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Hello everyone, I'm wondering if I could get some help, sort of ripping my hair out a little.
I have the EXOS2 PMC-8 and last night I used Sharpcap to Polar align, I set the mount up as usual and adjusted until Sharpcap said excellent Polar alignment.
So my next step was to do a 3 star alignment I was using a program called Explorer Stars to achieve this, so when I asked the mount to slew to the first star it was way off even in the finder scope, is that normal? So I used the controls to get the star in my FOV and centred it, it was the same for the next two stars way off. so I did the same.
I finished the 3 star alignment and when I asked the mount to find an object the object was not centred it was bottom right of the quick photo I took.
I've made sure all the information is correct by setting my Longitude and Latitude and also made sure my time is correct, the mount is balanced and level and pointing North. I think I'm doing something wrong somewhere.
Also may I ask should my counterweight bar be parallel with the mounts front leg? I have two little arrows on the mount that should be pointing at each other when in the home position but when they are the counterweight bar is pointing to the right quite a fair bit.
I'm new to all this only been doing it for a few months so a lot of things are quite confusing.
Thanks in advance Bill.
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?
I don't have a goto mount and I wondered if there was just a simple way of showing where the scope is pointing on (for example) stellarium or kstars.
I try and star hop to where I want to go and in the depths of the garden (or remote) I don't have any wifi to run nova.astrometry. so Im looking for a platesolving app that can run standalone on a windows laptop.
Usually I get to where I want to see, but sometimes I just get lost so Im looking for something that will help confirm or show where I have ended up so that I can manually adjust the scope to get where I want.
A typical use case would be....
Locate bright star near target.
Check collimation of camera, finder, telephoto (with dslr or gpcam + startcap)
Move/starhop to target.
… check whats supposed to be in the finder
… mhh its not there.
-- reverse steps and try again.
… mhh its still not there
check Im at the right start, double check target position and and try again
… mmmh still not there
Then what I want to do is
* take a photo and platesolve to find out where Im actually looking at this time
* correct manually using slow mo or some more star hopping
* hopefully get to the target, and enjoy the view.
Ive tried loading up 'all sky platesolver' which I can get to solve and give me location RA and DEC, but then I cant see how to get that location just to show on a star map.
Also I cant get any images to show on the screen from allskyplatesolver (although it says it should - image get not-found type error)
I don't really care what star map I use. I don't really want to start copying and pasting ra and dec into scripts at 3am.
The details of the platesolving apps seem to be mainly focussed on driving a mount to a location, whereas I just want to confirm where I've ended up pointing on a map so I can do the correction myself.
Any help please ?
PS - I might be trying to do this at 3am in a grassy field with the laptop balanced on an upturned crate whilst doing this ?
I am starting to get to grips with some of the features on Sharpcap and last time out I tried to use the focussing assistant using a bahtinov mask.
The first thing I noticed is how sensitive it is. The attached screenprint involved the tiniest movements of the fine focusser knob on my Star71, which caused it to sweep from +1 to -1 without stopping in the middle. I decided I wasn't going to do any better than that (but could probably waste an awful lot of clear-sky-time trying!), so decided to accept that. After all, from what I have read, an FWHM of about 1 is reasonable.
But now comes my confusion. I was assuming that the scale was FWHM units. But, as can be seen from the screenprint of the log, the FWHM of each individual sub was just under 4. I realise it would not necessarily be the exact same figure as when I was focussing, but 4x that figure? This would seem to suggest that either the scale on the focussing assistant is not FWHM units (so what is it?), or my focussing went way out during my imaging session. The resultant image can be seen here.
So, are these the sort of results I should be expecting? Or where am I going wrong?