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Experiments in extreme focal reduction

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#51 Eddgie

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Posted 22 June 2018 - 04:14 PM

Thanks Glenn for chiming in.

I guessed it was field curvature and astigmatism so looks like that's the case. On field curvature, I thought it was mostly a function of the focal length. A 14" f2.93 with 55mm afocal is running roughly 530mm focal length.

 

On my focal reduced 90mm refractor (with a simple 2" 0.5x reducer) at f/3.6 I am running 325mm focal length. But I don't see nearly the amount of field curvature or astigmatism present on the edge of the field:

 

iZkXnru.jpg

 

Given the very different setups what aspect of it do you think is causing the difference in amount of FC and astigmatism seen? My hunch is the Plossl is introducing it - given it may not be well corrected at that speed at the edge, but I don't have a 55mm Plossl to compare.

There is a goose in a wading pool in this picture!

 

Fun snap.  I like it very much!



#52 Jeff Morgan

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Posted 22 June 2018 - 07:42 PM

What I'd REALLY like to see here is a couple of photos through Ed's Boren Simon or Jeff's Tak Epsilon to SEE the difference an astrograph makes in field illumination. 

 

Hopefully that is coming after the next full moon.

 

My Epsilon is mounted non-tracking alt-az which will probably degrade the NiteCap stacking.

 

The good news is a Losmandy G11 is in the works (primarily for another scope, but you know how that goes). Just waiting on the next production run. Joe at Parallax is already making the required rotating ring set. So, that will eliminate the trailing and field rotation issues.



#53 Eddgie

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Posted 22 June 2018 - 08:05 PM

My phone camera is just not very good in low light.  I by not very good, I mean that even in the house, images are often blurred.

 

I will though try it to see... Maybe get Kelly's iphone 6, but that is not the best low light camera ever either. 

 

If not, I will see if my friend, the professional photographer can help. 



#54 moshen

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Posted 22 June 2018 - 08:33 PM

The iPhone 6 will work fine. Just got to figure out the NightCap app which isn't that intuitive.

 

I don't have a tracking mount, all my shots are alt-az but running wide FOV so it works.



#55 GlennLeDrew

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Posted 22 June 2018 - 08:46 PM

moshen,

We cannot base field curvature on the effective f.l. as derived from the use of a reducer and the effect of the eyepiece. (The following presumes a reasonably flat field produced by the NVD objective...)

 

The field curvature (f.c.) we ultimately suffer is the difference between the telescope objective's f.c. and the eyepiece's f.c. If both complement the other, there is no defocus due to f.c. Each telescope objective form/design, and each eyepiece, have their own degree of f.c. And so comparing systems which differ in design, dimensions and combination of components is expected to yield at least some difference in resultant f.c.


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#56 Mike Lockwood

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Posted 22 June 2018 - 11:04 PM

I see the same edge star aberrations with some of Lockwood's 55mm afocal photos that use a SIPS Paracorr.

Here's a link to one of them off his blog post (14" operating f/2.93):

They are less evident in the photos where he uses a slightly slower scope but still there. Here it his 20" operating at f/3.45

Perhaps Glenn or Mike could tell us what kind of aberration that is and if it's introduced by the eyepiece as it's not something I see with a focal reducer.

If you look at all the images, in some of my images it was present more than in others.

 

I believe that some of the inconsistency was due to unintentional defocus of the NV objective itself.  It's difficult not to rotate it and change focus when attaching and removing items from the NVD.  Clearly I need a reference mark to make sure the focus is consistent at the point that minimizes the aberration.

 

The remainder is from the 55mm eyepiece operating way outside its design envelope.  However, it's still a hell of a lot of fun to use at sub-f/3.0, so I can't complain, especially given its modest cost.



#57 GlennLeDrew

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Posted 23 June 2018 - 06:08 AM

Mike,

If focusing the NVD objective is a pain, if it's anywhere near close to focus you could go to the scope focuser. Unless you need to keep that locked down due to the weight of all the stuff hanging off it. ;)



#58 GlennLeDrew

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Posted 23 June 2018 - 06:12 AM

Mike,

If focusing the NVD objective is a pain, if it's anywhere near close to focus you could go to the scope focuser. Unless you need to keep that locked down due to the weight of all the stuff hanging off it. ;)



#59 Starman81

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Posted 24 June 2018 - 04:08 PM

Really great work and analysis to take NV observing to the closest level of aesthetic perfection as possible. There is so much to process in this thread.

 

Ray (GeezerGazer), your work is really amazing and the phonetography examples are absolutely critical in describing the different pros/cons of each setup. 

 

Yes there can be a varying amount of FC and/or astigmatism, and I have always seen barrel distortion in all pictures/videos through NV units. But as Eddgie said in a previous post: 

 

 

All approaches have their strengths and all have their compromises, and the fact is that there are many ways to get there with NV and no matter how we choose to do it, we are going to see amazing sights! 

 

So true!


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#60 GeezerGazer

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Posted 26 June 2018 - 12:43 AM

My Epsilon is mounted non-tracking alt-az which will probably degrade the NiteCap stacking.

 

The good news is a Losmandy G11 is in the works (primarily for another scope, but you know how that goes). Just waiting on the next production run. Joe at Parallax is already making the required rotating ring set. So, that will eliminate the trailing and field rotation issues.

Jeff, if you can select targets closer to Polaris, you should be able to stack for about 2.5-3.5 seconds at your native Epsilon FR.  Keeping the ISO as low as possible will help.  When you are using Night Cap, watch the screen as you make adjustments to the ISO & exposure... remember that if you have exposure set to 1/2 sec., the delay for adjustments in either ISO or Exp will take 1/2 sec to show up on screen.  I'm looking forward to seeing your results. I'm still traveling at new moon, but I'll be watching for your post in this thread.  waytogo.gif

 

There is so much to process in this thread.

 

Yes there can be a varying amount of FC and/or astigmatism, and I have always seen barrel distortion in all pictures/videos through NV units. But as Eddgie said in a previous post: 

Starman, that is for certain!  We all have different tolerance levels to distortions and there is a significant difference between using NV for photographic and visual use.  But either way, NV opens so many windows of opportunity.  This testing has shown me where my tolerance level resides, so in that respect it has been quite helpful.  I am, however, anxious to see results from the fast Newt astrograph, because it should show exactly what Eddgie has been proclaiming for a long time... full field illumination.  The brighter image will probably be mitigated by a slower ISO in imaging, but the field illumination is what I am really interest in seeing. 

 

Field curvature is the predominant distortion I've seen in my own photos, but I have found that a slower FR scope helps that issue.  When I get home in a couple of weeks, I'll try to take an afocal image with the 55P through my TEC 140 which is f:7.  I think it will show less curvature than the f:5 ST 120.  Here is a prime NV image through the f:7 scope that I took in May of M3, ISO 200, 1/6s, 15s stack:

 

IMG_1281.jpg

 

Field curvature isn't a problem for me in this photo... there aren't many stars at the edge of field!  lol.gif   Even in the next photo of Omega Centauri, using the ST 120 at f:5, I can live with what curvature shows up, ISO 1250, 1/2s, 4s stack 

 

IMG_1071.jpg

 

What I find objectionable is curvature that shows in the outer field as prominently as is shown by the 40 and 55 Plossls in my afocal images, especially with an added reducer.  If I had a PVS 7 instead of the Mod 3C, I'd be forced to live with that curvature.  Thankfully, the Mod 3C gives enough flexibility to go the prime focus route, which pretty much eliminates the problem... but at different scale than the afocal images.  The 55P is working at the same level as a .5x reducer in prime, which in my scopes vignettes AND shows field curvature.  So, reaching appropriate scale for me means that I will be changing scopes.  wink.gif  I'm OK with that compromise.


Edited by GeezerGazer, 26 June 2018 - 01:00 AM.


#61 Eddgie

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Posted 26 June 2018 - 02:07 PM

Why would you be forced to live with the curvature if you had a PVS-7?   It will focus anywhere a Mod 3 would and has the same kind of photocthode.  The only difference between 10130 and 10160 style tubes is that one has a fiber bundle that inverts the image (10160 if I recall correctly) and one does not.

 

Anyway, I am not sure why the PVS-7 would have field curvature.  It should be identical in terms of ability to reach focus with C mount and work in the same mode as the Mod 3 in telescopes. 

 

PVS 7 in dob.jpg

 

Great picture of Omega Centauri!


Edited by Eddgie, 26 June 2018 - 02:08 PM.


#62 Starman81

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Posted 26 June 2018 - 02:24 PM

Why would you be forced to live with the curvature if you had a PVS-7?   It will focus anywhere a Mod 3 would and has the same kind of photocthode.  The only difference between 10130 and 10160 style tubes is that one has a fiber bundle that inverts the image (10160 if I recall correctly) and one does not.

 

Anyway, I am not sure why the PVS-7 would have field curvature.  It should be identical in terms of ability to reach focus with C mount and work in the same mode as the Mod 3 in telescopes. 

 

attachicon.gif PVS 7 in dob.jpg

 

Great picture of Omega Centauri!

 

I'm pretty sure it was a typo and that he meant PVS-14 i.e. he would only be able run afocal and would have to deal with the field curvature observed in his images with the afocal setups. 



#63 Eddgie

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Posted 26 June 2018 - 03:22 PM

Ah..  Understand.



#64 GeezerGazer

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Posted 27 June 2018 - 11:11 PM

I'm pretty sure it was a typo and that he meant PVS-14 i.e. he would only be able run afocal and would have to deal with the field curvature observed in his images with the afocal setups. 

Thank you... that is what I meant. 



#65 Gavster

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Posted 29 June 2018 - 12:03 AM

Jeff, if you can select targets closer to Polaris, you should be able to stack for about 2.5-3.5 seconds at your native Epsilon FR.  Keeping the ISO as low as possible will help.  When you are using Night Cap, watch the screen as you make adjustments to the ISO & exposure... remember that if you have exposure set to 1/2 sec., the delay for adjustments in either ISO or Exp will take 1/2 sec to show up on screen.  I'm looking forward to seeing your results. I'm still traveling at new moon, but I'll be watching for your post in this thread.  waytogo.gif

 

Starman, that is for certain!  We all have different tolerance levels to distortions and there is a significant difference between using NV for photographic and visual use.  But either way, NV opens so many windows of opportunity.  This testing has shown me where my tolerance level resides, so in that respect it has been quite helpful.  I am, however, anxious to see results from the fast Newt astrograph, because it should show exactly what Eddgie has been proclaiming for a long time... full field illumination.  The brighter image will probably be mitigated by a slower ISO in imaging, but the field illumination is what I am really interest in seeing. 

 

Field curvature is the predominant distortion I've seen in my own photos, but I have found that a slower FR scope helps that issue.  When I get home in a couple of weeks, I'll try to take an afocal image with the 55P through my TEC 140 which is f:7.  I think it will show less curvature than the f:5 ST 120.  Here is a prime NV image through the f:7 scope that I took in May of M3, ISO 200, 1/6s, 15s stack:

 

attachicon.gif IMG_1281.jpg

 

Field curvature isn't a problem for me in this photo... there aren't many stars at the edge of field!  lol.gif   Even in the next photo of Omega Centauri, using the ST 120 at f:5, I can live with what curvature shows up, ISO 1250, 1/2s, 4s stack 

 

attachicon.gif IMG_1071.jpg

 

What I find objectionable is curvature that shows in the outer field as prominently as is shown by the 40 and 55 Plossls in my afocal images, especially with an added reducer.  If I had a PVS 7 instead of the Mod 3C, I'd be forced to live with that curvature.  Thankfully, the Mod 3C gives enough flexibility to go the prime focus route, which pretty much eliminates the problem... but at different scale than the afocal images.  The 55P is working at the same level as a .5x reducer in prime, which in my scopes vignettes AND shows field curvature.  So, reaching appropriate scale for me means that I will be changing scopes.  wink.gif  I'm OK with that compromise.

Is the FC when using a 55mm or 40mm plossl a result of the focal reduction given by these eyepieces in afocal mode? Eg if you used a panoptic 27mm which gives no focal reduction end is equivalent to using a mod 3 in prime mode, would the views be broadly equivalent with no FC?

 

I have a pvs-14 (living in the UK) and really like the afocal approach and the ease with which I can get focal reduction without significant vignetting. If I’m careful to get the focus right (as Mike Lockwood mentions above) I don’t see FC visually or in my phone photos.

 

I think I’ll try some direct phone photo comparisons of the 32mm plossl, 55mm plossl and 55mm plossl with 0.75 AP photo visual reducer in my f6.3 refractor to look at edge of field aberrations and vignetting. These systems will give f ratios of 5.3, 3.1 and 2.4 respectively.


Edited by Gavster, 29 June 2018 - 12:52 AM.


#66 GeezerGazer

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Posted 29 June 2018 - 09:27 AM

Good point Gavin.  I don’t have a 27 Pan to try but someone must.  

 

I know you’ve had good results with afocal, but as I’ve mentioned, these tests were solely with my ST 120.  When I get home after 7-11, I’ll try another comparison test with my 140.  Interesting about the focus... I only focused using the center FoV, and not as carefully as perhaps is needed. I’ll keep that in mind. 

 

Ill watch for some images from your tests.  Thanks Gavin.  



#67 Jeff Morgan

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Posted 29 June 2018 - 12:06 PM

Good point Gavin.  I don’t have a 27 Pan to try but someone must.  

 

I know you’ve had good results with afocal, but as I’ve mentioned, these tests were solely with my ST 120.  When I get home after 7-11, I’ll try another comparison test with my 140.  Interesting about the focus... I only focused using the center FoV, and not as carefully as perhaps is needed. I’ll keep that in mind. 

 

Ill watch for some images from your tests.  Thanks Gavin.  

 

I have the 27 Panoptic, never intended to use it because I prefer prime. But I will give it a go next time out.



#68 moshen

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Posted 29 June 2018 - 01:32 PM

It would definitely be interesting to compare photos taken with the exact same settings with prime vs 27 Pan afocal.



#69 GeezerGazer

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Posted 21 July 2018 - 08:51 PM

I think we need a little more testing.  

 

1.  Afocal with a 26 or 27mm eyepiece compared to prime, with and w/o a reducer.

2.  Afocal with 40 or 55 in a longer FR scope, with and w/o reducers.

3.  Prime with 2" .5x reducer spaced at different distances from the NV focal plane

 

Any others?  



#70 Jeff Morgan

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Posted 22 July 2018 - 08:11 AM

I think we need a little more testing.  

 

1.  Afocal with a 26 or 27mm eyepiece compared to prime, with and w/o a reducer.

2.  Afocal with 40 or 55 in a longer FR scope, with and w/o reducers.

3.  Prime with 2" .5x reducer spaced at different distances from the NV focal plane

 

Any others?  

 

I can test #2 in my 16" f/7 Newtonian. Unfortunately, with Arizona's monsoon season, it is very possible that won't be until September.

 

I'll pick a nice open cluster (Caroline's Rose?) and take photos with the cluster centered, and the cluster on the edge of field.



#71 GeezerGazer

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Posted 22 July 2018 - 11:12 AM

Jeff,

That would be excellent!  I thought I could also try that in my 140 frac at f:7, but better I think in a Newt, for the different optical system.  Have you figured out NightCap yet... for some images?  You probably have it mastered by now!  grin.gif

 

I can do #3.  Moshen did find a pretty good spacing for this generic 2" reducer (see his post and photo on page 2 at post #49)

 

The closest eyepiece I have to 26 or 27mm is a 24 Panoptic or a 30mm Ultrascopic.  I think someone else will have to do #1 comparison.  

 

Here in CA, it is not monsoon season, it is fire season in the Sierras, and particulate pollution is horrendous right now from forest fires in Mariposa (near Yosemite) and Yuba County (farther north).  I drove up to my green zone site in the foothills on Thursday after 10 pm, when the moon was down, and couldn't see a thing for the smokey haze.  Had to turn around and drive home.  Last year, the sky here was blotted out for almost 2.5 months from forest fire smoke.  This isn't meant to minimize the cost of damage to personal property and ecosystem damage... just sayin' sometimes we can't observe through the smoke.  undecided.gif   It may be a while for me to do additional testing too.



#72 Jeff Morgan

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Posted 22 July 2018 - 12:54 PM

Have you figured out NightCap yet... for some images?  You probably have it mastered by now!  grin.gif

 

Sadly, have not had a clear night since about the 6th.

 

I'm living vicariously through other peoples observing reports until probably September!



#73 Gavster

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Posted 23 July 2018 - 08:13 PM

I tried my Tec160fl with a 55mm plossl and an AP 0.75x reducer. This is pretty extreme focal reduction taking the f7 Tec down to around f2.5. I was pretty pleased with the results. The star shaped stayed sharp across most of the fov.

Here are 10 second iso 50 phone photos of the North American and eastern veil nebulae.

Attached Thumbnails

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#74 The Ardent

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Posted 24 July 2018 - 01:01 AM

Nice photos! 



#75 slavicek

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Posted 24 July 2018 - 08:32 PM

I tried my Tec160fl with a 55mm plossl and an AP 0.75x reducer. This is pretty extreme focal reduction taking the f7 Tec down to around f2.5. I was pretty pleased with the results. The star shaped stayed sharp across most of the fov.

Here are 10 second iso 50 phone photos of the North American and eastern veil nebulae.

Nice. FOV seems to be just right.

I assume you use Ha filter(?)




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