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Funny thing - I have an ASI1600 + EFW and am connecting to SW coma corrector which requires 55mm spacing. The camera is 6.5mm the EFW is 20mm, total 26.5, so 28.5mm spacer required - sound correct hopefully?
But the suppllied spacers are 21mm and 16.5mm which doesnt help (and doesnt seem correct even for a colour ASI1600 with no EFW) There is also a 11mm female/female adapter and a very short 1.5mm male/male adapter, so I guess I could use the 11mm F/F followed by the 1.5mm M/M + the 16.5 giving 29mm but this seems a bit complicated!
Just wondering what other folk do - I cant be the only one with this problem. Any thoughts please?
Over the years I have done a lot of Excel calculation to solve my astronomy problem when I try to make parts work together. Most of them has been my private and only in Swedish, some of them I have published on a Swedish forum and I believe some people have found them useful.
Now I decided to translate them into English and put them on my homepage for downloading.
It's always hard to understand Excel sheets that others have put together, but I hope that these ones with a little help of the instructions on my homepage could be interesting and useful to some of you.
Beware that it could be something wrong in the calculations, if I find something I try to correct it.
I don't take any responsible of it, you use them at your own risk !
650mm fl /130mm aperture reflector scopes are great for imaging with the fast focal ratio of 5, but they do suffer from coma aberration due to this.
My scope is no exception and seems to have especially bad coma. As coma should reduce with increasing focal ratio, as an experiment I cut a 108mm circlular hole in a piece of card and placed it centrally across the front of the OTA, effectively changing the focal ratio to 6 with the reduction of the aperture from 130 to 108mm.
Below are two single subs : one without the card(f5 at 120secs) and one with the card (f6 at 180secs). I was surprised by the great reduction in coma in the f6 image. (Almost full Moon so rather bright subs.)
Of course the reduction in aperture means an approximate 50% increase in exposure time, but it could be useful for brighter targets.
I have just gotten set up for the first time with astrophotography and have taken my first couple images. I have an Orion 8" Newtonian Astrograph F3.9. To the best of my knowledge I am collimated extremely well, I am still learning everything but I feel like my collimation is right on.
The problem I am having is with the coma I am getting in my images. I am using a HighPoint Scientific coma corrector but it doesn't seem to be doing much, I wanted to take some photos without the coma corrector to see how it would compare but I don't have the proper adapters to attach my camera and get it to focus without the coma corrector.
My question is, is my coma corrector insufficient and should I get a new one? My HighPoint Scientific coma corrector costs about $120, the description says that it is for newtonians with a focal range of F3 to F6 but optimized for F4.5. My telescope is definitely toward the lower end of that range and below what it is optimized for.
I have attached a slightly edited image to give you an example of the issues I am having for your thoughts. You can see around the edges, and even when zoomed in on the stars in the center, they exhibit pretty bad coma.
Just curious on what your thoughts might be.
I am looking into getting a coma corrector at some point. It might be after I get a new OTA, as that one will likely have more coma, and that coma isn't the biggest of issues right now. BUT, I've been surfing around, trying to figure out how coma correctors actually work and what other effects they might have, and it has been a bit confusing to me, especially concerning focus.
I've read several places that some CC's will either require in-focus or even out-focus, even out-focus in such a case where prime-focus, which wasn't possible before, actually can become possible. I'll quote what I read, but I'd like people to give some comments on how that would work out:
"I've found that the GSO requires about 10mm of focuser in-travel when using the supplied eyepiece holder and an additional 25mm of spacing between that holder and the optics unit. For every additional 4mm of spacing, you save about 1mm of in-travel required. So I guess if you put 110mm of spacing in there, you wouldn't change the focus point at all. Of course, the correction would be awful.
If you screw the same 70-75mm of spacing onto the back of your 2"-1.25" adapter, you'll need about 1.5 inches of out-travel to reach focus. Since my focuser won't go that far, I just pull the whole assembly up and out of the focuser about and inch or so. Thus, you'll always be able to reach focus with 1.25" eyepieces and possibly even with cameras setup for prime focus photography via the T-mount adapter that wouldn't be able to reach focus otherwise." Louis D -
This was interesting to me, as I am actually not able to achieve prime-focus as of right now with my current OTA (Celestron 130SLT), and was wondering if acquiring a CC now instead of later, would be able to fix this for me? And if so, does the focal-length attributes of the CC determine this? Like some CC's acting as a 0.9x reducer or a 1.15x. And what CC's would you recommend that I eventually get? My current plan for the new OTA, is a Skywatcher 8'' Quattro CF, so I'll want a CC for it eventually, but if the CC can also "fix" focus issues with my current OTA, that would kill two birds with one stone as I currently have to use a barlow to focus my Nikon D5200.
So any insight into this or general advice would be very welcome!
-Mathias M. M.