The old subject of focus

[Pitbull]

Hi all,

Apologies if this has already been cobvered but I have been trawling the forum for the answer to my question and found a number of answers but none of them seem to answer my question.

I have a Skywatcher 130m scope and a Canon EOS 550D DSLR, I also have a Phillips SPC880 flashed. last night I finally managed to get out and get some viewing doen thanks to the clear skies but I did suffer focus issues when trying to photograph.

Reading the forum, I see some have ovecome this by using a Barlow and this certainly improved matters. My question is, how do you focus cameras using these cameras without using the Barlow? Does the camera need to come closer to the mirrors (by modifying the eyepice mount someway), is an extension tube needed or is it a case of having to use the Barlow each time?

Many thanks in anticipation, if it turns out I need the Barlow then I wil need to invest in a better one than that supplied with the scope.

[Cornelius Varley]

The alternative method of reaching focus with a DSLR on a Newtonian is to move the primary mirror up the tube. You might be able to do this simply by using longer mirror locking bolts and recollimating the mirror. A bit more drastic method is to shorten the telescope tube and drill new holes for the mirror cell.

Peter

[Pitbull]

hmmm thanks for that, both sound a little drastic to me.

[Marcus daniel]

You could try a 35mm extension tube, I use one of these to give a bit more in focal length when using my DSLR.

[Stephen]

It's in-focus which is the problem, there isn't an elegant solution other than moving the mirror up the tube or mutilating the focuser

The other option is to get some longer bolts (locking and collimation) and some springs and reassemble the mirror cell. The new springs have the effect of moving the mirror up the tube.

This is what I did (for other reasons, but got the same effect.)

http://stargazerslounge.com/equipment-help/148344-replacement-mirror-cell.html#post1880554

[Marcus daniel]

what a nightmare then!!!

[Pitbull]

Thanks all, going to have to have a think about this. A little reluctant to mess about with the mirrors but maybe it's what I will have to resort to doing.

Am I correct in my assumption that the cameras are too far away to focus and Ideally need to move closer than the eyepiece adjuster will allow?

[wcgucfa]

Hello,

This is my very first post. I feel the urge to warn Pitbull not to

try and modify his telescope in any way if he has never had any

experience doing such a thing.

As Marcus suggested, if you are having trouble reaching focus with a Newtonian scope, try using an extension tube, available at many outlets like Orion Telecopes, etc. for very affordable prices. This should take care of your problem.

I've experimented with many cameras and have had no trouble reaching focus with my Newts. However, I do remember just reaching focus before losing tube length with a DSC camera (Sony DSC F-707). I know this is an old camera, but I specialize in unconventional astrophotographic methods due to my small income.

At the moment I am experimenting with a Canon T1i afocal, with a 120mm refractor. I will try using it with my Newtonian to get a feel of what you're up against. In the meantime, please think extension tube rather than telescope appendectomy!

Bill

[Cornelius Varley]

Hello,

This is my very first post. I feel the urge to warn Pitbull not to

try and modify his telescope in any way if he has never had any

experience doing such a thing.

As Marcus suggested, if you are having trouble reaching focus with a Newtonian scope, try using an extension tube, available at many outlets like Orion Telecopes, etc. for very affordable prices. This should take care of your problem.

I've experimented with many cameras and have had no trouble reaching focus with my Newts. However, I do remember just reaching focus before losing tube length with a DSC camera (Sony DSC F-707). I know this is an old camera, but I specialize in unconventional astrophotographic methods due to my small income.

At the moment I am experimenting with a Canon T1i afocal, with a 120mm refractor. I will try using it with my Newtonian to get a feel of what you're up against. In the meantime, please think extension tube rather than telescope appendectomy!

Bill

Extension tubes don't solve the problem of insufficient inwards focuser travel. The problem is that you cannot get the camera close enough to the primary mirror to achieve focus. The two solutions are to use a barlow lens to move the point of focus further away from the secondary mirror or move the primary mirror up the tube sufficiently to allow the required focuser movement.

Peter

[JamesF]

Aren't there some low profile focusers specifically designed to help with lack of inwards focuser movement?

James

[Pitbull]

Thanks all, I see the debate rages and that is what confuses me. There are those who say the camera cannot reach focus because you cannot get it close enough to hit the focal point and others who say you need extension tubes.

Looks like I am going to have to get out there on a clear night and experiment.

[wcgucfa]

I don't see where Pitbull said he was having an "inward" focusing

problem. I do see where someone else said this.

Anyway, if it's an outward focusing problem, Pitbull, you need an extension tube.

If your problem is indeed inward I still recommend you not try to alter the scope yourself if you've never done this sort of thing.

It's not worth wrecking your scope.

My apologies if I misread your original post.

Bill

[Stephen]

Set the scope up and once you've pulled the focuser out as far as it will go, release and continue to move the DSLR away from the focuser tube. I'm reasonably certain you won't find any distance away from the tube will give you focus.

Let us know how you get on

PS - Check this thread out re: 130 and DSLR click here

[Cornelius Varley]

The problem with Newtonian reflrectors is that they are designed for visual use, not photography. Generally it is a lack of inwards focusew travel. This is because the sensor is too far away from the secondary mirror for it to reach focus. By adding a barlow lens the focal length of the telscope increases and the point of focus is moved further out. Take a look at this video on youtube for further explanation attaching a camera to a telescope. The Skywatcher DS range of reflectors cured this problem by shortening the telescope tube thereby moving the primary mirror closer to the secondary.

Peter

[wcgucfa]

Peter and all,

Thank you, this is a great video, and a great site!

The instructor deals with shooting prime focus or no eyepiece. Have we established if Pitbull is using his camera in this manner or is he using the afocal method or eyepiece projection?

I have been using an Astronomics 40mm EP in my scope with varying

lenses on the DSLR. The DSC camera has a zoom lens which cannot be removed.

This setup has resulted in my never experiencing inward focus problems and only a slight outward focus problem on my Newtonians (remedied by and extension tube). No problems at all with refractors.

So my next question to Pitbull is: prime focus, or do you have lenses mounted on your camera plus the scope?

If you are shooting prime focus, it seems like the instructional video

may solve your problem.

Again, Peter, thanks for the link!

Bill

[Pitbull]

Thanks guys, sorry to set off a debate. My original question may have been confusing as I was unsure myself which way I needed to go to achieve focus, thanks for you input wcgucfa, I am a little uncertain about adjusting my scope anyway although being an engineer, I could manage from what I have read.

Kh3ldar you are indeed correct, I need to get out there on the next clear night and determine which way my correction is. Thanks for the link, it explains a few things.

Cornelius Varley Again thanks for the link, a useful video adding further belief that my problem is indeed inward focusing, I did get some results when I used the Barlow the other night.

Have also just received my copy of Making Every Photon Count and am currently reading that.

Will let you know how I get on if we get a clear night soon thanks again all for your help and input.

[Stephen]

Peter and all,

Thank you, this is a great video, and a great site!

The instructor deals with shooting prime focus or no eyepiece. Have we established if Pitbull is using his camera in this manner or is he using the afocal method or eyepiece projection?

I have been using an Astronomics 40mm EP in my scope with varying

lenses on the DSLR. The DSC camera has a zoom lens which cannot be removed.

This setup has resulted in my never experiencing inward focus problems and only a slight outward focus problem on my Newtonians (remedied by and extension tube). No problems at all with refractors.

So my next question to Pitbull is: prime focus, or do you have lenses mounted on your camera plus the scope?

If you are shooting prime focus, it seems like the instructional video

may solve your problem.

Again, Peter, thanks for the link!

Bill

With respect your camera is not like the OPs (I already googled and saw it had a fixed lens.) I felt the question was clear as to what the OP was trying to do?

[wcgucfa]

Only my old DSC has a fixed lens. My DSLR which is a Canon T1i, is, well...a DSLR.

Sorry, what's an OP?

Bill

[Cornelius Varley]

OP = Original Poster

Peter

[Stephen]

Only my old DSC has a fixed lens. My DSLR which is a Canon T1i, is, well...a DSLR.

Sorry, what's an OP?

Bill

OP is a lazy way of referring to the person who started the thread

I'm intrigued now, what scope are you using with the T1i (that's the 550D isn't it?) that requires an extension tube (at prime?)

[Sam]

I had this same problem with my 8" reflector and canon 400D. There was not quite enough inwards travel on the focuser to bring the image into focus. Like others have suggested I got some slightly longer collimation screws for the mirror cell and moved the mirror up by about 2cm. This was enough to give inwards focus travel to bring the image into focus. It was very simple to do and didn't require any permanent modification to the telescope.

[wcgucfa]

Thank you gentlemen, for the clarification of "OP."

I've always had trouble with abreviations!

The Canon EOS Rebel T1i is the 15.1 MP CMOS DSLR that I am using to introduce myself to digital single lens reflex astroimaging. It is also known as the "D500."

I am still using my old Sony DSC F-707 to image with. I get very good results with it, though of course, the pictures are noisy.

"The Sony" as I refer to it, is very versitile, as I have imaged everything from the Great Wall of China to the transit of Venus across the sun with it.

My most recent (past 6 years interest) is capturing incoming comets. The Sony can reach 13.5 magnitude with a 30 second single frame exposure under "good" conditions.

In reference to the T1i, I haven't yet tried using it at prime focus. My usual method is to use a 40mm eyepiece attached to a 120mm refractor afocal with either a 50mm lens on the camera or the 18mm-55mm kit lens it came with. My most recent "experimental" image using the 18mm setting is of Comet Garradd passing M15 in the same approximately 1.5 degree field. The T1i has reached 14.5 limiting magnitude, which I am very pleased with.

Experiments continue! Note: The Sony with its non-removable zoom lens and the Canon, reach almost the same focus on the 4.7" refractor. I wouldn't be surprised if the T1i needs an extension at prime although I know the Sony doesn't.

Bill

[Pitbull]

Well, got out tonight and yes it looks like I can only focus with a Barlow so need to either move the mirror or invest in Barlows and focus shifters to get any decent photography.

I guess if I want to get any decent pictures i am going to have to look at getting a refreactor.

Very frustrating.