the spherical aberration somewhat, and the 40° AFOV effectively means you don't see the worst of coma either).

 

A reducer only makes things worse, since the effective focal plane of the reducer+NVD is pushed even further out.

 

You can use the NVD, but you usually just ditch the Paracorr or SIPS and live with the coma of your scope and the extra aberrations caused by the reducer. Which is (given the small AFOV of the NVD) doable for a f/4.75, but less so for a f/3.72.

 

If you want something better corrected, you're better off removing the SIPS and using a TS Maxfield 0.95x coma corrector (which has a removable stop and can be inserted into the FT focuser). Or -- for more reduction -- the Starizona or TS coma correctors/reducers for 0.73x-0.75x, if you can ge them without a lip.

 

Once you start to barlow an NVD those woes with a P2 or SIPS disappear if you use a screw-on barlow°, since the barlow will move the effective focal plane closer to the telescope. And then the SIPS or the P2 is a boon since it's easy to ge the coma corrector on the right spot, whereas with screw-on coma correctors getting the spacing right is hard.

 

--

°e.g. a Baader VIP element in a M34-T2 adapter, or a Zeiss barlow element plopped into a Baader PushFit adapter with a M42->M48 ring on the Zeiss barlow's M42 threads


Edited by sixela, 05 August 2022 - 05:36 AM.

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#28 GeezerGazer

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Posted 05 August 2022 - 11:24 AM

Most NVDs can't be used in prime focus mode with the SIPS placed optimally...

Sixela, your explanation makes perfect sense.  bow.gif  Thank you.  

 

 

Ray,

Here’s a pic I took of the top of the TS and Nexus coma corrector/reducer. You can see the TS has a lip that gives vignetting when 2 inch eyepieces are used afocally. The Nexus doesn’t have this lip.

Gavin, thanks for the pics.  I suspected as much, because the top lens on my TS reducer/corrector has only a 35mm clear aperture as pictured.  When you do use the Nexus in afocal, let us know how it performs.  If it DOES work, the focal ratio of your 16" Dob would be approaching a crazy f:1.2 with the 67mm, or f:2 with the 41mm!  blush.gif   I'll keep my fingers crossed that it doesn't vignette or create unwanted aberrations.  As Alice said, "Curiouser and curiouser!!"  The Baader high speed filter is shifted for use at f:2...   I know you are going to try this soon.  lol.gif



#29 sixela

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Posted 05 August 2022 - 12:08 PM

Fortunately you can install the filter IN FRONT of the reducer ;-). But I think it's asking a lot for a modified Plössl to work well and not create aberrations at these extreme f/ratios behind the reducer. And I don't really see the need for a reducer combined with a 67mm Plössl, on a fast Newt with a sanely sized secondary the illumination of the outer field would be ridiculously low anyway.


Edited by sixela, 05 August 2022 - 12:53 PM.


#30 John Vogt

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Posted 05 August 2022 - 12:41 PM

Thanks, sixela for the additional education, much to contemplate!

 

I've only used my Mod3C afocally with the 55/67mm plossl so when going to prime

I'll follow your recommendations. 

 

If Gavin is successful using the Nexus with the 55/67mm I'm thinking that the paracorr lens could be removed while retaining the SIPS body and eliminate using the extension tube as that would give the

additional out0travel needed.



#31 sixela

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Posted 05 August 2022 - 01:18 PM

The SIPS assembly wants the original focal plane of the scope at its end (where it connects to the FT focuser). That means for prime focus use of an NVD you'd need to insert a Nexus 57mm into the focuser and get the NVD up to the photocathode into the focuser -- not going to happen.

 

So you're not likely to want to remove just the SIPS lens assembly, but the entire thing, using just the FT focuser onto the base plate (instead of the SIPS+FT). I replaced the grub screws between the SIPS and the FT focuser with thumbscrews for that very reason.

 

If you mean using it afocally, yes, the empty SIPS would work as an extension. But why would you use a 0.75x reducer and the 67mm Plössl? The 67mm already maxes out and shows the full 2" field stop in the NVD's FOV if you don't reduce, so you'd usually just get a lot more aberrations (it's a fairly simple eyepiece, even though the 67mm flavour has an extra group) and severe vignetting, and the f/ratio of an unreduced (or 0.95x Maxfield-reduced) afocal combo with the 67mm Plössl is usually already quite low.

 

Afocally I use this:

NVD_Afocal.jpg

 

It's a TS 0.95x Maxfield coma corrector in front of the 67mm Plössl. It's more convenient than a Paracorr but if you have a SIPS I'm not sure I'd bother, just for the 1.21x difference in f/ratio.


Edited by sixela, 05 August 2022 - 02:48 PM.


#32 GeezerGazer

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Posted 05 August 2022 - 02:21 PM

I knew I had made some notes about attempting to use the TS reducer/corrector in afocal.  These notes were in a response to a PM I received about 6 months after I actually experimented with the TS in afocal.  

 

"I did try it and it was awful for a variety of reasons.

The Ackermann reducer sold by TS Optics in Germany performs very well in prime focus where the NVD is attached directly to it without using the Envis, but it is not suited for afocal use, particularly with the 67mm eyepiece.  The TV 55/67 is very useful for afocal, but using it with an f:2.8 optical system just doesn't work well.  I did experiment trying to attach the eyepiece to the reducer.  Then you would have to attach the NVD with Envis lens to the top of the eyepiece.  It's a very long stack of heavy optics for the focuser, which is problematic at best.  Significant vignetting occurred, such that any gain in FoV was offset by the vignetting.  I would not recommend it for a variety of reasons.  H-a filters also have limitations which become very obvious with really fast focal ratios.  None of my H-a filters worked well with this experiment....

What I've found with my f:4 Newt is that reducing it to f:2.8 with the Ackermann provides a very good balance of speed with virtually zero aberrations; at f:2.8, filters work very well without a lot of vignetting from bandshift which can be introduced by faster speeds or narrower filters when rear-mounting H-a filters."

 

This experiment was before I purchased the Baader high speed 6.5 and 3.5 filters, so they might help.  And, it seems obvious that the 35mm top lens on the TS reducer may have been responsible for significant vignetting when using the 55/67 eyepiece.  

 

Besides these problems though, the focuser on my 8" Newt (inexpensive Crayford style) was simply not up to the task of supporting such a heavy stack of optics.  I do remember that problem specifically.  

Ray



#33 sixela

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Posted 05 August 2022 - 02:49 PM

I don't understand your comment about the H-alpha filters and the faster speed behind the reducer, given that you'd usually put the filter in front of the reducer, where the f/ratio is not yet reduced.


Edited by sixela, 05 August 2022 - 02:50 PM.

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#34 a__l

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Posted 05 August 2022 - 07:05 PM

I replaced the grub screws between the SIPS and the FT focuser with thumbscrews for that very reason.

 

 

The best option is to forget about SIPS in principle. Which is what I did. My regret for wasted money. There are too many rakes in this device. At least in mine.


Edited by a__l, 05 August 2022 - 07:09 PM.


#35 a__l

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Posted 06 August 2022 - 07:26 PM

The last stone in SIPS is the black paint peeling off inside, which got on the optics. I have previously shown photos of comparisons of PII (made in Taiwan) and SIPS (made in FT) on CN more than once. It would be better if they only deal with mechanics.



#36 GeezerGazer

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Posted 06 August 2022 - 09:43 PM

I don't understand your comment about the H-alpha filters and the faster speed behind the reducer, given that you'd usually put the filter in front of the reducer, where the f/ratio is not yet reduced.

What I was seeing was most likely vignetting from the 35mm restriction in the TS reducer and not from band shift.  Sorry.  As noted, the written description about observations was made 6 mos. after the test, so take it with a grain of salt.  




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