Celestron Field Flattener Follow-Up

This is a follow-up of a previous post.

I was able to image the other night with the short nosepiece. Recall that my goal was to get the system to match the presumed optimal f/6.3. It was f/6.0 with a long nosepiece/adapter in place, and by switching to a simpler nosepiece it allowed me to trim about 8mm off the FR/CCD separation. While the image is not very good, it's good enough to allow astrometry.net to plate solve it and calculate the pixel scale.

Here's the image:

NGC 7625 (small galaxy at center)

This was based on 30 minutes total exposure for each RGB channel, under near-urban sky. It could be a lot better, but sometimes one must take what the sky and gear gives.

The pixel scale is reported to be 1.51 arcseconds per pixel. This is really close to the value of 1.50 that corresponds to a focal ratio of 6.3--the error is only about 2/3 of one percent.

Using the Celestron Field Flattener with an SBIG ST-8300M and FW8-8300

I purchased a used Celestron 9.25" XLT SCT intending to use it primarily for planetary imaging at f/10 or greater. There was no need for a focal reducer/flattener. I did purchase a used Celestron 94175 f/6.3 flattener/reducer so I could, if I wanted to, image deep sky objects. As it turns out, deep sky objects have been the C925's main use.

Reducer/Flatteners like the Celestron 94175 work best when at a specific distance from the sensor. Can this distance be known? Let's use the calculator at  Wilmslow Astro and find out.

For a standard f/10 SCT, the separation to give f/6.3 (where we presume the field will be flattest) should be 105mm.

Two nights ago I imaged NGC 7008, the Fetus Nebula, using my CCD and the focal reducer. The stars were nice and round all the way to the edge:

NGC 7008 (click to enlarge)

Astrometry.net reported that the image scale was 1.58" per pixel, which translates to f/6.0. According to the above calculator this should happen at a reducer/CCD separation of 113mm**. 

If the reducer is flattest at f/6.3 I need to decrease the separation from 113mm to 105mm, or by about 8mm.

Happily, I can do that. The nosepiece I was using was a 2" focuser-to-TeleVue IS adapter + IS to T-ring adapter. I also have a nosepiece that's just 2" focuser to T-ring, and amazingly it will let the CCD get about 8mm closer. Just about perfect!***

The next night I'm out I'll try this configuration and see if it gets the focal ratio correct and also produces round stars.

* This suggests that the performance of the reducer is rather insensitive to the separation. Here I'm 8mm off and the stars look good. This has limits, though. In a previous image a 12.7mm spacer was included and the stars at the corners were plainly distorted. That image had an image scale of 1.65" per pixel, which corresponds to a focal ratio of  f/5.75. The calculator says this happens at a separation of about 123mm. I measured the separation as about 125mm. That image suggests the separation reduction should be 20mm, which is roughly the same as 12.7 + 7.3 (the spacer plus the ~8 mm suggested by the other image).

** One thing that's usually left ambiguous in reducer explanations is the point on the reducer from which the CCD distance is measured. In the case of the system operating at f/6 I measured the CCD to be 125mm from the front of the reducer. which is the same thing as 114mm from the center and about 100mm from the rear thread base. Because the center position in only 1mm different from the formula's value of 113mm, and given the inexactness of my measurement, It's reasonable to conclude separation should be measured from the center of the reducer.

*** Is this an accident? The Antares 50mm long 2" focus tube on the back of the SCT makes this possible, but it predates the ST-8300 + filter wheel and couldn't be designed to work with their backfocus requirement. But it works out nicely, right?

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What does this say about using a 0.5 focal reducer? The model I have is a SmartAstronomy 2", which is probably identical to the GSO 2". GSO says that the focal length is 106mm and the optimal separation is 53mm. The calculator puts it a bit larger at 56mm. Because this reducer screws into 2" nosepieces, the closest I can get it to my CCD is about 69mm (measuring from the center of the reducer). That puts it at about f/4. I'd guess that's too far from ideal to produce pleasing images, but it's worth a try. A Lumicon low profile nosepiece could trim 9mm off that, getting it close: f/4.7. Trying that will cost about $35.

Going a Little Deeper with M 57

When the first really deep images of M 57 started showing up, I was amazed at the wealth of structure that surrounded the familiar oval that gave the Ring it's name. As it turns out, many planetary nebulae show this kind of layered structure. The explosive events that drive the star's outer atmosphere into space aren't just a one-time event; instead, they can happen repeatedly, producing a set of expanding nested shells. The outer shells are older, farther from the central star, subjected to less intense radiation, more diffuse, and have had more time to cool, all of which means they are less luminous at visible wavelengths.

M 57 is quite bright and easy to image. Even the central star, elusive visually thanks to the diminished contrast in the Ring's interior, is easily imaged. But what about those outer shells? Can they be imaged by folks with modest gear?

Last night I made my first try at finding out. While waiting for another object to climb out of the trees, I had some time to spend on M 57. I used a C 925 SCT with a 0.63X focal reducer, an SBIG ST-8300M CCD camera shooting through a Baader Ha narrowband filter, and accumulated 39 minutes of 3-minute light frames. That's not very much, compared to some projects where one have a total exposure time of over five hours. But it was all I had time for last night.

The result was surprising, at least to me:

My image is on the left; at right is a deep image from the U of Oregon for comparison.

In both images the Ring is wildly overexposed, but you can see the second ring clearly in my 39-minute shot. Also visible are a couple of the brighter knots in the next outward ring.

The image scale here isn't the greatest, and only 39 minutes through an Ha filter leaves a lot of noise, but I think it's going to be worth going back to this and shooting for hours. 

If you haven't tried going deep on M 57, you might give it a try. The result may surprise you, too.

Clouds, WOW, Focal Reducer

The abysmal spring of 2013 continues with clouds, clouds and more clouds. My plan to attend the Wisconsin Observers Weekend this weekend has fallen through because of weather. The first night is being clouded out as I type this, Tomorrow night is forecast to be partly cloudy (at best) and Saturday night is forecast to be rainy (50% chance) and cloudy.

Worst. Spring. Ever.

But anyway, I did manage last week to shoot some test images with my .63 Celestron Focal Reducer.  I fiddle with my adapters until I got a spacing of about 104mm (I think), and here's what I got:

Flat Field test

This is taken using a Celestron 925 XLT and Celestron 0.63 Focal Reducer/Flattener. The camera is an SBIG-ST8300M with filter wheel (total back focus 38mm). Spacing was provided by a Celestron SCT-to-T adapter #93633-A (about 58mm).

The above full-field image is six 10 second exposures (2x2 binned) aligned and stacked, then given a stretch by Images Plus. The scope was not even roughly polar aligned and was unguided. I was also shooting through overhead power lines. Yechh.

I think it's fairly flat, at least flat enough to make me happy. Some vignetting is present, but it's about the same as found when imaging without the FR. I will probably tinker with the spacing in the future.

What's nice is that this setup gives a much more solid connection between the scope and camera. I look forward to using it SHOULD THE FREAKING SKY EVER CLEAR. Really, no, I'm okay, but thanks for asking.