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.

My 135 mm Tamron Delusion Ends, It's Plan B now

I really thought I could get away with using my old Tamron lens for imaging Integrated Flux Nebula (IFN), but it's not to be. The number of minuses kept growing as I spent more nights practicing with the setup. I realized that my desire to use the Tamron and was blinding me to the issues that doomed it.

The greatest difficulty was being unable to rotate the field of view in a reproducible manner. This happened every night I used the setup with one night being 45 degrees out of kilter from the others. Keeping the camera orientation consistent between multiple imaging sessions is essential; without this, some of the field will need to be discarded during stacking. It became obvious that the amount of lost field would probably be substantial, resulting in a much retained field of view. The advantage of the wide 135 mm field of view would be lost. I think the cause was the ring clamp I was using to join the lens and camera to the dovetail. To permit manual rotation of the camera this ring had to be loosened and retightened many times, resulting in misalignment.

Image quality was also not what I wanted. There was no reasonable solution for the back focus error and I would be stuck with strong aberration needing to be corrected by BlurXTerminator. I knew some residual aberration remained, and I was concerned that this might show itself during the aggressive processing I would use to draw out the IFN. 

I came to realize that the stepdown ring I was using to produce spikeless stars was causing severe vignetting. While flat frames could somewhat compensate for this, too much signal was being lost -- again diminishing the effective size of the field of view. Using the lens's internal blades to stop it down was an alternative that created large, flaring spikes around the stars. I found this unacceptable.

Plainly, the lens was not up to the purpose and it was time to move to Plan B.

Plan B

The fallback is to use the FSQ-106 + focal reducer operating at a focal length of 387 mm and focal ratio of f/3.65. This California Nebula image used that configuration and gives you an idea of what it can do. The advantages are many over the Tamron: 

• 17% increase in speed (f/3.65 vs f/4)
• A very flat field with modest vignetting  
• An actual rotation ring
• Amenities like autofocus, autoguiding and dithering, easier creation of flat frames, and automatic meridian flipping

This is a much heavier scope to tote around, but the only real disadvantage it has is the smaller field of view. Here's a comparison:

135 mm Tamron field (outer box) vs 387 mm FSQ field (inner box)

The star cluster is no longer in the FOV, which is fine as the IFN is the real target. Does the smaller FOV (about 3.5 x 2.3 degrees) include enough IFN to be worth imaging? This image of Polaris IFN by another imager has essentially the same FOV as my setup will produce. I think there's enough IFN there to make it worthwhile particularly if Polaris can be reduced in size. I'll also compose the image to have Polaris much closer to the north edge of the frame, making more room for the IFN. 

Other aspects of the comparison image are worth looking at.

The scope used was an f/5.5 refractor with a flattener that didn't affect the focal ratio (so far as I can discover). The camera, an ASI 6200 color camera, has a quantum efficiency about the same as my ASI 2600 mono camera. The total integration time was 5 hours and 10 minutes. My thought is that if I want a reasonably deep image with low noise I should try to get at least twice the total time that went into the image. I'll probably use a plan that requires two nights of imaging: 5 hours of luminance one night, 3 hours of Chrominance another night. That's roughly the equivalent of 11.7 hours of one-shot color gathered at f/5.5. 

Yes, I know, that's a LRGB ratio of 5:1:1:1 and instead of the usual 3:1:1:1. I just like working with luminance; one night a few years ago something glitched and I ended up shooting only luminance. The final ratio was 6:1:1:1 and the result was quite nice (in my opinion, of course).

The comparison image was created using 5-minute light frames, which is probably why star colors are muted and Polaris is bloated. I've had much better luck with shorter exposures, and may simply go with 90 s lights. Both of my linked images used 90 s lights exclusively.

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Tariff watch: the Rokinon 135 mm lens (Plan C) is holding steady at $449 and in stock at B&H.

Cabin Fever Dreams About Imaging IFN

Yes, it's mid February, the 2025 Nebraska Star Party mailing has arrived, and cabin fever is raging as temperatures are forecast to hit -18 F. Obviously that makes it a good time to play indoors with the imaging setup and ponder about the integrated flux nebula imaging I talked about last time.

The Setup

Here's a picture of the tentative setup that doesn't use guiding: 

Non-guiding setup

There's not much to it: camera, electronic filter wheel, lens, and "Frankenhub" for USB 3, power, and Dew control. This will be my "Version 1" to try at first. If it doesn't deliver adequate star shapes, I'll move on to Version 2 that includes guiding:

Same as in image above, but with guider swapped in for the hub

The hub will be a part of this, too. I'll attach it somehow to the underside of the dovetail or the G-11 saddle. This photo shows my old Orion StarShoot guider mainly because it can run off the ASI 2600's USB 2 port.

Note that in both setups the filter wheel limits camera rotation to a range of about 150 degrees, meaning there's a small (8%) chance I won't be able to get the exact composition I want. A suitable riser would fix this, so I may have to do some shopping.

I'm still eying that Rokinon 135 f/2 lens with a hefty measure of lust. Basic frugality stops me, and the truth is that I really am curious about how well the old Tamron lens performs for this task. The Tamron was about $90 in 1980; in 2025 dollars that's $342, close to the Rokinon's price before the tariff wars started up. While I've been writing this the price of the Rokinon has jumped from $368 to $409, an increase of 11%. 

PEC Training

This was a nice idea but after reading about what PEC is most useful for I doubt I'll bother with it. The focal length here simply isn't long enough for PEC to matter in any significant way. If tracking is fine unguided without training PEC, fine. If tracking isn't adequate I'll just use the setup that allows guiding. Many if not most commenters suggest that at short focal lengths guiding alleviates the need for PEC.

Guiding (if used)

The consensus seems to be that if one is using PHD2 and ASCOM guiding as I am (instead of ST-4) one should do PHD2 calibration by aiming the scope near the intersection of the celestial equator and the meridian. This is basically a do-once thing, only needing to be redone if you make a change in guider's orientation relative to the mount, such as rotating it around its optical axis. Once calibrated you can slew "anywhere" and it will guide properly.  I'm not entirely sure if it's possible to have guiding when aimed at a pole, or even at a declination like that of Polaris (3/4 of a degree from the pole). I'll find out if I decide to use autoguiding.

Dithering

Regardless of whether or not guiding is used, I'll want to dither. Even though it chews up some time I think it helps. If I'm guiding I'll try to use PHD2 dithering; if that works, fine. If not I'll dither with NINA. 

Acquisition

I would normally be tempted to use long exposures times for such a low surface brightness object like IFN. The problem is that long exposures tend to saturate stars; as a result, they lose their color. I want to keep that color as a contrast to the pale IFN. 

That means I'll need to use relatively short exposures of two minutes or less. Look at this image built from 90 s light frames to see what I think are ideal star colors. Yes, they're muted compared to what you see in a lot of images but I think they're more "natural" looking. (Never get me started on the rampant oversaturation of color in modern images!) 

Test images will reveal the optimal exposure length.

I'm going to shoot total exposure in the L:R:G:B ratio of 3:1:1:1. Maybe even 6:1:1:1. There's nothing magic about this, I just like to lean on luminance. That image I linked to was 1:1:1:1. Nice color, but not a lot of detail. More luminance might have helped. 

For convenience I'll probably shoot only luminance the first clear night under dark sky. That simplifies taking flats, focusing, and gives me some freedom about where I shoot the complementary color frames. Ideally those could get taken the second night under dark sky, but two consecutive clear nights in April in Minnesota? Ha ha.

If I only get one luminance night I can process that and see how well the IFN shows up in it. That will give me a sense of how much more is needed.

Any binning that gets done will be in post-calibration processing.

Composition

Using the celestial pole as frame center won't work well with NGC 188, which I want to include in the image. I'll ask NINA to put Polaris at frame center with NGC 188 in one corner. 

It should look sort of like this:

Planned IFN FOV (red box)

Combining Sessions

Given the hours of total exposure I'll want, this will certainly require multiple sessions scattered across several nights. This means I'll need to have NINA slew, center, and rotate consistently. Can NINA and my G-11 do this for a target so close to the pole? I'll have to find out.

Processing

Every new image brings new things to learn in processing. My Veil Nebula mosaic project taught me not only about mosaics, but also more about color calibration and background flattening. It also led me to acquire new tools like NoiseXTerminator, StarXTerminator, StarNet2, and the script StarReduction.  I expect this project will be no different!

The Weather

Camping may be deferred until May's new moon. Around the time of the April new moon the average daily low temperature is around 34 F. Great for keeping the camera cooled, but a little too chilly for this camper (I use a tent). Things improve in May, when the average lows are in the upper 40s. Still quite brisk, but much more bearable. I'll probably use April as my prep month, taking advantage of friend's warm house, and make camping reservations for May.

So that's my (over)thinking at this point. Before heading out to the state park I'll need to do a few things:

• See if NINA can slew/center/rotate for the intended composition
• Shoot some test exposures to assess unguided tracking, and switch to the guided version of the hardware if it can't. These exposures can also be used to judge exposure time, and if the lens suffers from distracting internal reflections
• Collect maybe a dozen or so frames of each color channel making use of NINA dithering and verify that star color is adequate
• It might be useful to verify that my stop-down ring is actually giving me f/4. This can be done indoors at any time using my flat panel

Aside from that last item all I can do for now is play with the hardware and wait patiently for warmer weather. Spring can't come soon enough!