An Imaging Platform is Born

In the spring of 2017 an anonymous donor (hereafter called 'AD') came forward with an offer to fund what was needed to make the Minnesota Astronomical Society's dark sky site at Cherry Grove a high-end imaging site. Basically the AD was volunteering to fund the purchasing of a new imaging platform. The money would come with a few strings:

• The platform would make use of an existing underutilized mount in the observatory. This mount was a Mathis MI-500, a huge mount capable of carrying 180 pounds of imaging gear. The mount is already outfitted with an Astro-Physics GTOCP4 control box and handset.
• The platform must be easy for beginning imagers to use.
• Purchased telescopes must to be able to fit under the roof. The purchase of large refractors like those installed at the club's other two sites was discouraged along with truss-tube scopes.
• A person or people need to be found who will maintain the platform
• Adequate training must be provided for those wishing to use the platform

Existing Facility

Cherry Grove Observatory (CGO) is a large roll-off roof building located in Goodhue county, Minnesota, about 23 miles northwest of Rochester and 20 miles south of Cannon Falls. While it's not a truly dark site (light green zone), it's far darker than the other club observatories. Horizon glow from the metro area of Minneapolis/St. Paul 60 miles to the north can be seen, as is a growing glow of light in the southeast from Rochester.

Cherry Grove. Warming house at left, observatory on right

The Process

The task of choosing gear for the Imaging Platform was passed to the small committee that oversees CGO. None of the committee members are imagers, and they thought it best to form a task force of imagers to make recommendations for what to purchase. That's when I entered the story.

Recruiting task force members went reasonably well for something like this. Some people agreed to help, others said they weren't really qualified or had other obligations. Five volunteers were quickly selected and I was designated the chairperson, probably because I had advanced some ideas for what might be done in an earlier request for help from the CGO committee.

The early going was hindered by a lack of any idea for what the AD was willing to spend. We quickly settled on a two-tier recommendation for what might be possible at around $10K and $20K.

Guiding Thoughts

Beginners will be using it, many with DSLRs but a few with CCDs or newer cooled CMOS cameras. Astrographs for which no flatteners are needed are simple and convenient. Beginners wouldn't have to be concerned about having the correct spacing rings.

What will people want to image? Probably the objects that image well from dark sites: Diffuse and dark nebulae, but not solar system objects. That suggests a telescope with short focal length and fast optics.


Many objects are small. After diffuse nebulae likely targets will be planetary nebulae, globular clusters, and external galaxies. The way to image them is with a longer focal length scope. Since these tend to have higher surface brightness than many diffuse nebulae optical speed is less of an issue.

Beginners will bring a wide variety of learning skills. Some will be happy to learn how to use software to control the mount and camera while others may be reluctant to use more than the mount's hand control. We should try to accommodate as many people as is reasonable.

Given all of these factors we decided to go with a platform that would provide two imaging astrographs in tandem-one short focal length, the other much longer for those tiny planetaries and galaxies.

As we looked over the available scopes it became clear we the low cost option of about $10K was unrealistic. In fact given the cost of all the accessories, shipping and taxes the higher tier of $20K would be inadequate, too! After a talk with the AD it was decided to go with a budget of between $30K and $35K.

At that point we were ready with our recommendation: A Takahashi FSQ-130ED as the short, fast scope and a Planewave 12.5" CDK. Along with the Tak we decided to get the 0.6X focal reducer and 1.6X tele-extender to provide three focal lengths: 390mm, 650mm, and 1040mm.

Current Status

The club's Board signed off on the purchase and we placed our order with Oceanside Telescopes.

The Tak has arrived and is on the mount for testing. So far it's living up to the Tak reputation: Exceptional optics and build quality. It's a great scope!

Takahashi FSQ-130ED on the Mathis mount (to be remounted when the Planewave arrives)

The Planewave is scheduled to arrive in July.

We hope to have the platform fully functional by autumn -- if this year's run of cloudy nights will ever relent!  I'll post images taken using the platform as they are created.

Imaging at the 4M

The 4M is the MAS (Minnesota Astronomical Society) Mini Messier Marathon. It takes place annually during the fall when it's the second time of the year one can see a large number of Messier objects in one night. In Minnesota the fall marathon usually has much nicer weather; in spring, the observing field can be under a foot or more of snow, or it can be a soggy mess thanks to snow melt. The wind chill can be -25F.

This year the 4M was very nice. Temperatures were in the 60s, the ground was soft but not too wet, and a gentle breeze kept the dew from forming. Mosquitoes were at a minimum--I noticed only a few, and that was only during the early evening. Clouds spread overhead at sunset and persisted until a little before 10 P.M., at which time it was a scramble to get my imaging gear going before I lost too much more time on the southern objects I was going for.
Because the transparency was poor and time was limited I opted to go for only one of my southern targets, Sh 2-46, a fairly bright (LBN brightness 3) emission nebula in Serpens Cauda. With almost no time for it I opted to image it only in luminance. I had to end imaging after 39 minutes because it was sinking into the low-altitude murk and starting to lose brightness.

I stayed west of the meridian to get LBN 113, another emission nebula that almost three hours of Ha could not catch in my back yard. This time I went with a different plan, L binned 2x2 and RGB binned 3x3. I used equal total exposure times for L and RGB, 15x180s. The results were better than I expected:

Top: Sh 2-46. Bottom: LBN 113.

2-46 is the unimpressive light patch just below the center of the image. 113 straddles the yellow star near the image's center. The bright blue star to its right is theta Aql.

These took me to almost midnight, so the evening was still young. By that time M45 was well up, and a difficult object  I wanted, IC 353, was right beside it. It didn't take long to acquire the object and begin shooting. 

However, I made a bad mistake at this time. The scope and camera had to move quite a bit to swing from west to east of the meridian, and in that flip the camera's sensor plane went out of orthogonality with the optical axis. Focus was no longer anything like crisp across the field, and there was some astigmatism introduced. I should have checked everything over, but in my haste I didn't.

After I was done with IC 353, I was getting tired and decided to image something bright. IC 405 in Auriga was up now, and that became my target:

Top: IC 353. Bottom: IC 405 (Flaming Star Nebula)

IC 405 deserves more exposure time, but that will have to wait until another night. By the time I was done with 405 it was 2:44 A.M., and I was ready to call it a night. all but a handful of marathoners had packed it in for the night by this time, so I decided to join them.

Three of these are new AL Bright nebula list images for me, and the image of LBN 113 replaces my earlier attempt. I'm left with only one shaky image, in my opinion, that of Barnard's Loop. It's plainly in the image, but it could look so much nicer!

What's next? The seasonal dry spell in AL Bright Nebular list objects will last for another month or so. Things really start to pick up in November. If there are some clear nights another trip to Cherry Grove may allow me to pick up some fading summer objects. Cherry Grove is only modestly dark, but it's dark enough to allow LRGB imaging. 

Focusing; imaging at a dark site

I recently posted that I was going to test the parfocality (if that's a word) of my Baader LRGB and narrowband Ha/SII/OIII filters. The test has been done, and it was less than conclusive. If you use a Bahtinov mask to do focusing, all the filters are essentially parfocal. That is after focusing for one filter, they're all in focus. They all have nicely centered spikes in the test images.

However, the centered appearance of the central spike when estimated by eye is only approximate, and the range of error is about the same as the range over which final focusing is done. I think you're still better off handling focus with the assistance of software that quantifies the sharpness of focus. Likewise, I think every time you change filters, it's time to refocus. At least that's what I'm going to do for the foreseeable future.

Last night I gathered another bunch of photons for my M101 project, this time from the dark-sky site of Cherry Grove. It's really nice to have a dark place to image from only an hour's drive away. The site has a warming house, too, which was nice. Imaging was largely automatice, so I could while away the hours reading and listening to the Twins game in comfort, as the temperature outside fell to around 0C.

Added note, 5/22/12: Further imaging with the Baader filters suggests that the LRGB filters are essentially parfocal when used with an apochromat. Likewise the Ha-SII-OIII filters.