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!

Fast Deliveries Mean an Unexpected Update: NEEWER to the Rescue

A couple of things came in so I'm doing this happy update:

135 mm Tamron & ASI in new mounting ring
atop a NEEWER dovetail atop an ADM vixen to Losmandy D adapter

The new 90 mm mounting ring arrived and it fits my ASI 2600 like a glove. Plenty of room for the entire optical train to rotate through 360 degrees, too. 

There's always a catch, though. The mounting ring comes with a foot appropriate to sitting on a quick-release tripod head rather than anything commonly compatible with astronomy mount saddles. If only there was an adapter to mate a dovetail to those two little holes on the ring? 

But there is, at Amazon. It's a NEEWER 9 inch Vixen dovetail. It mates perfectly to the ring using the ring's included bolts. The NEEWER slips into an ADM saddle adapter, so the whole thing will ride atop my G-11. Granted this does look like a bit of a kludge, but it's solid. 

I have a finder saddle to fit on the top of the ring, so I'll be able to guide and dither should I choose. If I make it to NNSF this will come along. I'll also bring a couple of other lenses for testing.

The last bit on order is a filter adapter that will act as an aperture mask for the Tamron. At this point it has departed China; delivery is still expected on the 18th.

Revisiting the Tamron 135 mm Lens.

Perfection is a terrible thing, being both difficult to define and impossible to achieve. It can stand in the way of getting something done, and lead a person to reject things that fail to meet your estimation of perfection. It has led me to consider getting rid of my old Tamron 135 mm lens. (Yes, I'm talking about that lens again. I could subject you to more battery talk, but no one really wants that, right?)

The Tamron was dropped from my integrated flux nebula imaging project for two primary reasons: the mounting ring I had made it all but impossible to consistently orient the camera, and it was showing too much aberration. 

I've now purchased a much better ring from Agena Astro. This ring and the dovetail it's on will also help with the limited rotation the old one allowed. The extended ring release knob release knob also eliminates the difficulty in turning the old knob, which was tight against the ring itself. I doubt the new ring will be "perfect," but it may be much more adequate that what it replaces. 

As for the aberration, I blamed that problem mostly on incorrect backfocus. I had assumed 45 mm was needed (44 for the camera plus 1 for the filter). My setup had backfocus of 12.5 (camera) + 20 (filter wheel) + 10 (canon to M42 adapter) plus 1 mm (spacer ring) for a total of 43.5 mm. This was 1.5 mm short, and the way people talk in forums that was seriously incorrect. 

But what if it wasn't? Watch this video. The imager is attaching an ASI 2600MM (my camera) with the same ZWO EFW as mine, a 135 mm lens (not the same as mine, sadly. She had the Rokinon 135 mm lens that I continue to lust after), used the same Canon-to-M42 adapter as mine, and a 1 mm spacer ring for a total backfocus of 43.5 mm (the same as mine). That gets her nice flat fields, so it's probably the case that my backfocus isn't the issue, it's the lens quality. 

A late-1970s Tamron at f/4 is no match for a modern lens like a Rokinon even if their inflation-adjusted prices aren't all that different. The obvious fix is to stop the Tamron down, perhaps to f/5.6. To do this without spiked stars requires a 58-24 mm step down filter adapter. (I found a 58-25 mm adapter on eBay ($2 + $15 for shipping), so I'll be shooting at f/5.4, which is close enough. Delivery date is projected to be August 18, two days before the Northern Nights Star Fest. 

These two issues settled (perhaps) the Tamron will continue to live.

If it arrives in time, I'll take it north and skies permitting put it to work on this:

Heart and Soul Nebulae with the Double Cluster

This is a very rich field ...

Above image with annotation

... with so much nebulosity and open clusters, too!

First priority will be collecting the green and red channel light frames to complete the IFN image. That should take only one night with time left over for more luminance frames. The Tamron will come out on the second clear night, assuming there is one. If the Tamron's aberration is still significant, I may switch to my 200 mm Olympus lens which has already proven itself to be flat-field at f/4. The Olympus will narrow the field to just the two nebulae.

Just in case NNSF produces four nights (!) of clear sky I'll bring along the 200 mm lens for some fun! And maybe even my 50 mm Olympus lens, why not.

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In terms of comfort sky clarity this summer has been one of the worst on record, with days and nights alternating between excessive heat and humidity, rain and severe weather, overhead smoke, and very poor air quality. In fact, half the days since mid-May have featured one of these unpleasantries (per the State Climate Office). My hope that the smoke will diminish before NNSF is only that, a hope. I'm fairly sure the fires will be much reduced by the time of the Iowa Star Party in October, but it would be nice if the new moon in September is also smoke-free!

We're currently in the midst of a multi-day air quality alert covering the entire northern Midwest. Yesterday morning the air smelled of sulphur and the sky has been a perpetual brown.

Air Quality Map @ 2 PM CDT 08/01/2025 (graphic from fire.airnow.gov)

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August 1 brings more tariffs, and it's time to check on that Rokinon lens. The "new" tariff on on South Korea is about the same as the existing one, so it's no surprise that the Rokinon is holding steady at $449.

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That's all for this time, next post is in late August with results from the NNSF (I hope!)