In this article Matt Harmston shows that, in theory, establishing ideal backfocus is straightforward: Insert spacers and accessories between the two surfaces such that their collective thicknesses match the manufacturer specified back-focus distance. However, as he details, in practice the fine-tuning of back-focus distance is a precision exercise.
A few months back, I mounted a Celestron C8 SCT side-by-side with a Celestron RASA 8 in my SkyShed POD. The goal was to complement wide-field RASA views with larger image scales coming from the C8. That said, I did not want to run the C8 at its native focal ratio of f/10. Rather, I wanted to “speed up” the focal ratio with a 0.63x focal reducer. Inclusion of a quality focal reducer/corrector would produce a wider field of view, smaller stars, and achieve comparable field illumination in less than half the time compared to the telescope’s native f/10.
Having had good experiences using their products in the past, I purchased Starizona’s SCT Corrector IV – 0.63X Reducer/Coma Corrector (see Image 1). This accessory is specifically designed to reduce the focal ratio by a factor of 0.63x and correct coma inherent in Schmidt-Cassegrain telescopes.
Image 2 is representative of targets I seek to capture with the C8. In this image, I have captured the hourglass structure in Messier 8’s core backdropped by billowing clouds of nebulosity. Though the reducer/corrector is threaded to accept 2” filters, this processed image represents an unfiltered live stack of 27 frames using my MallinCam DS26cTEC.
What is Back-Focus?
As with any reducer/corrector, achieving the manufacturer-specified target back-focus distance (here, 90.3mm) is important for successful use of the Starizona reducer/corrector. With my setup, optical back-focus represents the distance between the reducer/corrector’s rear-most optical component and the sensor of the camera. Should someone wish to determine back-focus distance with a digital caliper, measurement should be captured relative to the reducer/corrector’s camera-side thread base.
In theory, establishing ideal back-focus is straightforward: Insert spacers and accessories between the two surfaces such that their collective thicknesses match the manufacturer-specified back-focus distance. Coarse distances can be measured using a digital caliper or by relying upon advertised spacer/accessory measurements.
In each case, final fine-tuning of back-focus distance is a precision exercise. Regardless of initial steps, surprises may manifest that render finding the right distance a more complex, iterative process than anticipated. Or, in corporate-speak, we can experience much-maligned scope creep.
The Back-Focus Journey Begins
Direction from the primers is most readily applied when other image quality impactors have been accounted for. Cue the ominous background music. For sake of experimentation, I decided to systematically approach Starizona’s target 90.3mm back-focus starting from a distance shorter than ideal as determined by adding vendor-stated spacer and accessory thicknesses.
Instead of seeing the expected artifact of elongated stars radiating from center of frame, my frames contained entire fields of double stars all oriented in the same direction (see composite Image 3). Each frame in Image 3, heavily cropped to illustrate the double star artifact, has its approximate back-focus distance indicated at bottom-left. This artifact was impacted by back-focus distance. However, determination of the artifact’s cause and mitigation required quite a bit of sleuthing.
There are multiple potential causes of this double-star artifact given my side-by-side mounting. However, communications with Starizona confirmed that priority should be given to identifying tracking issues. Tracking could interact with improper back-focus to create the observed artifact. Upon review of guide logs, a subtle, repetitive tracking anomaly revealed itself. After this review, Losmandy advised me to ship the G11G mount to California for warranty adjustment. True to form, they really came through and eliminated the tracking anomaly.
While my mount was on holiday in California, I took delivery of a variable spacer enabling fine adjustment of optical back-focus. As well, I added a motorized focuser for fine, remote focus control of the C8. A third key enhancement was application of a new tool for dialing in collimation. The view of Messier 8 in Image 4 represents a single, unprocessed frame utilizing the adjustable spacer, motorized focuser, improved collimation, and newly adjusted mount.
Though artifacts still exist, note the substantial improvement in star quality compared to the cropped frames in Image 3. Regardless, Image 4 has some peripheral regions with star elongation radiating from center (suggesting further tweaks in back-focus may be warranted) and others without the artifact.
Assuming that the C8 was well-focused and collimated, asymmetric distortion at field edge suggests the presence of camera sensor-tilt. As confirmed by Siril software, some aspect of spacers and accessories used in my C8 optical train resulted in the camera’s sensor not being perpendicular to the telescope’s optical axis.
A Related Side Story: Sensor Tilt
Though image processing techniques can reduce the impact of some artifacts, I prefer having my optical train function at its full potential prior to any image processing. Thus, more recent clear nights have seen me trying to solve the sensor-tilt issue prior to finalizing my back-focus adjustments.
Spoiler alert: I am still working on the sensor-tilt issue. Even when securing against internal stops, my first two eyepiece holders had a small amount of “slop” that permitted the camera to droop. A recent purchase of Baader’s 2″ SCT ClickLock Eyepiece Adapter/Visual Back has resulted in a centered, rock-solid grip on my imaging gear. Though the “slop” has been eliminated, some sensor-tilt remains.
Having eliminated the eyepiece holder as a source of tilt, I have experimented with different orientations of spacers and accessories. Currently, I have the fewest number of threaded spacer/accessory connections available with my gear, including removal of the adjustable spacer. After doing so, I achieved a caliper-measured back-focus of 90.8mm… just 0.5mm beyond the target. Though this last round of adjustment further improved the view, some tilt continues to impact my images. At this point, there may be little benefit to additional back-focus adjustments. However, I cannot be certain until I get a better handle on the sensor-tilt issue.
In the End
Regardless of ongoing adjustments, the entire observing system’s performance shows much potential. Being close to the journey’s end, I am confident regarding several points. First, the Starizona SCT Corrector IV – 0.63X Reducer/Coma Corrector, in conjunction with my Celestron C8, brings a big smile to my face. Second, establishing back-focus can be an iterative process requiring repeated confirmation and/or refinement of other observing system characteristics, including (but not necessarily limited to) tracking, collimation, and sensor-tilt. Finally, the scope creep inherent in this journey has been well worth the additional effort.
Matt Harmston is an educational researcher whose appetite for the heavens has been whetted by increasing aperture over the years. More recently, Matt has immersed himself in video astronomy – a means of probing deeper into the night sky while making astronomy accessible to all ages and abilities. With this technology readily available, Matt is considering a career as a sleep-deprivation research subject.
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