TILT!

Yes, it's that enemy of flat fields: tilt in the optical train. 

Recall -- or don't, since I'm going to repeat the information anyway -- that my first choice for imaging the Polaris-area integrated flux nebulae was an old Tamron 135 mm f/2.5 lens that stopped down to f/4 is quite a nice lens. Second choice is to use my FSQ-106 with a 0.73X focal reducer that gives me a nice wide field. The FSQ is wildly higher in quality than the Tamron, but the only time I've imaged with the focal reducer it produced results that barely adequate: stars were noticeably elongated on one side of the image. 

I'll need to use the FSQ + focal reducer if for some reason the Tamron proves to be problematic. That means it's time to chase down the tilt problem and get it fixed.

What dawned on me recently was that there was enough back focus to allow me to use the tilt plate that came with the ASI 2600 camera I use for imaging. I had removed the plate to make the ASI compatible with my DSLR. The fact is that the ASI has largely made the DSLR superfluous; I'm  unlikely to image with it again. The tilt plate can therefore come out of storage and get back into action. Here's the present situation:

The configuration of interest here is the bottommost one. The reducer requires that the camera sensor be at a very specific distance from it: 72.2 mm. Currently there's a 12 mm M42 spacer in the optical train. Replace that with the tilt plate and a 7 mm spacer and it's all good (aside from some very thin spacers). 

Now, how to use the tilt plate? Let's start by verifying that it's the optical train and not the sensor that's out of whack. To do that correctly, I'd need to rotate the camera relative to the optical train and see if the effects of the tilt moved with it. I'm going to cheat a little and assume that if the field is flat for the native imaging mode (f/5) the sensor is fine.

Here's the ASTAP measurement of tilt in a single luminance image taken with native mode (f/5):

ASTAP report of field flatness for FSQ-106 & ATI 2600

So you're probably asking, "what the heck is that?" The numbers next to the yellow lines are the area-averaged half flux diameter (HFD) of the stars. The HFDs essentially measure how pinpoint the stars are, and they can be affected by focus quality, seeing, and aberration and tilt problems. A perfect telescope with perfect optics and perfect seeing would have very small numbers in this diagram.

Tilt will introduce variations in HFD that are in the direction of tilt. So how does one assess these numbers? ASTAP does that for you. If you look along the bottom edge of the above picture you'll see it assesses the tilt at 6%, which it judges to be "almost none". Great! This suggests that the native optics are adequately tilt-less--and by extension that the sensor is reasonably perpendicular to the optical axis.

Now here's the ASTAP analysis for a luminance frame taken using the reducer (f/3.65):

Same as above, but with the focal reducer

ASTAP says the tilt is 19% (moderate). It also confirms my eyeball judgement that the tilt is largely oriented along the long axis of the sensor. 

(Yes, you could deal with some star elongation in post-processing, but isn't it better to not have to do that? This brings us back the the first defense against tilt, a tilt plate.)

The ZWO tilt plate works using three adjustment screws arranged in an equilateral triangle, much like the tilt screws for adjusting a small Newtonian objective mirror. What I'll do is to orient the plate so that one of the adjustment screws is in the direction of the tilt, like so:

Orientation of ZWO tilt plate: blue arrow = tilt direction, red dots = adjustment screws

This way the adjustment screw at that left vertex becomes the primary one to adjust. Note that ASTAP has an option to provide a 3-point analysis which may be easier to interpret in some cases.

The process of correction is basically repeating the sequence of imaging, analyzing, and adjusting. It's another new skill to learn and put to use, which is good. There doesn't seem to be much to it; I'll let you know how it goes.

Unfortunately this won't work for the 135 lens; it has no spacers to swap out for the tilt plate. Should I find it needs correction, I'll probably use these things. I might get that and see if it's all I need for correcting the focal reducer tilt, too. If these shims work for that it would eliminate the need for a tilt plate and make it easier to change from one configuration to another. 

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At least it's March now. A few inches of snow is coming in the next day or so, just because it can. Almost time to make my reservations at Lac qui Parle!

[Added 5 March, it was more like 9 inches.]

Update: Risers

Last time I mentioned that my setup for imaging integrated flux nebulae won't allow a rotation range quite up to the 180 degree minimum I'll need. I mentioned a riser would probably be needed.

What should show up on Amazon Vine for review the next day but this: 

Car seat risers.

Yes, they're car seat risers for when you're not tall enough to drive your vehicle. This set comes with two 1" and two 1/2" spacers, nicely drilled through. The bolts are provided to fit the car model of your choice and are entirely too large for my purpose, so they go in the odd bolts jar.

One problem with these is that the edges of the spacers are too far from the hole and extend into the movement of the camera, keeping it blocked. When I realized this I thought all was lost because these felt as if they were made of metal and probably difficult to work with.

They're not!

What exactly they're made of is a mystery. An ordinary hacksaw cuts them with relative ease producing a sort of black, plasticky "sawdust," so I assume they might be made from some sort of composite material. In fact, it looks as if these might be created with a 3D printer. 

Whatever their composition, two minutes of hacksawing shortened one of the ends. With a quick trip to Home Depot for a M6 1x40mm bolt it was ready. It turns out the 1" spacer is sufficient, providing over 180 degrees of rotation.

Camera on riser. It may look a little precarious but it's solid with very little flexure

It's ready for the next step: assessing image quality. Will there be nasty internal reflections? Given that this is a new lens/sensor pairing will it produce a flat field at infinite focus? Will the lens to sensor separation need adjustment? Questions questions questions!

Spring is in the air -- today's high was at least 52F.  A week ago it barely managed to make it up to -2! I know, it's not April yet. Even worse, it's still February, the month that never ends!

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!

Happy 2025! It's That Time Again: Imaging Plans for the New Year

It's usually folly to plan a summer of imaging. Sure, clouds and smoke could spoil things, but why not give it a  try? 2025 might just turn out to be an exceptional year! State Park camping and two star parties! Let's go!

APRIL/MAY

The first new moon of the 2025 camping season is on April 27th, a week after Easter. So any time within a few days of that should be dark enough for deep imaging. Likewise the dark moon in May, which may be better (i.e., warmer) for camping.

The late spring sky doesn't offer much to image at the focal length of my FSQ-106 (530 mm), aside from groups of galaxies and a few clusters. There is one wide-field subject that has always intrigued me: integrated flux nebulae, or IFN. Here's a nice summary of what IFN is and is not, and where to find it.

Probably the best known and most frequently imaged IFN is in the direction of M81 and M82. Instead of that I'll go for the IFN near Polaris.

Imaging near the north celestial pole has some interesting aspects. Because the apparent movement of stars there is so slow it's common for people to suggest not using autoguiding when imaging at relatively short focal lengths. To do without guiding you need a near-perfect polar alignment; a well-trained periodic error correction is very helpful, too. PoleMaster provides alignment that's close to perfect, so that's covered. But I do need to train PEC and that will be one of the first things I do this spring. 

Associated with guiding is dithering, which I like to do. Is it possible to dither and not guide? Yes, with NINA's built-in dithering. I'll have to learn how to use this, and hope it works well. If it doesn't I'll probably just not dither.

I've never targeted anything so close to a celestial pole; A little testing suggests that my G-11 is fine with having NINA slew from counterweight-down it to the pole. What I'm not entirely comfortable with is NINA trying to center it there; will it get lost making excursions back and forth across the meridian? Is Polaris far enough from the pole to make centering a nonissue? 

Regardless of where the target is there's always the question of what imaging gear to use. The key characteristics of IFN are that it's very low surface brightness and extensive in size. That sounds like a job for short focal length and fast focal ratio, right? Here are my best options for the optics:

• Takahashi FSQ-106 (FL 530 mm, f/5);  Pinpoint stars, but smallish field and rather slow
• Takahashi FSQ-106 + focal reducer, (FL 387 mm, f/3.65); Pinpoint stars, very fast, but smallish field
• Tamron lens, (FL 135 mm, f/4.0); Fits NGC 188 in field, overmounted (not a bad thing), very fast, no autofocusing

Here are two trial images made using the Tamron. An H alpha image of the Lambda Orionis Ring from 2015 and an LRGB image of the Sadr area from 2016. Both used an ST-8300 mono camera, and I can't say my processing was particularly good (I was still using ImagesPlus). How well it works with my ASI 2600 camera is unknown right now. Will it need spacers? Will there be tilt problems? Will it show internal reflections when put on a relatively bright star like Polaris? I won't know until the weather warms up and I can shoot some frames. 

If the 135 doesn't work out I might go with the FSQ + focal reducer. Its ability to support autofocusing is some compensation for its small FOV. Maintaining the FSQ's focus will be important for dealing with the long all-night sessions (6+ hours in April, about 4.5 hours in May). With a small lens like a 135, it's easy to minimize thermal FL change and prevent dew formation (as suggested by KathyAstro) by wrapping a long dew prevention strap around it. 

Finally, there's the issue of exposure time. I'll probably use my standard 120 s @ gain 100 for luminance and try to collect as many frames as possible. If tracking is an issue, I can drop it to 60 s, but if stars stay nice and round, I might increase this up to as long as 300 s. Shorter exposures will be fine for the color channels; in fact, those might only be 60 s  @ gain 0 to keep stars from saturating. Some experimentation is needed!

JUNE/JULY

The Nebraska Star Party typically occurs close to the end of July, a bit too late in the year for imaging my desired target of the Antares/Rho Ophiuchi region. In June the situation is much better, and a good time for imaging it comes after the third quarter moon of June 18. The Antares and Rho Ophiuchi nebulae should look fine together at 135 mm. 

July is wide open for imaging much of the Milky Way, including the M8/M20 tandem. This pair will fit nicely into a single frame using the FSQ-106 with its focal reducer. Also available is M16, which nicely fits the FSQ native field.

LATE AUGUST

It's Northern Nights Star Fest time, and I'll continue collecting frames for the Soap Bubble. Recall that the Bubble had just started to show up with about six hours of frames collected in 2024. Continuing on this target in 2025 should put me over 12 hours of total exposure. Barring losing time to more Aurorae. 😁

SEPTEMBER/OCTOBER

The Iowa Star Party (2025 date TBD) will be a good time to gather more frames for either the IFN, the Soap Bubble, or some other target that suggests itself. Another possible target is IC 348.

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Obligatory Power & Battery Update 

As an Amazon Vine reviewer,  I have been able to get the components needed to replace my old 100 W solar panel and charge controller with a smaller, much lighter 50 W unit. Thank you, Vine!

Today wasn't ideal (high haze and tree branches) but it managed to supply 35 W. The charge controller seemed to work fine, and in an hour it boosted a 10 Ah LiFePo4 battery at rest voltage 12.96 V to 13.26 V. The meter suggested 20 Wh had been added to the half-charged battery. That's compatible with the rest voltages.

The real test will be when the sky is fully clear and the panel gets hot in the sunlight. I'll take it camping and see what it can do.

December Update

Winter Break Begins

The first Arctic blast of the winter has arrived and it's time to shut down imaging until spring. I've sent my Losmandy Gemini II off for its Level 6 factory upgrade, packed up all the optical gear and put it in the driest part of the house for winter storage, and have discharged all my lithium batteries to about 50% of capacity. The latter is for optimal battery health during several months of non-use. They'll get a recharge and second draining around February, and then get a full charge whenever spring decides to happen.

This treatment regimen includes the smaller LiFePo4 batteries that I'll probably haul up to the 2025 Northern Nights Star Fest for swapping/selling. 

The endless mosaic

I've decided to do a full reprocess of the mosaic starting from calibrated frames. Three of the panels have new light frames and will need to be re-integrated. So for the sake of uniformity and to improve the processing workflow I'll do it all again. Isn't reprocessing what winter is for?

Getting back into spectroscopy

My last activity of autumn was assessing the use of my AT65 astrograph for spectroscopy. I'm really unsure about using an astrograph for this purpose, as I'm concerned that putting the grating so close to the OTA's internal correctors will lead to smearing of the spectrum. Little did I know this was going to turn into a multi-night struggle as the mount and focuser had issues. Fortunately there were a few clear nights for fiddling with the focuser before the weather got too cold for me.

I had already adapted my old Pegasus FocusCube 2 to my AT65's focuser, but I had done it incorrectly. I had attached it to the fine-focus shaft. This led to absurdly fine focusing precision--and sometimes demanded too much torque from the FC2 causing it to seize. Putting it onto the coarse focus shaft solved the torque issue, and made it possible to rack out a couple of centimeters without having to wait half an hour.

Whether the change results in focusing that's precise enough to match manual focusing is something that won't be resolved until the spring. That's also when I'll configure it for backlash and autofocusing.

Maybe this is why the AT65 never appeared on the FocusCube compatibility list?

Stay warm everyone! Happy Holidays!

FrankenHub is Created & A Very Minimal Power Supply

Okay, it was Halloween yesterday. In keeping with that I'll show you my replacement for the Pegasus Powerbox that died in Iowa. 

My Powerbox was a USB and Power Hub with the capability of controlling dew straps as well. My replacement would need to have those capabilities.

I already had a powered USB3 hub and a small Anderson Powerpole-based power bus from past tinkerings, so it was only a matter of replacing the dew control capability. A nice little Pegasus Dew Controller took care of that.  Yes, it's manually operated, but that's fine.

The USB Hub is so old it's no longer sold (not a surprise) and now is a 7-port, smaller device.

What was left was how to put it all together. And that's how it became Frankenhub.  I decided the perfect "suturing" was silicone bathtub caulk. Silicone caulk is waterproof, bonds to almost anything, remains pliable, and can be removed. As an adhesive it's more than adequately secure when joining two flat, clean surfaces.

Frankenhub is three layers, a dovetail to mate it to the ad-hoc finder shoe on the scope OTA, the USB hub, and then the dew controller on top:

Frankenhub

Definitely not as pretty as the Pegasus box, but it works just fine. What it lacks is software control of the dew controller, but I can live with that.

Wait, where is the power hub? Caulked to the side of my G11 saddle:

Power Bus

Think of this as one of those big bolts in the neck of Frankenstein's Monster. You know, these things:

OEM Heavy-Duty 300,000,000 volt / 30,000 amp Connectors

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Speaking of power, here's a minimalist approach to supplying power: A 50 Ah battery and cute 12 V distributer:

Power and Distribution

The battery has all the power I need for several nights of summertime imaging and the distribution box has three 12V automotive sockets, all I need to run my setup. Here's a closeup:

Power Distributor

Note that it includes a voltmeter that can be turned on/off, various sockets for charging your mobile devices, and a 3-level light that you could easily cover with red plastic.

The included 50 A Powerpole connector probably couldn't handle the 30,000 amps that jolted Frankenstein to life, but it's more than enough for my rig's peak draw of  5 amps.

 

50 amps of gray

Next time I'll have another Frankenstein that I hope isn't monstrous: the Veil Nebula mosaic, using parts (images) from a few sessions at Eagle Lake Observatory, The Iowa Star Party, and a friend's back yard near Stacy, Minnesota. A real Frankenimage. 

 

First Pass at the Veil Mosaic

I finally had enough data to make a very flawed Veil mosaic. Here it is at 1/5 scale:

Click for 1/5 scale image

And here is the link to download the full-scale image. You can use the cloud site's .jpg to look at it, but I suggest you use your own image viewer. Minor advisory, this is a large image, about 10K by 10K pixels and the file size is 12.5Mb.

You should immediately see some interesting things in the full image, some of which I'll detail here. I'll start with the major flaws (all the images below are at full scale if you click on them).

1) What's the big circular thing?

Bad Flat!

This is in the upper left mosaic panel, and results from improper calibration (using an outdated flat, rookie mistake.)

2) What's all that noise?

So Noisy!

Again in the upper left mosaic panel and comes from not only the poor calibration but also insufficient data.

3) What's with all the black pixels?

Pepper Spray

This is found in the upper and middle left panel; it results from pushing the panel a little too aggressively in one post-processing stage.

4) That green thing can't possibly be right!

Streaks to Left of Center: Not Really Green!

So far as I know there aren't any green nebulae. For reasons I don't understand this feature shows up rather strongly in the green channel compared to the red and blue channels; it's quite strong in luminance. I've seen one other LRGB image of this area, and it didn't have these green bits, so I must conclude something in my processing is at fault here.

The Dastardly 52 Cygni

There was one other flaw I had to skate around. I used the script Star Reduction from Blanshan and Cranfield to perform star reduction. It requires a starless image; this can be made either using StarXTerminator (aka SXT) or StarNet2. I used SXT, and it had a lot of difficulty with Magnitude 4.2 52 Cygni (see above image). It essentially suppressed the northern end of NGC 6960! So I switched over to StarNet2 and it handled things fine. 

Notice that StarNet2 didn't handle it 100% cleanly either, giving it what look like (but aren't) diffraction spikes. Looks kind of cool, I think, but then I grew up as a Newt user.

Now for some positives:

1) No seams. I used Photometric Mosaic, and not only didn't it produce seams, but it avoided the pinched stars that GradientMergeMosaic plagues me with. 

2) Color calibration by SpectrophotometricColorCalibration (SPCC). I can't say enough positive about SPCC. My old workflow made color correction very fiddly, with the result depending on how I felt the day of processing more than any sort of objective measure.

3) Not narrowband. For good reason the Veil is usually imaged in narrowband, but that results in false colors. It's nice to see things more as they "really" are (aside from a Green nebula).

4) Not oversaturated. In my opinion oversaturation is very common now. I've said before that a guiding principle of processing should be that less is more; this idea applies to sharpening, stretching, and also color enhancement. I did bump this a half step using ColorSaturation, but pushing it further seemed like too much of a good thing.

5) Reasonably sharp. Here's a neat feature at the south end of of the East Veil. This looks good thanks mainly to BlurXTerminator (BXT). In fact, aside from the one issue seen in SXT, I think the triad of BXT, SXT, and NoiseXTerminator (NXT) is difficult to beat.

An Optical Spiral That's Probably Not a Real Spiral

   

I suspect this curly-thing at center is nothing more than an undulation in a plane seen edgeways, but it does look fun.

So, with those issues listed above this thing obviously isn't done yet. The other night I shot additional light frames for the two panels that suffered from noise (along with same-night flat frames), and I think I can make sure that pepper spray of black pixels can be avoided.

This means doing much the processing over again, but that's life.