Closed rzellem closed 3 years ago
pearsonkyle
commented
4 years ago Adding some references for later when I can test this with hardware
Mount TCS: https://www.celestron.com/products/nexstar-6se-computerized-telescope
https://s3.amazonaws.com/celestron-site-support-files/support_files/1154108406_nexstarcommprot.pdf
rzellem
commented
4 years ago From Leon on Slack #instructions:
The Celestron SE series is really designed at providing a computerized observing experience for visual observers who are willing to spend for convenience, but at an affordable price. It can technically be used for astrophotography however there are some important things to consider.
1. The tracking. From experience, the 6SE/8SE mount is designed to keep the object in frame so you can look at it. Keeping it precisely on a single point isn't really what it was designed for, and you will inevitably notice trailing with slightly longer exposures. Even if you take planetary images, for which you usually attach a barlow ( = >3m FL), you will notice that the planetary target drifts out of frame and manual correction with the hand controller is required. This issue is less severe with the 6SE over the 8SE due to:
2. The weight capacity. The mount, according to Celestron's website, is capable of holding 5.44kg (12lbs). The 6SE optical tube fortunately only weighs in at 3.6kg, so accessories like a DSLR or camera with filters can safely be attached, also if required a dew shield. However the 8SE weighs in at 12lbs. This means that if you intend to attach a heavier accessory such as a camera which shifts the weight balance of the telescope, you will lose performance. This is extremely noticeable with a DSLR or dew shield. This can be fine for visual observing, however the severe loss in accuracy makes it unsuitable for photometry. The 6SE is more affordable, and while it cannot see targets as faint as the 8SE, it is much more accurate. Better choice for photometry.
3. The f-stop. The SCT design, at least without an additional reducer, is f/10 or higher. This unfortunately limits it to the brightest targets, as even if the optical tube is theoretically capable of capturing good data on fainter ones with long(er) exposures, the mount won't support it. First due to the non-astrophotography grade tracking, and second due to inevitable field rotation.
4. The single-arm mount. Having used the 8SE, it was rather flimsy. It held up the telescope up alright however the slightest vibration or even a very gentle tap at a tripod leg would significantly shake the image. It is again fine for visual observing, and I certainly do understand why Celestron did not include dual-forkarms for better stability (keeping the price low, no photography demand beyond planetary imaging in most cases), but nevertheless this is an issue. When on grass or stone, a small step would register as image shake even several meters away.
5. The edges. Celestron makes two SCT versions currently; one more expensive "EdgeHD" version aimed mainly at astrophotographers, and one "normal" version, which the SE series falls under. The normal version suffers from imperfect correction of the image. This means without an additional corrector installed the image will have strong vignetting anywhere but the center. It is somewhat acceptable around the center and extremely poor at the edges. I highly recommend the 0.63x SCT Reducer/Corrector from Starizona, it is far better than the Celestron one. This however is an additional costly expense.
On to the solutions.
To mitigate 1:
Only consider brighter targets so exposure times can be kept low. The OTA can in theory resolve fainter stars, yet the mount is not designed for this to be exploited with cameras.
Every few minutes, check if the mount requires a correction. The target star must be kept as centered as possible. I suggest if at all possible to use a remote software on a Laptop which is outside, so that the status can often be checked from inside.
Keep the weight low. No heavy additions onto the optical tube. Do not use a dew shield unless you really need to.
Use the 6SE. The 8SE is great for visual use if automation is valued, but due to it being at just over 99% of the mounts weight capacity, tracking inevitably suffers.
To mitigate 2: (edited)
4:46
Only thing you can realistically do is use the 6SE over the 8SE, or use it on a different mount. It can work if the equipment you add is light enough, although I would strongly not recommend it. Follow advice for 1# as well.
Also make sure that the mount is very well balanced, even if this takes a few minutes.
To mitigate 3:
Consider the use of a focal reducer, if the budget allows it. Otherwise keep exposure times short, and prioritize brighter targets.
De-rotate the images in post using software. Stars must be near the center to make this viable, though this still doesn't allow for longer exposures.
To mitigate 4:
Any type of vibration suppression pad is nearly a must, though not necessarily the ones by Celestron. Quality homemade ones will likely work
Keep the mount isolated and away from any moving surfaces or objects (including persons).
Make sure the tripod is not extended at all.
Make sure all cables are taped or tied to the tube, and everything is tight and secure.
To mitigate 5:
The target star and any comparison stars must be kept as close to the center as possible. Follow advice for 1# as well, and check regularly. It is best to place a cross-hair overlay on any capture software being used
If budget allows it, seriously consider a focal reducer, or an EdgeHD telescope.
General advice:
Always use the 3-star align. Before finishing alignment, make sure that you center the star using the RIGHT and UP arrows to avoid backlash.
Make sure the tripod is level. Use a water scale preferably on the accessory tray.
If there is a way, try to platesolve. It will make the process much easier, especially considering that the pointing accuracy isn't that great in the first case.
Use an APS-C sensor (or slightly larger) - anything smaller will make alignment, keeping the target centered, and go-to an extreme difficulty, borderline not usable at all, though theoretically possible.
Do not buy the wedge. It has little use beyond keeping planetary timelapses from field rotation. The mount accuracy really isn't that great, and instead of purchasing the wedge, it's better to switch the mount or telescope altogether. You will not get field rotation sure, but you are still extremely limited by the forkarm which you attach the wedge to. The wedge does make sense for the dual forkarms mounts, though I imagine a CEM25/HEQ/EQ6-R will still put it to shame.
If you do not use a dew shield, be aware that stray light may easily enter the optical tube. Make sure your surroundings are dark
Consider off-axis guiding. This is not all that expensive to add onto the 6SE, and is a massive benefit for longer exposure times. The mount will still be limited due to field rotation unfortunately. Though the off-axis guiding can continuously be used should there be a mount switch.
Perhaps consider not using the SE series if you intend to do photometry primarily, with visual observing not being the main goal. A smaller setup at the same price will be significantly easier to use and provide much more accurate results. Really isn't what it was designed for in the end of the day, unless you move the optical tube assembly onto another mount and correct+reduce it.```Might be mitigated by off-axis guider that is on-order.
It looks that the tracking of your Celestrons is not perfect. We should explore on-chip guiding, where any changes in the location of a bright field star (or the target itself) are compensated by sending movement commands to the telescope mount. Could be part of the real-time data reduction code.