Recently Browsing 0 members
No registered users viewing this page.
Q. When using Star Sense, why does my telescope go all around the houses on some of its GOTOs? I'm sure I set 'Cordwrap' to 'OFF'??!! A. Because Star Sense reset it to 'On' after you did your alignment.By StarGazingSiouxsie
Now, I know that Cubble likes to keep fit. I try to keep all of my telescopes healthy and feed them only the best planets and DSOs.
But - why is that Cubble just LOVES to go all around the houses on some of his GOTOs?! Talk about taking the long way home!!! I could have sworn that I had 'Cordwrap' set to 'Off'.............
I have set Cubs up with a totally cordfree / wire free system and therefore find this going 340 degrees around to an object that is 10 degrees away incredibly annoying and time wasting.
Well, I think I may have found the answer today. Some of you may already know this but for those that don't, I hope it helps relieve some of your frustration.
What I disovered is that the Star Sense hand controller resets the Cordwrap setting (MENU>>>TELESCOPE>>>CORDWRAP) back to 'ON' each time a new alignment is completed. So, one can spend time seemingly making sure that it is set to 'OFF' (if that's the setting you want, of course) only to find that once you have completed an alignment, the hand controller defaults the Cordwrap setting back to 'ON'.
So, if you don't want or need Cordwrap set to 'ON', make sure you change the setting after completing each new alignment.
Celestron's EdgeHD product line features an integrated field flattener. As with all flatteners, these produce optimal results when the imaging plane is a specific distance from the flattener. In the EdgeHD whitepaper Celestron describe the optimal backfocus of 133.35mm (5.25") for the 8" model, and 146.05 (5.75") for the 9.25", 11" and 14" models. They suggest that the image plane should be placed within 0.5mm of this distance.
When putting together an imaging train it can be quite hard to determine the actual backfocus. You could add together all the optical lengths quoted by manufacturers, you could get calipers and actually measure each part or even try to measure the entire thing (although it can be quite hard to figure out where to measure from. At some point, you have to trust some manufacturer spec (unless you fancy risking your sensor).
Once all this is done you might, however, find that things vary ever so slightly; everything from the tightness of threads to the T-ring not quite giving exactly 55mm. How do you work out if you've done it all correctly?
In a table in the whitepaper (page 13), focal lengths are given for each OTA (for example 2125mm for the 8" model). Hypothetically then, it should be possible to measure whether or not you're at optimal backfocus by plate solving for your image scale. In the same table, an image scale is given for a sensor with pixel size 6.4 micron but you can use a calculator (such at the astronomy.tools one) to work out the expected image scale for your particular sensor. This does require that your image is as close to perfect focus as possible.
Putting all this into practice. I used my calipers to try to get the image train as close to 133.35mm as I could and then plate solved some resulting data taken with a DSLR with 5 micron pixels. From my calculations, if I'm in focus at the correct spacing, I should have an image scale of 0.485"/pixel.
However, my astrometry.net solves gave an image scale of 0.495"/pixel. Working backwards, this indicates that I was at a focal length of 2083mm, quite a way inside 2125mm. Although I can't find a reference I've read that, for an SCT, the focal length changes by approximately 3mm for each 1mm of backfocus, this implies that my sensor is 14mm too close!
Now, I'm no expert with calipers but I feel like I couldn't have been more than a few mm out, and if anything I thought I was too far. I suppose I could have been a bit out of focus but surely not ~10mm.
Is there a mistake in my logic of aiming for 2125mm focal length?
For the first time in 57 years I'm having the itch to look up , I'm fed up with looking at people wearing masks, moaning there's no loo rolls and 2mtr distance markings on the floor, so I think me needs to take a deep breath and look through some quality glass and see the amazing views above the clouds. The problem I'm having, after watching many YouTube videos, is lots of different views on what your first scope should be, so, I need your help please.
My budget started at around £500 but I'm starting to think I need around £700 to get something I'm going to be pleased with, unless I come across a used one of course. I'm pretty sure it's going to be mostly visual star gazing I'll be doing and then perhaps as my interest and curiosity grows, I'll have a dabble into astrophotography, although this side of it does look very complicated, not to mention much more expensive. I have noticed in many videos that there is a push for newbies to get a reflector scope but they do seem a bit too big and cumbersome, and storage is an issue where I live, so I'm thinking more of a refractor.
There's a few nice 80 ed scopes out there but as I understand it, the moon will look nice but I'll have a hard job making out the planets like Saturn at maximum power. 102's and 120 seems a good starter but the jump in cost to an ED version is BIG, so I'd like some help on whether or not it justifies the extra money please to remove much of the Chromatic Aboration that many complain about of the cheaper scopes. I've also noticed that some scopes out there, although different in colour and name, the build is exactly the same and it's said by some, but it's pretty obvious it is, that they are just re-branded and come from the same factory in Shenzen. So, can you help/advise please, is a 102 or a 120 a good choice, should I pay the extra for ED, and which brands should I consider, i.e. Skywatcher, Celestron etc. Also, any advise on what extras I will need, I can see most advise on upgrading the 10mm eye piece that comes with most scopes for a better one, and a decent 2x Barlow, but anything else I'll you'll think I will need please and any tips and tricks for a very keen, getting on a bit, newbie, thank you very much.
Just so you know, I've made my first purchase, it's a planisphere and I'm learning how to use it from a nice chap I've found on YouTube here: https://www.youtube.com/watch?v=Jbz3QnAbzFM&t=962s&ab_channel=AstronomyandNatureTV
Take care, stay safe and clear skies as they say