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First Nebula Session with 8" f/2.8

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#1 Eddgie

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Posted 11 August 2020 - 08:41 AM

Finally had a night were I could try out the Orion 8" imaging Newt at f/2.8.   I had extended the screws on the primary to raise it up a few weeks ago and had a quick test to make sure it would reach focus with the ASA corrector, but at the time I did not really do more than test it.  Last night was the first time I had a chance to view any nebula with it.

 

Conditions were so-so. At 1x with the binoculars, the Milky Way was easily visible with a of structure, but I could tell that the transparency was a bit off.  The view was just a bit dull. Summer in Austin is almost always a bit hazy and humidity, while not high in absolute terms (probably about 40%) was high by Austin standards. Ambient temp was about 90 (sad for 10:00 PM and the tube I was using was not the lowest EBI tube I had, but last night, sky glow and transparency were the limiting factors). What all of this means is that it as not a great night for nebula but it was what I had, so I used it.

 

As with the 6", the ASA corrector really does produce a very nice well illuminated field of view with sharp stars across most of the field.  I did a vignetting test and I estimate that the fully illuminated circle is probably about 12mm in diameter and drops off by maybe 25% at the edge of the field.  Now this is not the scope...  This is the filter that does this.   I use a filter wheel, and at f/2.8, the filter opening is far enough in front of the photocatode that at f/2.8, it becomes the limit for off axis illumination.   For general work, I always use the filter wheel because of the convenience.  I have a 1.6x Barlow position, open, 610nm (which I really need to change to a 650nm or 695nm, a 12nm H-a and a 6nm H-a. (I really need to bump the 610nm up a position and fill that position with an agressive long pass).  For dedicated nebula under dark skies, I ditch the filter wheel and move the filter to very short 1.25" nose and this can allow almost full photocatode illumination, but when I am observing from home, I always run the filter wheel.

 

Given the so-so conditions I was pretty pleased with the performace of the 8".  While the power is only a little more than 25% higher, I could see the difference in several objects.  I won't bore with repeating all of them, but the Trifid was the best example.  In the 6", the nebula is rather small and the dark lanes are not separated out so well that they appear stark and black, but with the larger scope, I do feel like the extra scale gave me a view that was much brighter than in the 12", and while the scope was probably 50% magnficatgion of the 12", it was still good enough to provide a very enjoyable view.   Cresent was another case where even though the change in scale is small, it does make a nice difference.

 

Where the scope did really well was on rich fields.  The 6" does great under dark skies, giving fields that are dense with stars but under city skies, the inherent baseline limiting magnitude of a 150mm aperture is always going to dull down star clouds.   The best example where I saw a big benefit on non nebula was the Ink Spot and the surrounding area.  The Ink Spot itself was not darker in the bigger scope, but it was larger which I think gave the impression that it was a bit darker, but the field around it appeared richer than when I have viewed it with the 6".  Same for Sagittarius Star Cloud. The field is a bit smaller, but the 8" does produce better star density simply due to the greater limiting magnitude of the larger aperture.

 

The good news though is that modifying the scope for use with the ASA corrector was easy and the results were excellent. Buying an 8" on the used market and adding the ASA corrector is a fraction of the price of buying an 8" Boren Simon.   The only problem with going with an 8" is that few Alt-az mounts will handle this setup well.  The IOptron Az Mount Pro does great with it, but it would not work for a 10", and since there are no 10"f/4 dobs on the market, there is not an easy solution to getting a 10" to work other than a GEM or one of the new convertible GEMs so it can be alt-az mounted. 

 

Easy to make the ASA work with the 8" imaging Newt though and I am guessing it would be the same for a 10".


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#2 GeezerGazer

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Posted 09 September 2020 - 01:50 PM

Nice report Ed.  My own results with an f:4 ES 8" CF Newt + ASA .73x reducer is about the same, except I don't use a filter wheel.  I see full field illumination using 2" filters in prime.  The ES did not require longer screws to advance the primary; it came to focus with the ASA without any alteration to the scope.  The 6" Orion f:4 Newt that I had in 2019, did require minor modification (shorter light path by 2cm) to bring the ASA reducer to focus. The ASA reducer does seem to perform great with a range of ~f:4 Newts/Dobs.  

Ray



#3 Eddgie

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Posted 09 September 2020 - 04:28 PM

Nice report Ed.  My own results with an f:4 ES 8" CF Newt + ASA .73x reducer is about the same, except I don't use a filter wheel.  I see full field illumination using 2" filters in prime.  The ES did not require longer screws to advance the primary; it came to focus with the ASA without any alteration to the scope.  The 6" Orion f:4 Newt that I had in 2019, did require minor modification (shorter light path by 2cm) to bring the ASA reducer to focus. The ASA reducer does seem to perform great with a range of ~f:4 Newts/Dobs.  

Ray

 Yes, it is a great reducer/corrector for getting some extra speed out of these f/4 and f/3.9 imaging Newtonians.  



#4 GOLGO13

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Posted 10 September 2020 - 08:43 AM

  I see full field illumination using 2" filters in prime.  

Geezer,

 

Can you expand on this comment and talk to how your setup looks using 2 inch filters for prime focus?

 

I've been all 1.25 inch so far, but will consider getting 2 inch filters at some point. When I use 1.25 in prime I have a c mount to 1.25 thread adapter and I have a scope stuff c mount to 2 inch adapter for using a 2 inch reducer (which has a 1.25 thread inside of it for installing filters). 

 

Just want to ensure I'm not cutting myself off at all. My scope of interest for this discussion is my 6 inch F4 newt. Though I believe my 10 inch dob has a pretty large secondary also.

 

Thanks!


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#5 GeezerGazer

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Posted 10 September 2020 - 03:46 PM

Can you expand on this comment and talk to how your setup looks using 2 inch filters for prime focus?

 

Just want to ensure I'm not cutting myself off at all. My scope of interest for this discussion is my 6 inch F4 newt. Though I believe my 10 inch dob has a pretty large secondary also.

GOL, going 1.25" or 2" comes down to personal preference and purpose.  In 2017-18, I observed with mostly 1.25" setups for visual use.  I was using a Baader T2 prism diagonal, with a 1.25" filter wheel attached between it and the Mod 3.  I used this for a variety of optics, including a 60mm f:3.8 VersaScope achro, my f:7 TEC 140 and f:5 ST 120 achro (often reduced to f:4)... all refractors.  In 2018, I fell away from my refractors in favor of a 6" f:4 Newt and then an 8" f:4 Newt.  These scopes used in prime focus, offer some benefits that I find attractive, both for phonetography and visual use, one of which is full field illumination of the NVD sensor.  At f:2.8 using the ASA reducer, there is a tiny bit of vignetting that occurs because of the steeper light cone, but nothing like I saw with the refractors.  Most of the f:4 Newts are made to fully illuminate camera sensors, which makes them a natural for fully illuminating the NVD sensor, as most have an oversized secondary.  Eddgie has written about this benefit many times.

 

I prefer having 2" connections between the scope and the NVD because they are more robust than 1.25", plus they allow for a wider light cone to extend nearly all the way to the NVD sensor.  In an ideal world, the C-Mount connection of the NVD would be replaced with a T2 connection instead... then we would not have to worry about the C-Mount restriction clipping a fast light cone.  But overall, even with the C-Mount restriction, there is less vignetting at the NV sensor using 2", or at least T2 connections, right up to the C-mount flange on the NVD.  Small amounts of vignetting are not a major issue for visual use.  But in photos, even a little vignetting can become prominent.  That's why it's an issue for me.

 

Here's the thing... using a 1.25" prime setup for visual use is not bad.  It offers a compact system, using less expensive filters, that offers different benefits, especially if a filter wheel is included for quick looks without the time consuming effort of manually changing filters.  But for photos, I would not recommend that system because it can cause vignetting, depending how far from the sensor the filter wheel is located.  The other issue I found with my filter wheel was that I often encountered reflections that I don't see when filters are connected directly to the ASA reducer.  

 

So, preference and purpose... if you are using a 1.25" system currently and are happy with its performance, I'd recommend you stay with it.   If you are using your Newt at native f:4 with 1.25" filters, it seems unlikely that vignetting would be a problem visually, depending on the distance between the 1.25" filter and the sensor.  On the other hand, if you are bothered by vignetting in that system, then moving to a  2" system might help solve that problem. 

Ray



#6 GOLGO13

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Posted 10 September 2020 - 04:03 PM

Thanks Ray. I have not seen vignetting much using prime focus. As far as I know at least. I do have a 1.25 inch filter wheel I used once. But I probably should break it out again.

 

When using Afocal I notice more edge issues (possibly vignetting). But for the most part it works also. I actually think the edge issues I see are from field curvature of the Envis lens. Because even with a coma corrector I still had poor edges of the view. 

 

If I stick with using the 6 inch F4, the filter wheel works well. But if I am using a few scopes it's not as useful. Though I do have quite a few filters nowadays. I have 610, 642, 685, 12nm, 8nm, 6nm, and 3.5nm. So I could just use the 642 and 8nm in a different scope, and in the filter wheel I could have the 610, 685, 6nm and 3.5nm.

 

It would be nice to not spend the extra money of the 2 inch format. However, it also could be useful to get the new 67mm afocal setup or something like it. Today my afocal setup is the 40mm plossl which I sometimes pair with the .7 Antares reducer.


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#7 GeezerGazer

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Posted 10 September 2020 - 04:22 PM

BTW, it was Eddgie who pointed out vignetting in my old photos taken through my refractors.  When a nebula extends beyond the EoF, where you know it is bright... but in your visual image, the nebula trails off in brightness before it reaches the EoF, then you are dealing with one of two issues that can cause this vignetting:  physical restrictions prohibiting full field sensor illumination OR H-a filter band shift.  Solving one of these issues does not necessarily solve the other... but it could! 

 

For example, using a longer focal ratio optic, creates a "narrower" light cone than a shorter focal length optic having an equal aperture.  The steeper light cone of the fast (short FL) optic is more apt to be clipped by a physical obstruction in the optical system; IIRC, Eddgie once wrote that any light cone faster than f:3.3 would be clipped by the C-mount restriction of the NVD.  Which is why very fast optics often vignette with NV.  We also know that the slower the optical system is, the less effect band shift presents in the NV image.  So under these circumstances, the "fix" can have an effect on both a physical restriction and band shift.

 

You can solve band shift by moving to a wider band pass filter, which may reduce or totally eliminate vignetting caused by a very narrow H-a filter.  But if a physical restriction exists, like a 1.25" filter cell that is too far from the sensor, it can still interfere with, and clip, the light cone, which causes vignetting.  

 

I hope that makes sense.  

Ray


Edited by GeezerGazer, 10 September 2020 - 04:57 PM.



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