Focus problems

[gcraib]

I was viewing the sun today and saw a few sunspots but no prominences.

When I put the camera on (via 32mm EP) I found that it wasn't possible to get all of the sun in focus, just the area in the middle of the screen (live view).

This meant I could capture some sunspots but the rest of the sun was blurry.

I'll try and post up the photo's but have had problems in the past with that.

link from flickr, doesn't seem to work so I've attached the photo.

[Viv]

seems strange that - i've moved the thread to imaging, so you'll get some advice,

[Bizibilder]

If that is eyepiece projection then the most likely cause is the camera not being square to the telescope optics. If you have an "old" refractor then it may be caused by the focusser tube being a poor fit and the weight of the camera causing it to droop?

[JamesF]

I agree that the camera probably isn't perfectly square to the optical axis of the scope. But assuming you're using the 127 Mak here, I think you're trying to do it the hard way. Mine has had several owners I think so I'm not sure how original it is, but the visual back has a male T thread. If yours is the same then you can get a EOS to T thread adapter and put the camera straight on the back of the scope, using it like a 1500mm prime lens. With my 450D the camera has to go straight onto the visual back of the scope, otherwise the image is too large to fit on the sensor.

I did exactly this on Friday and used APT (Astrophotography Tool) to capture a sequence of images to stack as per Roger's (Bizibilder's) "howto" here:

http://stargazerslounge.com/topic/153712-simple-white-light-solar-imaging/

I'm quite pleased with the results:

http://stargazerslounge.com/topic/160715-first-ever-solar-white-light-image-1st-september/

I think that's a lot easier than trying to take an image through the eyepiece. Getting focus is the tricky part, but APT allows you to display the live view image on the screen and you can tweak the focus to get the best possible image before you start. If you don't have a computer to connect up to the camera to worth with then focusing using live view and taking images using a programmable remote is probably the best bet.

James

[gcraib]

Thats a nice image, what I saw yesterday but nothing like what I captured.

Yes I was using the mak, via a diagonal. perhaps there could be an alignment issue, I will try again without the diagonal.

Have tried prime focus before but the image was too big, so using the 32mm EP made it a nice size for the camera.

Did have focus problems before but I think it may just have been because it was difficult to find the right focus point due to the image shake.

The EP has a movable tube, perhaps this wasn't aligned properly.

I'll try again next time there is a clear sky and I have time.

Left the telescope all set up yesterday hoping for a clear sky last night (but alas NO!) so at least all I have to do is just carry it back out into the garden.

I took the laptop out and connected it up to the camera (first time I've done that) but couldn't do anything as the clouds had rolled in by then. At least I got a feel for using it with camera, although it was quite difficult to see much on screen as it was so bright. Even with the brightness at max it was still difficult to see, maybe I need to clean my screen too.

[Montana]

I have no idea about the focus but are you imaging in white light? if so, you will not be able to view prominences in this band only spots. Also, try using an UV/IR cut filter as sometimes if you are using a refractor which has poor colour correction then there can be an out of focus UV or IR image blurring your true image.

Regards

Alexandra

[gcraib]

Yes, it was imaged in white light.

The image above is taken through neodymium filter. I had one taken without filter but it was over exposed (didn't realise that at the time) so I then switched to the filter before realising the exposure was too severe. Didn't manage to get any without filter as the clouds had started to come in.

[JamesF]

I think you should be able to fit the image on the sensor at prime focus, but you can't have much between the scope and the camera. I had the T ring converter on the camera screwed straight onto the visual back on the scope and the image just fitted on. If I put even 20mm of extension between the camera and visual back then the image was too large. The 550D sensor is fractionally bigger than my 450D, but not by enough that you'd notice.

I'm debating setting up this afternoon as the sun is bright, but there's a lot of cloud in the sky. If I do so (or if the promised clear spell arrives this evening so I can take some moon shots) I'll take a picture of the arrangement I'm using so it's clear. I think generally the more you can remove from the optical train the better.

James

[Bizibilder]

If you find the image is too big for the sensor you can always take two (or more) overlapping images and use freeware like Microsoft "ICE" to make a mosaic.

[JamesF]

This is how I have my 450D connected to the Mak for solar imaging. I have no problem reaching focus with this configuration:

James

[GlassWalker]

The original problem looks like field curvature to me. With the sunspots in focus near enough in the middle of the sample, the image appears to defocus more as you go away from that point. So the closer upper edge is less defocused from the lower edge which is further away. This can be confirmed if you move the image around and look at the degree of blur.

No direct solution to that as far as I'm aware. looking it up, the scope is 1500mm focal length, which is a bit long to fit it in one go. I have a 1325mm mak used at prime focus, and depending on how close the moon is that does or doesn't fit in one. And the sun might be fractionally bigger in apparent size than the moon. A mild focal reducer might be a way around this if you want to get it in one shot, or like others have said, you can mosaic it in two parts.

[jezhughes]

I can't advise on the focus problem but as JamesF said, you should stay away from using the angle. its an unnecessary extra surface and can cause problems, perhaps like what you're having.

direct on to the Mak is the best way and is how i do all my solar/lunar stuff

[JamesF]

I guess if you still want to do afocal there's no reason you can't try putting the eyepiece straight into the visual back without the diagonal. I suspect that if the focus problem is related to alignment though that the cause is the visual back not holding the diagonal/eyepiece square to the optical axis because of the amount of weight on it. Removing the diagonal but keeping the eyepiece may reduce the effect, but quite possibly won't remove it altogether.

James

[Zakalwe]

If you remove the diagonal, then you might need an extension tube to achieve focus (to make up the length that the diagonal took up).

You will get better results by mounting the camera, using a T-ring, directly to the scope, as per JamesF's picture. This will remove any "slop" in the imaging train which could be causing your issue. If the resulting image is too big to fit onto the chip, then take two or more images and mosaic them in Microsoft ICE or iMerge.

[JamesF]

If you remove the diagonal, then you might need an extension tube to achieve focus (to make up the length that the diagonal took up).

With the Mak and a 450D I don't. Perhaps the OP's 550D is slightly different though. The problem with adding extensions is that the image gets bigger the further from the back of the scope that you get. With 60mm of extension I can't even get close to getting all of the sun on the sensor. With 20mm it's close, but still too big. Right on the visual back it just fits, but I bet there's less than a couple of hundred pixels in it.

Anyone know if the 550D is unusually deep compared to the 450D in terms of the depth of the sensor from the lens ring?

James

[GlassWalker]

James, the distance between mount and sensor is the same on all EOS Canons. If not, lens compatibility would be a nightmare!

I also wonder if there's some confusion going on in this thread. The extension to get the right spot for prime focus and extension with afocal projection are different...

[JamesF]

James, the distance between mount and sensor is the same on all EOS Canons. If not, lens compatibility would be a nightmare!

That's an exceptionally good point

I also wonder if there's some confusion going on in this thread. The extension to get the right spot for prime focus and extension with afocal projection are different...

The thread has wandered a bit, partly because of me...

The OP was after sorting problems with focus for afocal imaging, but I suggested using prime focus instead and he replied that he'd tried that but the image was too large to fit on the sensor whereas with afocal it was fine. As I'm using a 450D which has near enough the same size sensor (fractions of a millimetre smaller) at prime focus on the same scope and can get the Sun onto the sensor with a bit to spare, I'm trying to work out why he wouldn't be able to do so. Going to prime focus would seem to be the easiest way to resolve problems with alignment of eyepiece and diagonal.

I'm a bit stuck for ideas now though. Unless there's something fundamentally different about his 127 Mak and mine (which is possible, I guess, since mine is fairly old).

James

[GlassWalker]

Only a guess, as I've not looked up the orbit of the Earth, but could this be due to when you try? I'm only going by the moon since that's my main experience with my smaller Mak, and at 1325mm focal length, depending on if the moon is closer or further it may or may not fit. The sun is in a similar ball park in angular size so similar may apply. The Skymax 127 seems to be 1500mm from what I've looked up, so it's going to be tight if it fits at all. Unless they made different configurations of that through the ages.

[JamesF]

The Mak's image scale at 1500m is just over two arcminutes per millimetre and the Sun's apparent size only varies by about 0.1 arcminute, or 0.05mm on the image plane. Based on my images I'd say that makes it quite tight when it's largest, but still achievable. The figure I calculate for the maximum size of the Sun on the image plane is just under 14.3mm -- about 0.5mm smaller than the height of the sensor. The moon at its very largest should be 14.9mm on the image plane which looks like it's going to be too much.

It's entirely possible I guess that they might have changed the design of the mirror carriage and the new design won't slide back quite so far on the baffle tube, but I'm guessing...

James

[gcraib]

Thanks for all the comments, I haven't been back out to try again.

I think it is down to the size of the object, field curvature as previosuly mentioned. Can this not be corrected with coma correctors or am i way off there?

What about collimation? I thought only reflectors needed this done but there are 3 collimating screws in the back of the mak.

have also read about IR-cut filters but as I'm using a baader neodymium, I thought this did IR-cut?

James, the differences in our setup may be down to how the camera is mounted. I'm using a canon camera adaptor threaded into a t-ring and inserted into scope tube.

http://www.firstlightoptics.com/adaptors/t-rings.html

http://www.firstlightoptics.com/adaptors/flo-2-inch-t-mount-camera-adapter.html

Yours looks slightly different, so maybe that affects size and focus? My t-ring may be too long?

I don't mind cutting off parts of the object (rather not though) or stitching, but this still needs uniform focus!!

[JamesF]

Ahh. The visual back on my Mak has a T-thread on the end, so I take it off by unscrewing the collar and screw it into the T-adapter directly, then screw the entire lot back onto the scope. I don't use any other adapter at all. Is yours different?

James

[sologuitarist61]

I get the same problem when I use cheap projection EP's such as the GSO (£50). The centre is in focus and the edges are blurred just like in you picture, yet when I use the Baader prime or zoom EP's on the same scope and with the same camera all is fine. So I would guess that either the EP isn't up to the job or that it is the collar that the DSLR attaches to just isn't holding the camera in line to the image train.

The strange thing is that if I use the scope terrestrially, then the image is in focus from corner to corner, so have just given up using it for solar. Thought that it was a cheap option but ended up wasting £50.

[gcraib]

Here is a picture I took of the moon, it has uniform focus so must be the EP.

2010-12-24 Skymax 127 Second Moon Shot by gmcraib, on Flickr

Its (sort of) prime focus, I had only just bought the DSLR so had no adaptors or t-rings, just a steady hand!!