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Fast Newtonian Options: ASA Reducer CC vs a True F/2.7 Build

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#1 careysub

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Posted 21 November 2021 - 02:14 PM

Looking at the options for pursuing NV (I have the Mod3C) with fast Newtonians - I had recently found that Waite was offering F/2.7 mirrors in small sizes, at the expected high price point.

 

But I have seen discussion here of the ASA 0.73x reducer coma corrector that shortens the focal length, not lengthen it, like the visual coma correctors (HRCC, Paracorr) that I already have. Thus a (far cheaper) F/3.7 becomes an F/2.7.

 

What is the tradeoff/limitations here? If it is for NV use is there any drawback for the ASA-F/3.7 option?

 

On the side, how does the ASA 0.73x reducer coma corrector perform visually?



#2 scooke

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Posted 22 November 2021 - 11:15 AM

With a f2.7 parabolic primary, you will still need a coma corrector and that is pushing it even for a Paracorr type 2.  I have no personal experience with the comparison of views of whether the 3.7 with the ASA reducer would be better or worse than a 2.7 with a Paracorr type 2 but that would be the relevant comparison.  A 2.7 without a coma corrector would be unusable except the very center of the field.



#3 careysub

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Posted 22 November 2021 - 12:01 PM


 

With a f2.7 parabolic primary, you will still need a coma corrector and that is pushing it even for a Paracorr type 2.  I have no personal experience with the comparison of views of whether the 3.7 with the ASA reducer would be better or worse than a 2.7 with a Paracorr type 2 but that would be the relevant comparison.  A 2.7 without a coma corrector would be unusable except the very center of the field.

My initial post was poorly composed, which I cannot account for.

 

I know about using coma correctors, and have the Paracorr II (with is 1.15 FL increase factor) and the Explore HRCC (1.06 FL factor) for scopes much slower than 2.7, (I consider them essential at F/4.5).

 

I should have compared an F/2.7 with the Paracorr II, giving a final F/3.11, with an F/4.25 and the ASA Reducer/CC giving the same final F/3.11, though in practice it would really be an F/4 (a commonly available FL) giving a slightly faster 2.92.

 

The question is not about not needing a coma corrector, it is whether simply shortening longer FL Newtonians with the ASA rather than springing for a very expensive specialty ultra-fast mirror is a good idea. Is it an even trade-off? Is the ultra-fast mirror of value, given that the ASA is available?

 

The ASA is expensive, $860 from Teleskop, but a 9" F/2.7 is $3100. And I would be able to use it with multiple telescopes including my existing F/4.42 13.1" (which gives F/3.22).

 

Searching on CloudyNights for discussion of the ASA does not bring up much discussion, though I see multiple references in this forum for its use with NV.

 

I see essentially no discussion of its use for purely visual observing.

 

On the astrophotography forum I see that it has been criticized for extreme sensitivity to precise collimation, which matters most for imagers with long and repeated exposures which does not concern me. Also that if requires a lot of back focus, but since I am considering it for custom scopes (and I can easily adjust the focal path in my custom F/4.42) I can build it as I need it.



#4 Mazerski

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Posted 22 November 2021 - 12:46 PM

Carey,

 

Not sure if this helps… I have the TS Boren- Simon 8” f/2.8 with included ASA and f/4 without. 
I have a 12.5” NewMoon Dobsonian and had an SCT and the ASA does not allow focus in either scope.

I use the BS 100% visual with night vision with filter wheel at both Focal Ratios. 
I do not have a paracorr but do have a Badder MPCC. 


Edited by Mazerski, 22 November 2021 - 02:24 PM.

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#5 ButterFly

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Posted 22 November 2021 - 02:07 PM

The ASA needs about 90mm of backfocus, with about 65mm of spacing from the top lens.  That's asking a lot for a visually oriented scope!  I have been looking at the Starizona Nexus, and mostly wondering.  It needs about 55mm of backfocus, which I can actually manage.  It seems better suited for my f/4.17 mirror.  Mind the image circles for afocal use.  The 67PP can be used down to about f/3 before the eyepeice without wasting aperture.  The ASA specs it at 18mm for f/4.

 

As always, check pictures on Astrobin and the like for relative performance.  NV is fundamentally imaging.


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#6 careysub

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Posted 22 November 2021 - 03:49 PM

The ASA needs about 90mm of backfocus, with about 65mm of spacing from the top lens.  That's asking a lot for a visually oriented scope!  I have been looking at the Starizona Nexus, and mostly wondering.  It needs about 55mm of backfocus, which I can actually manage.  It seems better suited for my f/4.17 mirror.  Mind the image circles for afocal use.  The 67PP can be used down to about f/3 before the eyepeice without wasting aperture.  The ASA specs it at 18mm for f/4.

 

As always, check pictures on Astrobin and the like for relative performance.  NV is fundamentally imaging.

A problem with my question here is that I am interested in both direct visual and NV, so it is a "fish and fowl" question, so no forum cover the concerns completely.

 

If I spend $3100 on an F/2.7 mirror it will be in part to replicate the visual experience that Mel Bartel references here:

https://bbastrodesig...7Telescope.html

and also to get a really fast relatively large aperture compact optical system for use with NV.

 

As an ATMer the backfocus issue is not a problem for me as all of the scopes I am interested in using this with are yet to be made, or are builds I can modify as I please (my F/4.42 13.1", my F/4 6"). I built my F/4 6" to use the HRCC as an integral part of the design.

 

The 18mm spec seems not be a problem for the Mod3C (others are already using if for that, and I think that is about the Mod3C sensor size - I calculate 18.85 mm), but I suppose that means it limits FOV with any EP with a field stop larger than that for visual use.

 

That would seem to be the key limitation of the ASA approach then (which is the information I was looking for here), thanks -- I cannot use it for the widest angle views, which is one of the objectives of the fast small scope.

 

I can usefully use EPs with field stops up to about 28mm with fast scopes like this. My pupil has sadly declined from >7 mm in my youth to 5.2 mm now. I hope the decline has stopped here, but have no reason to actually believe that it has. This is an additional reason for my interest in NV which no longer requires wide pupils to see faint objects.

 

Sounds like I should get an ASA then, since I can use it on scopes I have now (with easy to make mods at most) and possibly for an NV optimized reasonably priced compact scope.



#7 careysub

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Posted 22 November 2021 - 03:51 PM


 


I do not have a paracorr but do have a Badder MPCC. 

How do you like the MPCC?



#8 ButterFly

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Posted 22 November 2021 - 04:39 PM

Two considerations to weigh:

 

 

The 18mm spec seems not be a problem for the Mod3C (others are already using if for that, and I think that is about the Mod3C sensor size - I calculate 18.85 mm), but I suppose that means it limits FOV with any EP with a field stop larger than that for visual use.

It's very difficult to predict where the hard stop to the FOV will be without knowing everything about the corrector/reducer, as well as the eyepeice behind it.  There will be vignetting for sure when the field stop is 46mm, and quite possibly a hard stop stopwhere.  Without specifics, which are likely not availble anywhere, that's as close as one can get.  For prime, 18mm isn't bad at all.  The 13 Ethos' effective field stop is spec'ed at only 22.3mm.  That's not the actual field stop size, because there are field lenses before it.  Again, it's a matter of specifics that you may not find anywhere.  For visual, it's not likely not going to be terribly noticable with the available eyepiece options (see below), but with afocal with the 67PP, wth its ~46mm field stop, something has to give.  How much is the mystery.

 

 

I can usefully use EPs with field stops up to about 28mm with fast scopes like this. My pupil has sadly declined from >7 mm in my youth to 5.2 mm now. I hope the decline has stopped here, but have no reason to actually believe that it has. This is an additional reason for my interest in NV which no longer requires wide pupils to see faint objects.

You have to consider the eyepiece's focal length as well then.  If 5.2mm is your pupil, with an f/2.7 output to the eyepeice, that's a ~14mm (5.2*2.7) upper limit to eyepiece focal length without wasting some aperture.  With the P2 as the visual corrector, it's about f/3.1, pushing it up to around 16mm eyepiece focal lengths.  So that's fine for one of the 13-14mm 100 degree options, just shy of a 17 Ethos.  A 17 Ethos isn't a terrible waste of aperture, just a little, but we are all getting older.
 

An f/3.7 mirror, with a P2 as the visual corrector would juice that up to f/4.2.  That would allow a 21 Ethos!  Whether that is a benefit at all depends on the relative effective focal lengths.  For the same sized aperture, comparing a net f/4.2 to a net f/3.1, the added field (ratio of focal lengths, assuming same AFOV) needs to beat the ratio 4.2/3.1 (~1.35).  For a 13 Ethos to a 21 Ethos, the 21 Ethos clearly wins (21/13 ~1.6).  For a 17 to 21, it's a slight loss.(21/17 ~ 1.2).

 

There are all the other combinations of aperture, f/ratio, and CC used to consider.

 

Extension tubes for visual using a P2/HRCC are in your future if you build around the ASA, but so what?  Those have tunable tops ready to go.



#9 careysub

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Posted 22 November 2021 - 06:14 PM


...

 

An f/3.7 mirror, with a P2 as the visual corrector would juice that up to f/4.2.  That would allow a 21 Ethos!  Whether that is a benefit at all depends on the relative effective focal lengths.  For the same sized aperture, comparing a net f/4.2 to a net f/3.1, the added field (ratio of focal lengths, assuming same AFOV) needs to beat the ratio 4.2/3.1 (~1.35).  For a 13 Ethos to a 21 Ethos, the 21 Ethos clearly wins (21/13 ~1.6).  For a 17 to 21, it's a slight loss.(21/17 ~ 1.2).

 

...

 

Extension tubes for visual using a P2/HRCC are in your future if you build around the ASA, but so what?  Those have tunable tops ready to go.

Thanks.

 

This is getting into visual, not NV (fowl, not fish) but:

 

I have a spreadsheet calculating Bartel's etendue figure of merit for a large number of actually available mirror sizes, focal ratios, coma correctors, secondary mirrors and EP combinations that discounts the basic Bartel Etendue FOM (BEFOM) for the integrated light loss from the secondary (which combines the effects of CO and uneven illumination - now helpfully provided by Mel's new improved telescope designer calculators), and the effect of light loss from an excessive exit pupil.

 

So I can try to optimize for what he has experimentally discovered (and later found to be due to etendue) for optimal wide field observing. Given my pupil limitation my existing 13.1" F/4.42 BEFOM-wise does pretty well with my Explore 25mm 100 degree EP (1982 discounted BEFOM) and 1.50 degrees TFOV which is only 15% less than the best thing I have on my spreadsheet (DIS-BEFOM 2341) - the 9" F/3 with the Paracorr II, Ethos 17mm, and a 82mm secondary, giving a 2.33 degree TFOV.

 

Due to the 18mm limitation in field, if I have understood it correctly, the ASA does not really help visual astronomy.

 

But maybe I haven't understood it correctly. The Teleskop site states that it really has a 30mm field that astrophotographers can use by compensating for the drop off with a flat. The human eye is also able to accommodate drop-off pretty well up to 0.5 magnitudes with little effect on perception (Mel calls 0.3 "hardly noticeable" and even 0.5 "is not readily detectable on brighter objects"). Is the effective usable field with actually substantially larger than 18mm at F/4?

 

It does look good for NV optimized Newtonians though. (There! Brought it back on forum topic!).


Edited by careysub, 22 November 2021 - 06:19 PM.


#10 ButterFly

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Posted 22 November 2021 - 06:30 PM

Is the effective usable field with actually substantially larger than 18mm at F/4?

For visual, no doubt about it.  Vignetting is very hard to notice visually.  The 2" Paracorr 2 only fully illuminates less than APS-C sensors.  No one has any issues putting a 31 Nagler in them.

 

NV can't use flats, so there is some balance point.

 

For off-the-shelf scopes, it's the backfocus more than the fully illuminated field that will get in the way.  The focuser just won't be able to go far in enough to get to focus.
 



#11 Mazerski

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Posted 22 November 2021 - 06:31 PM

Ok, I would buy the Boren-Simon scope again and I use it at f/2.8 and f/4... for an 8" scope with NV (prime mode) it does well. The MPCC seems to perform well and I also use it in the NewMoon at f/4.5. My use is 100% visual.

 

I say seems to perform well for as you know, NV bloats bright stars and that may / may not be that noticeable depending on where in FOV the bloaters are.

When I picked up the NewMoon at Ryan's shop, we tested and Ryan threw in a Paracorr 2 and man, that thing is a beast and I can live without it.

I'm still balking at Afocal setup due to the number of pieces and length / weight in focuser.


Edited by Mazerski, 22 November 2021 - 06:31 PM.

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#12 Deadlake

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Posted 05 January 2022 - 11:02 AM

The BS ASA reducer/corrector is no longer in production.

Are there any alternatives to this or is it Envis lense scenario again and buy one as fast as possible?



#13 DavidWasch

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Posted 05 January 2022 - 01:33 PM

Take a look at the Starizonia .75 Nexus. @$495

 

https://starizona.co...-coma-corrector

 

Here's a review of it here on CN:

https://www.cloudyni...or-first-light/



#14 GeezerGazer

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Posted 14 January 2022 - 08:58 PM

The .73x reducer used in the B-S scope was originally marketed as an ASA reducer. About 2018, the B-S reducer suddenly changed names to the Ackermann reducer... even though the physical reducer that was always supplied with the B-S scope did not change.  I'd guess it might have been a patent infringement or something similar.  The Ackermann reducer is still used in the B-S scopes and is available through TS in Germany, but is pricey at ~$700 found here:

https://www.teleskop...hotography.html

 

When you look at this reducer, note how similar it appears to the brand new, just-released, Starizona Nexus reducer that was previously mentioned and sells for $450 as a .75x reducer.  The Nexus is a little heavier and offered with 2" threaded connection, whereas the TS Ackermann is slightly lighter weight and comes with M42/TT connection.  

 

I have used the TS "Ackermann" reducer in my 8" f:3.9 ES Newt since 2018 with excellent results using NV with proper spacing (like in photography).  But I have never attempted to use it for visual with glass eyepieces.  l suspect it could be done if spacing was about right and consideration was given re: the eyepiece field stop, but I have never tried.  I like this reducer because it performs so well with NV in the f:4 Newt to become f:2.8.  I have used this drop-in reducer with two other f:4 reflectors (6" and 12") with NV showing the same flat FoV as seen in my gallery images (my gallery image indicate the optical system used at the bottom of the image when they are opened).  

 

Mazerski, Edggie, Longbond and I, have all used the TS Ackermann reducer, and each of us has been pleased with performance.  

 

I hope this may be of some help.  

Ray


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