Iowa Star Party Results

Back from the Iowa Star Party, and my first field use of my Samyang lens. Mostly the results were good; shooting with the Samyang at f/2 appears to have great potential, but I need practice to make it really great.

The first night of the three-night party was on-and-off cloudy with enough breaks to keep visual observers happy, but it was not sufficient for the deep sky imaging I wanted to do. I collected some frames mainly to practice with the system.

The second night was almost perfect. Thunderstorms had moved through during the day and were gradually drifting off to the east as the night progressed. Frequent lightning flashes illuminated the eastern sky at for a while but diminished so that by the time I stopped (around 3 AM) they were not a factor. I used the first three hours of darkness to image the Heart Nebula / Soul Nebula / Double cluster area:

Heart and Soul Nebulae, Double Cluster

Next up was an hour on the central Orion area (20 minutes per channel). I didn't do a good job with processing this.

Central Orion (M42, Horsehead, Flame)

To illustrate what the lens can do, here's the Orion Nebula from the above image, post-processed for higher dynamic range:

M42 from above image

It's pretty, but it's only 1293 x 1326 pixels, so at 300 DPI it's only good for a 4 x 6 inch print. 

It was getting into the wee hours of the morning, but I couldn't stop--I get so few opportunities to image in Orion. I just had to image the Lambda nebula around Orion's head, something I had imaged eleven year earlier in H-alpha on an unusually warm February evening. This is based on a ridiculously short 30 minutes of total exposure (10 minutes per channel) and really illustrates the power of shooting at f/2: 

Nebulosity at the head of Orion (Betelgeuse is at lower left)

(All three of the above full images are at half-scale. For full scale images, see my AstroBin gallery.)

Alas, the third night was not to be. The cold front came through and the day was windy and brisk. late in the afternoon it started raining; online sources suggested that clearing might happen around midnight -- or later. I decided to call it a star party and get some good sleep before the return drive the next morning.

After an imaging session it's always good to reflect on what was learned, and this trip had some lessons beyond learning that my sleeping bag advertised as being good to 20 F was definitely not.

Rotating the system proved very doable. It was awkward at first, but so is everything. I found it almost impossible take the focusing belt on and off without messing up the focus, but the penalty for that is minor. My NINA advanced sequence design with added pauses seemed to work out fine.

The first night I found that R-G-B-Dither cyclic acquisition was problematic. I was doing this to make drizzle processing possible. Mostly this worked, but occasionally PHD2 went wildly unstable in RA and suffered death-by-overcorrection. I'm not sure if this is a settling time issue or what. The second night I turned dither completely off and guiding was fine. The ASI 2600MM seems not to need dithering, but if I'm to drizzle, I'll need it.

Look again at the above images, and notice the slightly darker circular area at the center of each image: a donut hole. This arose because of bad flats; here is the stretched value of the ratio between older, correct flats and those from Iowa. 

Lovely radial symmetry about the image center, but this should be a nearly uniform field aside from any changes in dust. 

I tried using the older flats in place of the new ones, but that failed. Because my system is currently almost dust free, I tried skipping the flats and using PixInsight to correct the vignetting. That also failed. My next attempt will be to use Photoshop to manually correct this issue. 

My guess for what caused this is improper focus. I deferred shooting flats until I returned from Iowa, and in that time the zero reference point on the EAF was lost. This made it almost impossible to reproduce the focus used for the light frames, resulting in the poor flats. One clear night should make it possible to shoot some new flats at proper focus.

The weather forecast is mildly optimistic for the first week of November with highs in the 60s; will that warmth will bring the clear night I need? 

In preparation for the Iowa Star Party I'm doing a some imaging housekeeping. The guider needs a fresh calibration and the filter offsets need a good redoing. More complicated are the changes needed for a basic imaging sequence using the Samyang lens. Because rotating the camera/lens requires the EAF belt to be disengaged and then re-engaged, one needs to put some breaks into the sequence to make opportunities for working with the belt. Here's what I'm setting up for the Iowa party.

The following instructions are for the NINA advanced sequencer. If you're not using that, you really should be! Also note that this is specific to a short focal length system for which you intend shoot RGB suitable for drizzle processing.

• NINA's Advanced Sequencer (If you are not, you should be)
• Your EAF position is zero when the lens focus is at the infinity stop

All good? Great. Here's what you need to do if you plan to orient the imagining camera while it sits in an Astrodymium cradle. It's a little complicated by the need to disengage the EAF belt while turning the lens.

Rotating the lens requires that you add two Message Box (MB) instructions to the sequence. The MB instruction is found in the Utility section. Obviously, if you don't care about the camera orientation, you can just skip the following.

If you're doing a Slew, Center, and Rotate (SCR) and have connected the Manual Rotator to NINA, place one MB just before the SCR command and another just after it. When the sequence reaches the first one, it will stop. You should then disengage the belt from the lens, being careful to not disturb the focus. Then click away the Message Box. NINA will slew, center, and tell you the amount and direction to rotate the lens. Do this until NINA is happy. It may also do some additional centering, but eventually it will reach the second MB and stop. Re-engage the focus belt. Verify that focus is good enough for autofocusing, and click away the second MB. The sequence will now resume normal operation.

If you're using Slew and Center (SC) and simply eyeballing the orientation, just use one MB immediately after the SC instruction and a second SC right after that*. When sequencer get to the MB, Disengage the EAF belt and manually orient the camera using snapshots. When the orientation is correct, re-engage the belt, check focus, and click away the MB.

*The second SC is needed to insure the target is at frame center; it acts as the post-rotation recentering performed by SCR but not by SC.

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I still wasn't happy with the tilt I was seeing, and I wondered if maybe it was coming from the way I had the camera attached to the Samyang -- that partially attached 5 mm spacer seemed iffy. I sent a note to Nic at ThinkableCreations asking if he had some advice about how to remove about 2 mm from the M42 threads on the adapter he sells. He didn't, but he expressed some concerns about the shorter thread length not providing a solid connection. I thought that would not be a problem and went ahead with my plan to sand off the end of the threads.

I used 600 Ultrafine "Wetordry" sandpaper from 3M for a while and made little progress, so I switched to 400 and it went nicely. Eventually I was able to attach my 7.5 mm spacer to the Thinkable adapter, giving me 45.5 mm of backfocus, close enough to the 45.0 desired.

Sounds good, but the reality was that this change once again made the lens unable to focus infinity.

Swapping off the 7.5 mm spacer for a 5 mm, back focus was now 43 mm and focus was restored at a EAF position of about 3200. Still bad tilt, possibly because I didn't sand the Thinkable adapter down far enough. I added two spacer shims to bring the back focus up to 44 mm. The focus position was now around 2300. One last tilt tilt-correcting shim took it up to 44.2 mm and an EAF position of 1400. Possibly I'll add another thin spacer in Iowa.

Evidently the ability of a lens to reach focus depends strongly on backfocus, something I didn't know. 

EAF position (vertical axis) vs. Backfocus (horizontal axis, mm)

(The zero position in the diagram corresponds to the maximum lens movement in the direction of infinity focus. The negative position at 45.5 mm is a guesstimate.)

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Anyway, here I am a few days away from the Iowa Star Party and I'm going to shove my perfectionist nature to the curb and settle for "good enough." I'm going to rely on BlurXTerminator to handle aberration and tilt issues.

Here's an idea of what the lens can do. This is about a 16th of the full image area, unscaled. M31 straddles the right edge of the image. This is a single 20 second frame of luminance; click to see it at 1:1 scale.