Help! I can't get sharp astro-photos in my refractor

[Annamalai]

I have a Skywatcher 80mm Shorttube refractor and Microsoft lifecam HD 3000 webcam.I adapted my webcam to refractor for astrophotography with 90 and 45 degree diagonals and also without diagonals.I can't get sharp image,i tried focusing the camera about an hour,finally,no results.But the setup i made worked well for my 76mm reflector telescope,i can get a perfect image.Please help me in taking some photos

[ronin]

The SW 80mm is an achro so will give a fair degree of CA, as that is colours at different focal lengths it translates as an image that will not be exactly sharp - there is no sharply defind position for the image. I do not know what depth of focus is defined for that scope, something manufactuters do not supply, but I would expect a reasonable depth and that translates as a non-sharp image at a defined plane.

You will also be getting SA where even the same wavelengths are not at the same focal plane = fuzzy image.

The focuser is basic, not a 2 speed so getting the best position is difficult.

This scope may be worse then average or the previous may have been better then average.for the particular model.

Ultimately a short tube f/5 achro is not the best for imaging.

Has the webcam had the IR filter removed? If so replace it. IR will be seen by the sensor and focused to a different plane then visible by the scope, so again more lack of sharp focus.

[Annamalai]

The SW 80mm is an achro so will give a fair degree of CA, as that is colours at different focal lengths it translates as an image that will not be exactly sharp - there is no sharply defind position for the image. I do not know what depth of focus is defined for that scope, something manufactuters do not supply, but I would expect a reasonable depth and that translates as a non-sharp image at a defined plane.

You will also be getting SA where even the same wavelengths are not at the same focal plane = fuzzy image.

The focuser is basic, not a 2 speed so getting the best position is difficult.

This scope may be worse then average or the previous may have been better then average.for the particular model.

Ultimately a short tube f/5 achro is not the best for imaging.

Has the webcam had the IR filter removed? If so replace it. IR will be seen by the sensor and focused to a different plane then visible by the scope, so again more lack of sharp focus.

Yes I have removed the IR filter.But,If i replace the IR filter it will change from prime focus to afocus,which means i can use my iphone for taking photos.Why should i use webcam??

[Jannis]

I think you missunderstand.

When you remove the webcam lens, wich most likely contain the IR filter inside, you need to replace the IR filter itself.

He don't mean to put the webcam lens back on, but to put for example a 1.25" IR-block filter between the webcam and the scope.

Without an IR filter you risk that the picture will be very unclear, as IR light that passes through the optics reach focus at a different point.

This means that while you'll have the "visable" light in focus, the IR is still out of focus. The IR light is "visable" for the camera, so it will therefor blur your picture.

By putting an IR-block filter in front of the sensor, the IR light will not pass and blur out the picture - and create a much sharper final picture.

Edit: As for reflector vs refractor, i don't know exactly why one turned out good and the other not. But a refractor will give you more CA, wich alone can ruin a whole picture if you ask me.

It could also just be good and bad seeing conditions causing it. Do you have any sample pics to upload? A bit easier to see what might be the cause if we can see some examples.

Use a focus mask, like a batinov mask to focus. This will get you in perfect focus even if it's had to determin with the naked eye.

[Annamalai]

I think you missunderstand.

When you remove the webcam lens, wich most likely contain the IR filter inside, you need to replace the IR filter itself.

He don't mean to put the webcam lens back on, but to put for example a 1.25" IR-block filter between the webcam and the scope.

Without an IR filter you risk that the picture will be very unclear, as IR light that passes through the optics reach focus at a different point.

This means that while you'll have the "visable" light in focus, the IR is still out of focus. The IR light is "visable" for the camera, so it will therefor blur your picture.

By putting an IR-block filter in front of the sensor, the IR light will not pass and blur out the picture - and create a much sharper final picture.

Edit: As for reflector vs refractor, i don't know exactly why one turned out good and the other not. But a refractor will give you more CA, wich alone can ruin a whole picture if you ask me.

It could also just be good and bad seeing conditions causing it. Do you have any sample pics to upload? A bit easier to see what might be the cause if we can see some examples.

Use a focus mask, like a batinov mask to focus. This will get you in perfect focus even if it's had to determin with the naked eye.

Yes,I have.The first one is taken with ST-80 and 2nd one is with 76mm reflector

[Jannis]

Wow, that's a HUGE difference. I dubt that much difference can be caused by the IR filter alone. Almost looks like the ST-80 was having dew on the glass?

[Annamalai]

I have checked that too,it is.perfectly alright.Also I don't have an IR filter for camera,I have only the camera lens.

[astrokoma]

Try a bahtinov mask to fine focus, adjust the right exposure, and study photoshop....i agree that short achromatic refractors is not the best instrument for astroimagine....

[Jannis]

Could it possebly be dew on the sensor or diagonal mirror? On a reflector the sensor/diagonal is exposed and will get good ventilation, but on a refractor it's sealed.

It could be it's completly none-dew related though.

Have you tried it in the daytime?

[Annamalai]

Could it possebly be dew on the sensor or diagonal mirror? On a reflector the sensor/diagonal is exposed and will get good ventilation, but on a refractor it's sealed.

It could be it's completly none-dew related though.

Have you tried it in the daytime?

I have tried imaging in day time time too but,the result was same.I pointed my scope to a Mobile network tower approximately 800m away,it was too blurry.It was not because of atmospheric distortion because I got a good clear image in my eyepiece.

[Jannis]

Have you try without the diagonal?

[Annamalai]

Have you try without the diagonal?

No,because the focuser is not long enough to focus the image.But,I have tried with a 2x barlow which reduces focuser's focal length,then too i got blurry image

[Annamalai]

Have you try without the diagonal?

Also Light gathering power in refractor is comparatively less than my reflector.In my old scope(reflector) for full moon i use -11 exposure value and in refractor i use -6 exposure level.

[Jannis]

It's not uncommon that a refractor in geneal have a lot less light gathering as it usually have a lot less aperture then a reflector compared to the focal lenght. You'll see this by the F-ratio of the scope. But that shuoldn't affect the final picture in this way though.

Thechnically, if you look through the EP and it's clear and sharp, then replace the EP with the camera, and it's unclear, and you know it's nothing wrong with teh camera - then it's more or less put down to UV/IR filter missing, as that's the only extra thing the camera pics up that your eye does not. As far as i understand it at least.

[Annamalai]

It's not uncommon that a refractor in geneal have a lot less light gathering as it usually have a lot less aperture then a reflector compared to the focal lenght. You'll see this by the F-ratio of the scope. But that shuoldn't affect the final picture in this way though.

Thechnically, if you look through the EP and it's clear and sharp, then replace the EP with the camera, and it's unclear, and you know it's nothing wrong with teh camera - then it's more or less put down to UV/IR filter missing, as that's the only extra thing the camera pics up that your eye does not. As far as i understand it at least.

Where do i get IR/UV Filter? Is there any software for focus correction?

[Jannis]

You can't correct focus properly with a software, it's all in the hardware. You might correct the focus to some degree in software, but it will never be the same as if you got the optics in focus. You can get software to help you reach exact focus though.

I personally just use a batinov focus mask, and it works perfectly. Always spot on focus in just a few sec.

UV/IR-block filter is available anywhere you can buy astro gear. FLO for example, the sponsor of this forum:

http://www.firstlightoptics.com/uv-ir-filters/baader-uvir-cut-filter.html

QHY also have a cheap IR-block filter, but i don't know how it performs:

http://www.modernastronomy.com/filtersOther.htm