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

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#1 Vondragonnoggin

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Posted 01 July 2019 - 04:24 AM

Tonight I took my little 120ST to some new speed for me.

 

I’ve been experimenting with afocal configurations to try and get some extreme reduction for brightest nebulae views and to smooth out my fixed gain signal using 3nm Ha filter

 

I hit the jackpot tonight. My last session was my AT72ED with 56mm Plossl, and Antares 2” .5x reducer screwed on the bottom of the 2” plossl. I had my 2” 3nm Ha filter screwed on the reducer. I had previously taken my pvs-7 out with a 135mm F/2.8 Vivitar lens on it and then swapped to my NVD Micro (VG Tube, P43 phosphor) on a Nikon Pistol Grip trying out the 135mm F/2.8 and a c-mount 75mm F/1.4. I loved the bright images but too small so I setup the AT72ED with 56mm Plossl afocal and 2” .5x reducer + 2” 3nm Ha on the Plossl. I also tried my 42mm Superview as base afocal eyepiece. 

 

My calculations and observations showed I was getting F/2.67 with the 42mm Superview and reducer and F/2 with 56mm Plossl and reducer.

 

Tonight I went out with my 120ST and first the pvs-7 with 2” nosepiece, 2” .5x reducer, and 2” 3nm Ha filter in the diagonal for prime focus viewing. Image scale is great but compared to what I saw in the AT72ED at F/2 and F/2.67, it was definitely lacking brightness and had more scintillation. I decided to swap to afocal with 56mm Plossl, reducer, 3nm Ha but ran into the issue I predicted having a Linear Bearing focuser with 50mm drawtube - not enough front focus to reach focus afocally. The 50mm drawtube is just not long enough. I have full sets of extension tubes so I figured a 35mm extension tube on the Plossl, then the 2” .5x reducer, then 2” 3nm Ha on the end of that would alter focal point enough to fall in the range of the 50mm drawtube. I also figured I would get more reduction by putting the 35mm extension between the Plossl and the reducer.

 

I was right. Unbelievable how bright it was and such a smooth view with 3nm filter. Never seen it that smooth except with my nifty fifty, the ENVIS, and my 75mm F/1.4 lenses.

 

The reducer on the 35mm extension plus using the 56mm Plossl as the afocal base brought me to F/1.7

 

Nebulae I had missed before was now popping pretty good and just walls of dust everywhere.

 

I used to just hit Lagoon, Triffid, Eagle, Omega then move over to Cats Paw, War and Peace. I’m getting true .7x reduction with the .5x reducer on the nosepiece at prime focus and that brings the 120ST to F/3.5 which is still bright, but this was unreal for my 120ST.

 

I’ve never seen so much nebulosity and fine detail. Even though I had the 3nm filter before the reduction, I still had some bandwidth shift rolloff on the edges, but the center 70% view was unbelievably bright and the view was as smooth as my nifty fifty or as smooth as my variable gain pvs-4 with the gain turned down a bit. Not much scintillation really. Like a longpass at F/5.

 

Tonight was a lot more noticed and easily recognized as separate objects but there was so much to see and Ha glowing between the brighter nebulae. Best 120ST session so far.

 

Im a firm believer in afocal viewing and this was as smooth and pleasurable a view as my fast slr lenses.

 

My list for this evening:

 

IC4678
IC4685
NGC6559
IC1275

IC1274
IC1283
IC1284
IC4701
NGC6596
IC4706
IC4707
IC4701
Eagle Nebula
Star Queen Nebula
NGC6625
Veil Nebula
NGC6979
NGC6974
NGC6995
IC1340
Network Nebula
NGC6871
NGC6883
Crescent Nebula
NGC6910
Gamma Cygni Nebula
Elephant Trunk
North American Nebula
Pelican Nebula
IC5070
IC5068
Dumbbell Nebula

 

I was out about 3 1/2 hours before getting some clouds coming in. Some of these showed distinct structure where previously I had only seen cloudy nebulae extending. Actually separate objects. Really impressed me what the 120ST was capable of when reduced by .34x

 

I used Luminos to setup FOV with scope at Native F/5 and added a .34x reducer to equipment which brought the FOV to what I was seeing in relation to size of objects.

 

I normally get .7x at prime with reducer on the nosepiece on all of my scopes, and on my 150ST with low profile adapter, I get true .67x reduction from the .5x reducer on a 35mm extension on the 2” nosepiece.

 

With Plossl and reducer afocally, the reducer on a 35mm extension, brought it to F/1.7

 

I still had about 15mm back focus left on the linear bearing focuser 50mm drawtube.

 

An amazing session with some extreme reduction on the 120ST. A lot I’ve never seen before in that much detail.

 

Next time out - the 150ST at F/1.7

 

I think I can get there easily with my 150ST also. Linear Bearing on that scope too.


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#2 Vondragonnoggin

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Posted 01 July 2019 - 04:32 AM

Forgot to put Cats Paw and War and Peace in that list. The really surprising bit was next to the Lagoon is IC1274, IC1275, IC1283, and IC1284 and I usually just see the Lagoon extended out but this was so much detail and structure, it looked like another Cats Paw nebula.

 

Crazy good views. 

 

cool.gif

 

100% worth the tall stack to me. With 2” reducer and 35mm extension, plus 3nm filter on the end of all that, pretty much the entire barrel of the Meade 56mm plossl was above the diagonal. A whole lot of silver barrel showing.

 

Props to Gavster for getting me interested in digging into afocal viewing again! I would have never thought I could get my telescopes to give me as smooth and bright of views as my slr lenses. With greater image scale than the slr’s by far. Combined with 3nm Ha, really good S/N on Ha signal.


Edited by Vondragonnoggin, 01 July 2019 - 04:55 AM.

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#3 Vondragonnoggin

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Posted 01 July 2019 - 04:35 AM

I am going to revisit using the Baader zoom afocally also. Too many great globs out this time of year to pass up some longpass views magnified a bit more with the zoom.



#4 cnoct

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Posted 01 July 2019 - 05:13 AM

Hard to top one's first NV astro experience, me thinks you just leveled up to no limits NV astro, attaining NV astro enlightenment.  


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#5 Vondragonnoggin

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Posted 01 July 2019 - 05:29 AM

lol.gif

 

I never thought I’d see my scope with a 3nm Ha having that little scintillation.

 

It was a Nirvana session for sure. Can wait get the 150ST to that level.

 

The craziest confirmation of something Glenn has been trying to drive home in his posts about focal ratio - the brightness of extended objects is the same in any scope if the f ratio is the same. Doesn’t matter if a 50mm objective at F/1.8 or a 150mm objective at F/1.8 - the brightness is constant. That’s what I saw.

 

Want the same benefit but longer focal length to bring that image scale up.



#6 Gavster

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Posted 01 July 2019 - 09:59 AM

Excellent stuff Vondragonnoggin. I love running a fast sub f2 speeds afocally for exactly the reasons you set out above. It works a treat with very narrow ha filters for brightness and detail even in LP areas.
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#7 Starman81

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Posted 01 July 2019 - 01:19 PM

Forgot to put Cats Paw and War and Peace in that list. The really surprising bit was next to the Lagoon is IC1274, IC1275, IC1283, and IC1284 and I usually just see the Lagoon extended out but this was so much detail and structure, it looked like another Cats Paw nebula.

 

Crazy good views. 

 

cool.gif

 

100% worth the tall stack to me. With 2” reducer and 35mm extension, plus 3nm filter on the end of all that, pretty much the entire barrel of the Meade 56mm plossl was above the diagonal. A whole lot of silver barrel showing.

 

Props to Gavster for getting me interested in digging into afocal viewing again! I would have never thought I could get my telescopes to give me as smooth and bright of views as my slr lenses. With greater image scale than the slr’s by far. Combined with 3nm Ha, really good S/N on Ha signal.

 

Wow--sounds like it was one for the ages!  cool.gif

 

About the tall stack, I remember the absurd scene of my own focally-reduced afocal setup on the AT60ED... In that, I was prioritizing maxing out the TFOV while going as fast as possible, but I never did try doing the opposite (maxing out speed on larger apertures for max brightness but with image scale).

 

Your blockbuster session with the 120 has me intrigued... I'm sure the edges weren't anywhere near perfect, but that's not really relevant. At f/1.7, you're 4x as bright as f/3.3, which is roughly what I was getting with the 80/120/150 achros with the 0.5x focal reducer at prime focus. Afocally, with focal reduction added to the long f/l eyepiece (TV 55 Plossl in my case) should get me to the goal of sub f/2.0.

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  • Mod 3 C TV 55PL afocal stack in AT60ED.jpg

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#8 Starman81

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Posted 01 July 2019 - 01:26 PM

This was my stack up... Ideally, I would have had the 2" filter threaded onto the 0.5x focal reducer, but I have some older, different version 0.5x that doesn't have filter threads on the bottom. What's worse is that the bottom lens is convex and if you set the focal reducer down on the bottom the glass will touch whatever you set it down on! I think I need to pick up the newer version to remedy that...

 

 

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#9 Vondragonnoggin

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Posted 01 July 2019 - 01:55 PM

Even with filter sandwiched between reducer and eyepiece barrel, you shouldn’t have as much bandwidth rolloff as attached to the ENVIS. You are probably getting the filter in while still F/3.x, then the 55 plossl will take it the rest of the way to sub-F/2 speeds. I’m using the Antares 2” .5x reducer. 

 

I found the Antares to be a good inexpensive solution for reducing without vignetting.

 

The edges weren’t bad. Some seagull on very edges but there is a shadow caused by the eyecup on the Meade using the Baader Microstage. I can flip the eyecup down and the shadow goes away but also susceptible to stray light reflections creeping in between NVD and plossl if Im next to any reflective surfaces on my back deck near my house. It needs a little space to between plossl and ENVIS for best view. Putting my hand up to block reflections also works if I’m in a spot where house reflections are coming back towards the scope. I think I’ll make a little shroud to wedge in the Microstage Adapter where it clamps the plossl. That should take care of stray light reflections and still maintain optimal spacing between plossl and ENVIS. I have some protostar flock board left over that would make a nice stiff shroud to wedge in the adapter and rise up enough to cover the space between plossl and ENVIS.

 

Its not a big deal with the shadow ring anyway though because of bandwidth shift rolloff. You don’t look at the edges. The best view is center to 70%. The last 30% has some rolloff of filtering. I imagine the Televue/TNVC Adapter combo would not have the same stray light issues at all. I’m in no hurry to buy any new eyepieces though. I still have to get my M57 to T thread adapter to use my 42mm Superview like the TV/TNVC coupling.



#10 Vondragonnoggin

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Posted 01 July 2019 - 02:12 PM

The nice thing about afocal setup using the longer fl eyepieces is it is opposite of the reducer in focal point needs. The afocal setup will require front focus and the reducer requiring back focus will bring the stack back to a middle spot on the drawtube.

 

It might be possible in my 150ST to have more than a single 35mm extension between eyepiece and reducer. My 150ST already requires an extension to use prime focus and I place a 35mm extension on the nosepiece before the reducer, then another 35mm after the reducer and the filter on the very end of that. That’s at Prime Focus. It’s because I shave several inches off the fl by using the ScopeStuff low profile adapter for 150mm Synta scopes (replaces stock focuser collar) and a short GSO linear bearing on it. 

 

With afocal, I should be able to get at least a 35mm and a 20mm extension in between the eyepiece and reducer and will probably get me to F/1.4 - I may need that much just to have the stack come to focus with room on either side of focus for adjustment.

 

I’m excited to try it. Last time I tried afocal with the 150ST was with stock focuser and stock collar and only could put the reducer right on the nosepiece. The setup now with low profile adapter and linear bearing is a full 2” shorter than stock. Much more back focus available. Not enough to put a reducer before the diagonal still, but enough that it needs the reducer sandwiched between two extensions at prime focus.



#11 Doug Culbertson

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Posted 01 July 2019 - 02:25 PM

I wish that I had thought about this kind of setup before I sold my AT72EDII. I couldn’t reach focus with my Mod-3c and a focal reducer so I sold the telescope, but now I wonder if an afocal setup would have worked. Nice write up! 


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#12 Vondragonnoggin

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Posted 01 July 2019 - 02:49 PM

I wish that I had thought about this kind of setup before I sold my AT72EDII. I couldn’t reach focus with my Mod-3c and a focal reducer so I sold the telescope, but now I wonder if an afocal setup would have worked. Nice write up! 

The first night out trying this with my AT72ED I didn’t sandwich an extension between eyepiece and reducer which was why I was at F/2, but there was a lot of back focus left and I think I could easily put the extension between eyepiece and reducer and get the AT72ED to sub F/2. 

 

Its a pretty tall stack for the little AT72ED, but not unmanageable. I’m using mine on a Benro S8 fluid head mounted on a Manfrotto 117b tripod and the Benro S8 has a counter balance dial that keeps it very manageable with the extra weight.

 

The experiments with AT72ED prompted me to try it with the 120ST. The only issue is at 430mm fl, reducing the AT72ED that much makes image scale pretty small. Still better scale than SLR lenses. I’m finding the brightness making up for smaller image scale in pleasing views though. 

 

My 120ST setup is on a Vanguard Auctus Plus 323AT with geared center column and a Stellarvue M2 Head with steering handle. Also very manageable.

 

A real treat would be a larger aperture scope reduced this much because the focal length would still be long enough to bring objects in for some closeups while maintaining brightness of a 50mm slr lens. The 120ST scale was still pretty good, but my 150 will be better still. A scope in the 8-12” range at sub F/2 would be great for image scale because the focal length long enough to get some closeup details.

 

If my back wasn’t shot from previous injuries, I’d put this to the test with some larger aperture, but I’m still amazed to get what I get using 6” or less scopes. They are convenient to setup too. 



#13 Stargazer3236

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Posted 01 July 2019 - 03:12 PM

Star Queen and Eagle Nebula are the same nebula



#14 Stargazer3236

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Posted 01 July 2019 - 03:14 PM

Also, I tried using the Meade F/6.3 reducer / corrector on my 120ST telescope and I got sharp stars from edge to edge. Gave me about F/3.15.



#15 Vondragonnoggin

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Posted 01 July 2019 - 03:23 PM

Star Queen and Eagle Nebula are the same nebula

My Luminos App had both names listed but put the Eagle name in the center and Star Queen on the edges. Probably just because I have several catalogs enabled to show names.

 

How did you have your Meade 6.3 reducer setup? Most people can’t use the reducer before the diagonal unless they start cutting tubes, so the true reduction is not the stated reduction of the reducer. Like the case for my .5x reducer. I can’t use it before the diagonal where it would give me true .5x reduction. On the nosepiece of my NVD at prime focus I get true .7x from it instead.

 

I get true F/3.5 on my 120ST with the .5x reducer on the 2” nosepiece of my NVD’s (5 x .7)


Edited by Vondragonnoggin, 01 July 2019 - 03:25 PM.


#16 Starman81

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Posted 01 July 2019 - 03:45 PM

Also, I tried using the Meade F/6.3 reducer / corrector on my 120ST telescope and I got sharp stars from edge to edge. Gave me about F/3.15.

 

I had read about this here recently... Wondering how exactly the reducer was able to be used with a refractor since it has threads for SCT use? confused1.gif



#17 Stargazer3236

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Posted 01 July 2019 - 05:25 PM

You buy an adapter from Scope Stuff: http://www.scopestuff.com/ss_taep.htm



#18 Vondragonnoggin

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Posted 01 July 2019 - 06:01 PM

You buy an adapter from Scope Stuff: http://www.scopestuff.com/ss_taep.htm

I see - you use cameras and reducers. Yes, reducers with cameras give true reduction when placed where they are supposed to. NV eyepieces are put in diagonals like regular eyepieces and not enough back focus to use reducers right in the focuser. We have to get a little creative to get massive reduction with a diagonal still in the optical train.


Edited by Vondragonnoggin, 01 July 2019 - 07:30 PM.


#19 Vondragonnoggin

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Posted 01 July 2019 - 10:16 PM

I’m ready to experiment tonight with the 150ST. I made a little shroud to go around the plossl and NVD where there is space between ENVIS and plossl eyelens. Happened to have the cotton black bag my sons sheets came in and it has Velcro on it, so I got full wrap around eyepiece and NVD, then Velcro to keep it in place. I spent some time to perfectly align the Micro over the 56mm plossl too. 

 

Tonight I try two extensions between reducer and eyepiece. Ideally I could use no extensions and put the reducer before the diagonal but I seriously doubt that will work. Will try it first though.

 

Another hour....



#20 Vondragonnoggin

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Posted 02 July 2019 - 02:51 AM

FOV circle in Luminos matched to eyepiece view showed me using a reducer that was .28x on my 150ST

 

I checked the FOV against Lagoon, Triffid, IC1274, IC1275, IC1283, IC1284 all in the same view, then against Eagle and Omega centered, then checked against North American Nebula, then Gamma Cygni main giant butterfly looking part although there are three major nebulous structures called Gamma Cygni in the Luminos app. The brightest is the giant butterfly looking one though. Nebulosity extends between them all though and all the way to the Crescent.

 

Luminos allows all kinds of equipment to be added, AFOV of eyepieces, barlows, reducers, etc. 

 

The 150ST looked right with a .28x reducer. 

 

That’s F/1.4. bigshock.gif

 

It was 100% awesome.  

 

cool.gif

 

Will post a pic soon of the stack in the focuser. I had to use two 35mm extensions then reducer and then 3nm filter.

 

My little shroud worked good. Pic has the shroud over the space where the ENVIS sits over the plossl.

 

The other all kinds of awesome was removing the plossl, and readjusting with Baader Zoom in the last hour. I used the 640nm Longpass and no filter but preferred the view with longpass on. 

 

I had had one of my son’s friends interested and he got looks of Sagittarius nebulae and some globulars.

 

My son actually took a few peeks at nebulae too. Pretty awesome to get his interest up again.

 

I’ll post a little more info and a few pics later. Need food now and some coffee. 

 

That was unbelievable. It was even more revealing than last nights session with 120ST at F/1.7

 

Very close in views but you could easily see what the faster f ratio was doing to some of the fainter nebulae. 

 

The higher powered glob views were pretty amazing too, but enjoyed the high power on the Sagittarius Star Cloud just as much as the globs. The whole area towards Galactic center was just incredible viewing.

 

Amazing!


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#21 Vondragonnoggin

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Posted 02 July 2019 - 03:39 AM

Here is a pic of the stack with shroud on it. Notice where the Plossl barrel ends and extensions begin. It was seated fully in the 2” diagonal.

 

Notice the ScopeStuff low profile Synta 150 Adapter and how much I have left on my linear bearing focuser’s 50mm drawtube. The focuser is literally butted up to the end of the tube by that adapter. I needed that much extension to come to focus but since the reducer is on the end of the extensions, it put a lot of space between reducer and afocal base plossl, giving me even more reduction. Yeah, the plossl barrel has some scratches on it already. Didn’t notice until I looked at the picture. What a performance boost though. Like getting new tube upgrades again for emission nebulae views.

 

 

FC883669-579D-43BB-B6C7-6BF8D7C56EAE_zps


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#22 Vondragonnoggin

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Posted 02 July 2019 - 10:28 AM

I made some approximate view images of the Lagoon/Trifid and Gamma Cygni regions

 

Red/White zone border. Gamma Cygni is over Anaheim area and white zone while the Lagoon and Trifid area is more on the white/red border. To the right of the Lagoon about 20° is more red zone as it hangs over the ocean more

 

Lagoon/Trifid

https://i100.photobu...zpspzucvtuz.jpg

 

Gamma Cygni

https://i100.photobu...zpszytqqxw3.jpg

 

Omega Nebula (representing brightness and detail)

https://i100.photobu...zps2qklvmiv.jpg

 

 


Edited by Vondragonnoggin, 03 July 2019 - 09:32 AM.

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#23 Starman81

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Posted 02 July 2019 - 05:09 PM

Here is a pic of the stack with shroud on it. Notice where the Plossl barrel ends and extensions begin. It was seated fully in the 2” diagonal.

 

Notice the ScopeStuff low profile Synta 150 Adapter and how much I have left on my linear bearing focuser’s 50mm drawtube. The focuser is literally butted up to the end of the tube by that adapter. I needed that much extension to come to focus but since the reducer is on the end of the extensions, it put a lot of space between reducer and afocal base plossl, giving me even more reduction. Yeah, the plossl barrel has some scratches on it already. Didn’t notice until I looked at the picture. What a performance boost though. Like getting new tube upgrades again for emission nebulae views.

 

 

I take it that the low-profile adaptor is this one (?):

 

#FRC6 - Adaptor Ring for Crayford Refractor Focuser on 5.4" ID Tubes - Low Profile (150mm Skywatcher, Celestron, Orion, Konus, etc)

 

I took a look at the my 150R and my ST120 and see that the ST120 has a very short adapter, so I'm thinking the double focal reduction (afocal + focal reducer) experiment should work with it. But the 150R has the stock adapter/collar that might take me out of that sweet spot. I got your point on how afocal wants front focus while focal reducers require additional back focus and how the two can balance out, so maybe I should just try it out first...

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#24 Vondragonnoggin

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Posted 02 July 2019 - 05:30 PM

I take it that the low-profile adaptor is this one (?):

 

#FRC6 - Adaptor Ring for Crayford Refractor Focuser on 5.4" ID Tubes - Low Profile (150mm Skywatcher, Celestron, Orion, Konus, etc)

 

I took a look at the my 150R and my ST120 and see that the ST120 has a very short adapter, so I'm thinking the double focal reduction (afocal + focal reducer) experiment should work with it. But the 150R has the stock adapter/collar that might take me out of that sweet spot. I got your point on how afocal wants front focus while focal reducers require additional back focus and how the two can balance out, so maybe I should just try it out first...

Yes, that is the adapter. Also - yes, we are opposite. It should work on your 120ST. I have the stock collar on my 120ST and 96mm GSO and the low profile on my 150ST with 86mm GSO. I also have in a drawer, the low profile adapter for the 120ST but don’t have another 86mm focuser for it. I originally bought the low profile for the 120ST and had to buy the focuser with it, but Markus at APM put a TS Monorail linear bearing on my 120ST when I bought it (same as GSO linear bearing)

 

I was able to reach focus at prime with .5x reducer on the nosepiece with stock collar on my 120ST, so I put the adapter and focuser in my drawer for years. When I bought the used 150ST this year, I ordered the low profile adapter for the 150 and used the 86mm linear bearing that was in my drawer. 

 

All this influx of afocal with more people using pvs-14’s and TNVC Adapter got me thinking I need to experiment more.

 

Can’t hurt to try the double extensions on your 120ST. If you don’t mind the height of it, then maybe get the low profile adapter for your 150. Even at prime you’ll get more reduction. I was able to sandwich my reducer in between two 35mm extensions at prime focus.

 

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#25 Vondragonnoggin

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Posted 02 July 2019 - 05:46 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

 

I thought about a 1.25” diagonal but then couldn’t use the 56mm plossl and would defeat the purpose.

 

It does allow a binoviewer to work without GPC if using a 1.25” diagonal. I have the Williams Optic BV and can get low power views by leaving out the GPC. Only limited by the 22mm clear aperture of the BV’s but my low power eyepieces with 23mm or less field stops work for low power bino viewing in the 150ST with the short adapter. It has dual purpose for extreme reduction with NV or using a BV at low powers with regular eyepieces.

 

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It did take a little time experimenting to get the Baader Zoom to work afocally. No reducer of course, so had to play with extensions a bit to find the combo that would still land the focal point within the 50mm drawtube length. Once I found it, it was great for zooming on globs. It was quite a bit of magnification at 8mm setting. I actually preferred the look at around 10 - 20mm settings for globular views. I still think my mak 150 at F/12 does better globular views, but they are both good.


Edited by Vondragonnoggin, 02 July 2019 - 05:49 PM.



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