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Why why why???


philhilo

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So why did I have to pursue something so dang hard? I have got some reasonable data in the past and slowly introduced new things, usually with a struggle. However for tonight I thought I would introduce simple things, a mains power lead into my Canon 700D and a coma corrector into the system. So after a huge struggle to get a 1 star align (I have a telrad waiting to be installed) the camera just messed about and I eventually put the battery back in. When I finally got aligned and got Capella dead centre I thought I am close to giving up, the clouds are rolling in, lets at least get some images with the coma corrector and …… everything is out of focus despite the bahtinov/APT/live view suggesting everything is sharp. Recheck with bahtinov, sharp, reimage and all out of focus (or so it looks)....gave up. Second night of frustration and these possible nights are few and far between. Single__0002_ISO800_30s__20C.CR2Single__0002_ISO800_30s__20C.CR2

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Yes, put up a JPEG. It will do fine for our purposes here.

The smaller stars look miles out of focus and seem to show the shadow of the secondary. Could it be the distance between the chip and the CC which is causing this? A specific distance will have to be respected here and will be stated by the CC maker. This has to be done before any attempt is made to focus.

Olly

Edited by ollypenrice
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Thanks folks for the responses to my rant last night. I finally have a chance to reply. Finally got a CR2 file converted into a JPEG using REA and it has done something horrible to the colour (its gone orange), brightness (bright) and even the sharpness (loss of definition), however I have attached it.

What I did was put the SW coma corrector for f5 Newtonians into the system between the SW 200PDS and my Canon 700D. I focused with this in place using a Bahtinov mask and APT. The APT tool told me focus was close and looked as good as any other time I have used it and got sharp images. I took a test frame of 30 seconds and got donuts. I checked the focus again with the mask and APT and it was still good. I took another test image and got the same result, donuts (at that point after 4 hours of battle I surrendered, however there might be another chance tomorrow evening so keen to resolve this). So baffled as to why it was in focus but produced out of focus images when I changed nothing and nothing moved. 

Single__0003_ISO800_30s__31C.jpg

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The cc to sensor distance should be 55 mm and is achieved by the t-ring that attaches it to the dslr. For most dslrs, the sensor to flange distance is 45 mm, and the thickness of the t-ring is 10 mm, giving the correct distance. This distance is not as critical as eg a field flattener's. Unless you have more rings/filters in your imaging train, you should be fine.

The colour cast may be due to light pollution, exaggerated by the image conversion software. My favourite raw conversion software is Rawtherapee, which is free and available for windows, mac and linux.

On sw telescopes with the standard focuser, locking focus can lead to focus shift. One way around this is to use a motorised focuser, where the holding torque of the motor will keep the draw tube in place.

Hope this helps. 

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You got similar donuts centrally and in the corners so your field appears flat so the distance between camera and CC is probably correct. So most likely your focus was either off even if you though you nailed it or the focuser slipped after focusing. I agree with Alan that the best way to focus a DSLR is to use live view at the highest ISO and 10x zoom on the creen. Focus on the smallest stars (the ones you only see when you have perfect focus). Just do not forget (as I sometimes did) to turn down the ISO before you start exposing.

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I used to use a SW 150pds and had a few exactly the same donuts without the use of a coma corrector. Later I found out the reason is because the shift of focus, mainly due to insufficient cooling. 

I used a Bahtinov for helping focus as well. I can see from the sub frames that the focus of the first image is spot on, and the second one shows slightly more blurred stars, the 3rd, the 4th I can see the donut started to grow larger and larger. 

After this experience I made sure I left the OTA outside for at least 20min before focusing with Bahtinov and this never happened to me again. Also make sure the focuser is locked in place and not slipping.

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Can you take an image with your Bahtinov in place and in focus and post it here?  A high ISO and sort duration shot will be fine.

(If you enable the R+L in the Qlty box in APT you won't have to convert your raw files to jpg).

I use the SW coma corrector with its associated 48mm adapter (takes all the guesswork out of getting the correct distance to sensor) on a Canon 700d (admittedly on a SW200P rather than a PDS) and find  the Bahtinov works OK so this should be fixable.

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  • 3 weeks later...

Thanks for the further replies, sorry I hadn't picked these up. More great reading and more things for my checklist. I see some folks responded to my most recent image so can see the focus issue seems to have gone - however I am now better prepared if it shows up again. Cheers!

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