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120ST at F/1.7 - Afocal + Reducer on Extension - oh yeah!

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#26 Starman81

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Posted 02 July 2019 - 06:33 PM

I should also mention that I was only about 10mm away from being able to use the .5x reducer right on the diagonal inserted in the scope. I could see the defocused stars but just shy the back focus to work at that setting. If I would have had that extra 10mm or so, I could have taken the scope to F/2.5 then another .5x reduction from afocal setup so about F/1.25

 

 

Glenn had mentioned that the limit for afocal reduction is the f/ratio of the ENVIS objective, which I believe is f/1.2, so that would have put you basically right at the theoretical maximum focal reduction threshold... But don't feel bad about only being at f/1.4, I'm sure you're not missing out on too much! 


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#27 Vondragonnoggin

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Posted 02 July 2019 - 06:50 PM

Glenn had mentioned that the limit for afocal reduction is the f/ratio of the ENVIS objective, which I believe is f/1.2, so that would have put you basically right at the theoretical maximum focal reduction threshold... But don't feel bad about only being at f/1.4, I'm sure you're not missing out on too much! 

I’m pretty happy with the F/1.4! Happy with F/1.7 for that matter. Each scope gave me better nebulae views than I’ve ever had with the amount of detail and brightness of the image and least scintillation with an Ha filter.

 

One thing I haven’t tried yet with such a large amount of back focus, is using a field flattener with prime focus viewing. 

 

I thought I had bought an AT2FF flattener years ago, but checked my drawer and pulled out the Orion field flattener for short focus achromats. It’s supposed to be for scopes 400 - 650mm focal lengths, but if I use a reducer at prime, then instead of the extensions on either side of the reducer, use the field flattener after the reducer, it should work. It has T threads on top and I already have the T thread to c-mount adapter. Might make for some nice views at prime focus running at F/3.3 

 

It might even work without a reducer. It’s really for the 120ST size scope but should work at prime focus I without reducer on both the AT72ED and 120ST at native focal lengths. Some views at native fl are really great with a longpass in. I will do some more experimenting.

 

That is one of the biggest complaints I hear from others on the fast achromats - the field curvature. It’s never really bothered me but might make for some interesting views if it actually flattens the view to the edges.



#28 Vondragonnoggin

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Posted 05 July 2019 - 01:52 AM

The Orion Field Flattener for short refractors requires more back focus than the Antares 2” .5x reducer on the ScopeStuff 2” nosepiece.

 

I did not try it yet in my 150ST with low profile adapter, but in the 120ST with 96mm Linear Bearing and stock collar, it would not come to focus. It bottomed out on the focuser probably a good 10mm short of focusing. I had the 120ST out already so I dropped it in my Meade 5000 2” 2x TeleXtender and had 12mm left on the drawtube at focal point. Good for globs at that magnification. Was F/10 so narrowband Ha not very pleasing. Too much scintillation and noise. 640nm Longpass brought out some good globular views though.

 

I’ll try it in the 150ST next time I’m out with it. I’m debating still on whether I should just buy the 86mm linear bearing to use with the 120ST low profile adapter I already have. If the flattener works with the 150ST it might prompt me to use the 120ST low profile adapter. 

 

Despite it being at F/10, the field was flat in the 120ST. Sharp stars across the entire field to the edge. I did enjoy the view although I tend to not be picky about edges anyway. I don’t know how I would get the flattener to work with any afocal setup. Strictly prime focus as the flattener has the T threads at the top and I used my T thread to c-mount adapter to couple it to my pvs-7. The flattener has 48mm filter threads on the bottom but I put my filters on the 2” TeleXtender barrel.

 

If things were ideal, I would have enough back focus to use the FF with the 2” Antares .5x reducer screwed on the bottom of the FF and filters screwed on the Antares reducer. We will see if it’s possible in the 150ST but doubt it. I have enough back focus now at prime focus to use a 35mm extension, Antares reducer, then another 35mm extension, then 2” filter on the last extension. I’m getting approximately .67x reduction with reducer at that spot. Leaves me about 15mm left on the drawtube of the focuser. I guess I’ll find out next clear night I get if it works using FF and reducer. If it does have enough back focus, it should put the 150ST somewhere around F/2.8 - F/3 at prime focus. That’s fast enough for me to get some pleasing views in narrowband and a bonus if I actually get a flat field at that speed. Would be worth it as an option on nights I want larger image scale and don’t need the extreme reduction of afocal viewing. I won’t be surprised though if it falls short of focusing.

 

The other option might be to get an adapter that couples 48mm filter threads to T threads, then an adapter from 48mm to c-mount and sandwich the Antares reducer between NVD and FF instead of on the bottom of the FF. I would need less back focus, but not sure how it would affect the flat field the FF gives by putting a reducer between FF and NVD.

 

Just thinking out loud here on options to try.

 

I had anticipated the FF requiring some back focus but not that much.

 

Will update the thread when I try the 150ST with it.



#29 Vondragonnoggin

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Posted 09 July 2019 - 10:11 PM

I think I have a few hours clear tonight. Pulled out the Nikon D3300 and Litton dslr relay lens. Have my Litton M942 with Omni IV tube hooked up to the camera right now. I’m charging up my little separate lcd monitor battery right now for the little lcd extension I have. My lcd on the camera doesn’t swivel out but the little mount I bought for the second lcd does. Connects up by hdmi cable and should be able to get a live view with it swiveled at 90° to the camera. Going to try stacking reducers on my 150ST and going straight through with 2nd lcd swiveled 90° so I can get a comfortable view with it. I haven’t tried this stuff in a few years and never tried the 2nd lcd. ISO goes high enough in camera to focus during live view, then set to bulb mode and use my wireless shutter remote. I think if I can get the scope reduced to anywhere from F/1.2 - F/1.6, I can get sufficient low power and bright image to take some sub 1 second shots without too much star trail and nebulae blur with my manual mount. Last time I did this I was at F/2.5 or F/5. I can’t remember it’s been so long. F/1.4 or so should allow me to get sufficient light in sub 1 second to still use ISO of 800 or less to keep noise down.

 

No tracking on my mounts so these are always fun experiments. Will be monochrome shots. Hopefully can get this all to work quickly. If not, I have my 56mm plossl hooked up afocally to my NVD Micro and a diagonal ready to drop in so I can at least get some good views. Might end up trying some prime focus at F/3 with a field flattener in place also.

 

I’ll see what I can come up with. Don’t want to spend all night experimenting either. So few clear nights I just want to view sometimes too and not experiment all night. I can get a few things setup while still light though. pvs-7 attached to field flattener, NVD Micro attached to Meade 56mm plossl, M942 attached to D3300, then just swap out configurations and play with spacing using extensions and reducers.



#30 Vondragonnoggin

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Posted 10 July 2019 - 03:55 AM

Well, did some experimenting and then just plopped the whole plossl/Micro stack in to actually get some viewing in without experiments. F/1.4 for viewing through the 150ST tonight. 

 

My camera experiment:

 

Straight through with field flattener and focal reducer - would not come to focus. Ran out of back focus.

Imaging train was D3300 - Litton Relay - Litton M942 - c-mount to T2 Adapter - Orion FF - Antares .5x reducer - 640nm Longpass. Probably 15mm shy of focusing. Could see out of focus stars.

 

Straight through with two .5x reducers - would not come to focus. Ran out of back focus

Imaging train was D3300 - Litton Relay - Litton M942 - Scopestuff 2” c-mount nosepiece - 35mm extension - Antares 2” .5x reducer - 35mm extension - GSO 2” .5x reducer - 640nm Longpass. Should have tried reducer direct on nosepiece and one 35mm extension between the two reducers or a smaller 15mm or 20mm extension between them.

 

Straight through with two 35mm extensions and .5x reducer - this worked and came to focus

Imaging train was D3300 - Litton Relay - Litton M942 - Scopestuff 2” c-mount nosepiece - 35mm extension - 20mm extension - 15mm extension - Antares 2” .5x reducer - 640nm Longpass.

 

With final configuration using 70mm of extensions and the Antares 2” .5x reducer, I swapped out filters for my 3nm Ha filter. After focusing with stars at ISO 800, I cranked up the ISO to 1600 to get live nebulae views. My 2nd lcd tilted 90° from the camera worked good for viewing but ISO 1600 is noisy. All my single shots were a noisy mess with that Omni IV tube, aggressive filter, and high ISO.

 

The unexpected bonus - I was not in bulb mode and I think my infrared shutter remote battery is dead or I was at the wrong angle or it needs bulb mode. I can’t remember but think it works when not in bulb mode. I’m unfamiliar with my camera and doing this again because it’s been so long. I accidentally took 2 short 60 frame per second 1080p movies and did not discover this until after I put almost everything away and replaced the battery in my camera. I was checking the stills and saw two short movies. I should have taken longer ones. As soon as I started the playback, the noise went down considerably and I got two mini movies showing a very bright Lagoon with a good deal of scintillation still but tolerable for how bright the nebulae was. I need to experiment more with this. Still possible I can reduce further with the right combo. I was around F/2.25 from the looks of the FOV. If I can get it to F/1.2, the live movie mode will actually come out pretty decent with my manual mount. In one of the movies I switched ISO while filming and even at ISO 400 I could see some nebulosity in real-time. At 1600 it was very bright but a bit noisy but nothing like the noise of the stills. I think some further experimenting might get me some real-time movies of nebulae viewing and very bright. If I can reduce to F/1.2 - F/1.4 I can bring down ISO to 400 and still show very bright nebulosity without as much scintillation. I was almost there with what I thought to be an acceptable live nebulae view video. I hesitate to use a less aggressive filter, but would rather concentrate on getting f ratio down more.

 

Next time it’s clear maybe and clear all night. Forecast said clouds around 12:30 and was correct. Clouds started rolling in. If I can get a full night of no clouds I can figure out best setup for some nebulae video.

 

I did not try my pvs-7 with field flattener and reducer. I don’t think it will work anyway. Ran out of time. First hour and 20 minutes running my experiments, then swapped out to afocal F/1.4 setup and relaxed while viewing some great nebulae.

 

The more I think about this, the more I’m convinced I can get the focal reduction down below F/2 and lower the ISO. At ISO 1600 it also picks up IC1274, IC1275, IC4685 next to the Lagoon pretty well defined. I want that much detail but a little smoother in the video. 


Edited by Vondragonnoggin, 10 July 2019 - 04:18 AM.


#31 Vondragonnoggin

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Posted 12 July 2019 - 01:40 AM

It clouded up really fast tonight. I was going to setup the 150ST but glad I didn’t. 

 

The optical train made me nervous with the weight. I found the 1x lens for my M942 and setup the M942 with Litton relay connected to my Nikon D3300 over the 56mm plossl using the Baader Microstage II

 

Each component by itself is not heavy, the relay lens being the most weight, but put them all together clamped to the 56mm plossl and it’s pretty heavy.

 

I gripped the barrel of the 56mm plossl and held it completely horizontal and there was no sag with the Microstage II. In fact, the arm of the Microstage II provides extra stiffness over a longer span, but I would have been adding extensions and focal reducer to the barrel of the plossl too and it makes a long moment arm with the weight.

 

I decided to skip testing tonight with the 150ST and instead put on a 75mm F/1.4 lens and put the setup on my photo tripod with a 3nm Ha on the front of the 75mm lens. I took three videos. 32 seconds of the Lagoon and Trifid area and then 2 videos at 52 seconds and 2 min 14 seconds of the Gamma Cygni area and North American/Pelican area.

 

1600 ISO was still the best brightness even at F/1.4 - I tried 800 and it was a lot less noisy but too dark. Nebulae was still visible but just the brightest areas. At 1600 ISO the dust in between the brightest nebulae showed up pretty good. All those areas are absolutely full of dust visible between brightest nebulae. This is what I experience visually at F/1.4 in the 150ST also. All kinds of interesting in between dust shows. Visually the noise is not there and scintillation is low at F/1.4

 

I also tested out some views with the Litton 1x lens on the M942 with no filter and quite a bit of Milky Way nebulosity shows up. I can take ISO to 100 and still get billowing Milky Way views through the live view screen. Very low noise at 100 ISO.

 

I had the camera set to monochrome and the views show up like white phosphor. It’s an interesting change from green.

 

I did not try the 640nm filter, but next time out I will try my 50mm lens with 640nm filter and low iso. It’s only 2x magnification, but quite a bit of interesting things show with the Milky Way at those low powers and very mild filtering.

 

Fun to play around with when it is an easy setup. I will eventually try the afocal with camera on the 150ST. I just need longer time with clear skies.


Edited by Vondragonnoggin, 12 July 2019 - 01:43 AM.



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