Back From NNSF: Tamron 135mm Verdict

NNSF 2025 was better than average; I had two solid nights of imaging. I might have had three if I had stayed over on Sunday night--but I'm not really sure if it was clear that night.

The first night (Wednesday) was a dusk-to-dawn session and I used my FSQ-106 to collect the green and red channel data I needed to complete my integrated flux nebula image. Plus a few more luminance and blue frames, just because I could. I'll be processing that data in the next couple of weeks.

Thursday night was lost to clouds, but Friday night was fine and I went until about 3 A.M. using the Tamron for wide-field views. The Tamron was stopped down to about f/5.4, basically reducing it to a one-inch f/5.4 telescope that no one in their right mind would use for imaging. It wasn't surprising that the results were less than adequate. Here's the Veil:

Final result, significantly cropped to hide flat frame errors

This is based on about 60 frames in the LRGB ratio of 2:1:1:1, with each frame a 120 s exposure. Bias, Dark, and Flat frames were applied; processing was in PixInsight.

That's not completely terrible if you only look at the much scaled-down size. But the faults are still there.

There's an overall blue cast to stars, due to problems with the red focal point and probably chromatic aberration. 

The stars are quite bloated, probably because of the tiny aperture used. I tried to reduce them as best I could, but you can see how the Veil is almost lost in all the stars.

There was good news, the new mounting ring made rotating the camera easy and reliable. And stopping down the lens and processing with BlurXterminator made the stars nice and round even in the corners:

PixInsight Aberration Inspector Panel

The red focal point was substantially different from what NINA expected based on filter setoff training. The Astrodon LRGB filters I use are essentially parfocal, so I have to assume this is some sort of lens fault.

Final judgement: I don't want to do wide field at a sluggish f/5.4, I don't like bloated stars, and autofocus is something I can no longer live without. This image just isn't what I consider acceptable, and I also like to guide and dither. Although the above image used neither and turned out fine -- the G-11 tracks well enough at 135 mm -- I think some dithering might be helpful. So: the Tamron will be relegated to daytime use and I'll need to find a replacement. How about that Rokinon? Or maybe the identical Samyang? Why pay an extra $50 for the brand name?

Here's the kit to go with a 135mm f/2 Samyang: A cradle from Astrodymium that will let me attach a guide camera and a ZWO EAF, giving me my precious autofocusing!

Now let's address the backfocus issue, something that has plagued my use of Canon lenses with the ASI 2600. The desired backfocus is something like 45 mm (44 mm + 1 mm to compensate for the filters I use). Conventional wisdom says you must be within +/- 2 mm, and that being within +/- 1 mm is preferred.

Solution 1 is to use a Canon to M42 adapter with maybe a 1mm spacer ring: ASI2600 (12.5 mm) + ZWO EFW (20 mm) + Canon adapter (10 mm) + spacer ring (1 mm) = 43.5 mm backfocus. That's off by 1.5 mm, which might be enough to show up visually. It would be nice to slip in another space ring, but there just isn't enough thread depth for it.

Here's a video by someone with exactly the same gear I would be using, and she's happy with it.

I don't know if that will hold true for me, though. Maybe variations between lenses might be enough to require a closer backfocus match. Maybe imaging at f/2 changes the amount of filter backfocus compensation? It would be nice to have some options.

Which brings us to Solution 2: ASI2600 (12.5 mm) + ZWO EFW (20 mm) + spacer ring (1 mm) + (extension spacer) 5 mm + spacer ring (0.9 mm) + M42 adapter (5.5 mm) = 44.9 mm. (I don't have two 1 mm spacers.)

There's even a Solution 3, since I have a 7.5 mm extension ring: ASI2600 (12.5 mm) + ZWO EFW (20 mm) + (extension spacer) 7.5 mm + M42 adapter (5.5 mm) = 45.5. I actually prefer this as it it eliminates the spacer rings and allows all the threads to engage more fully.

All the orders have been placed, and things start arriving in only two days! Plenty of time to get this assembled and running before the Iowa Star Party in October.

Next time, the IFN image and a look at the new Samyang!