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:
Star Queen Nebula
Gamma Cygni Nebula
North American 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.