A coma corrector conundrum

[billyharris72]

Hi all:

Here's my problem. Some time ago I chopped 26mm off the focuser tube of my Skywatcher 130 P-DS to accommodate my Nikon DSLR better. Since then, I've acquired a ZWO ASI 1600 MM and filter wheel, only to find that, thank to my earlier DIY, it will not reach focus. It can reach focus if I rack the tube right out and then pull the coma corrector halfway out, pinching it at the end, but I don't want to be picking my nice new camera off the floor anytime soon.

While I could buy a new focuser, I i) now have no money, and ii) would be back to square one with the DSLR, which I'd like to keep as a one shot colour option if possible. To me, that suggests an extension tube. Problem is that the Skywatcher coma corrector won't accommodate anything between itself and the focuser as far as I can see.

So my question is, would it be okay to put an extension tube between the camera and the coma corrector, or will this cause problems? I reckon the current sensor distance at around 42.5mm and I'd like to add about 10mm to that. Is that going to make the image worse, or would that be okay?

Billy.

[Adam J]

47 minutes ago, billyharris72 said:

Hi all:

Here's my problem. Some time ago I chopped 26mm off the focuser tube of my Skywatcher 130 P-DS to accommodate my Nikon DSLR better. Since then, I've acquired a ZWO ASI 1600 MM and filter wheel, only to find that, thank to my earlier DIY, it will not reach focus. It can reach focus if I rack the tube right out and then pull the coma corrector halfway out, pinching it at the end, but I don't want to be picking my nice new camera off the floor anytime soon.

While I could buy a new focuser, I i) now have no money, and ii) would be back to square one with the DSLR, which I'd like to keep as a one shot colour option if possible. To me, that suggests an extension tube. Problem is that the Skywatcher coma corrector won't accommodate anything between itself and the focuser as far as I can see.

So my question is, would it be okay to put an extension tube between the camera and the coma corrector, or will this cause problems? I reckon the current sensor distance at around 42.5mm and I'd like to add about 10mm to that. Is that going to make the image worse, or would that be okay?

Billy.

2 inch draw tub extension might be a solution with the cc sliding into the extension.

The cc spacing is critical it will not tolerate being 10mm out of wack.

[Adam J]

something like this but make sure the barrel of the cc is less than 35mm if you use this exact one.

http://www.365astronomy.com/TS-2-Extension-Tube-with-35mm-Optical-Length.html

[wimvb]

You could try to get the same type of focuser you already have (2nd hand from someone who has upgraded from a SW focuser?), and use the drawtube of that for your ZWO, and the shortened tube for your dslr.

[billyharris72]

Looking at the FLO site ive just seen that the coma corrector needs 55mm back focus. Does that mean the distance from the corrector to the chip? Does that mean that it would actually benefit from an extension between the camera and the CC? I've remeasured and make it ~43mm at present.

[geordie85]

From camera sensor to coma corrector glass needs to be 55mm. 

Did you not get a 2 inch extension tube when you bought the coma corrector? 

1 hour ago, billyharris72 said:

Looking at the FLO site ive just seen that the coma corrector needs 55mm back focus. Does that mean the distance from the corrector to the chip? Does that mean that it would actually benefit from an extension between the camera and the CC? I've remeasured and make it ~43mm at present.

 

[Adam J]

1 hour ago, billyharris72 said:

Looking at the FLO site ive just seen that the coma corrector needs 55mm back focus. Does that mean the distance from the corrector to the chip? Does that mean that it would actually benefit from an extension between the camera and the CC? I've remeasured and make it ~43mm at present.

No 55mm is optimal if you make it longer or shorter than 55mm you will make the image worse in either direction the tolerance is about 2mm. The thickness of the t-ring is about 10mm so unless your using a low profile t-ring then you should end up with a 55mm ish spacing to the sensor itself (don't measure to the filter in front of the sensor). 

[billyharris72]

2 hours ago, Adam J said:

No 55mm is optimal if you make it longer or shorter than 55mm you will make the image worse in either direction the tolerance is about 2mm. The thickness of the t-ring is about 10mm so unless your using a low profile t-ring then you should end up with a 55mm ish spacing to the sensor itself (don't measure to the filter in front of the sensor). 

That's how I calculated it:

Sensor recess distance 6.5mm

Filter wheel 20mm

Optical length of t to M42 extension tube 16.5mm.

So I was thinking the setup as it currently is probably does not have enough back to us anyway. If that is the case and it needs a bit more that should also help with the focuser issue ( I hope).

 

[billyharris72]

Hi geordie85. I did get a 2 inch extension tube. Trouble is the chopped focuser only has about 20mm play, and the tube I have is too long. If i could find something about 20mm it would be perfect, but have not been able to find a drawtube extension that short (whereas there are plenty of M48 extenders in that range).

Billy.

[Adam J]

ah ok well in that case if you can increase the spacing so that you acheive the required spacing 

 

[pushrod]

On ‎23‎/‎06‎/‎2017 at 12:46, billyharris72 said:

Hi geordie85. I did get a 2 inch extension tube. Trouble is the chopped focuser only has about 20mm play, and the tube I have is too long. If i could find something about 20mm it would be perfect, but have not been able to find a drawtube extension that short (whereas there are plenty of M48 extenders in that range).

Billy.

Hi Billy, can i ask did you get your problem sorted?

I am just about to cut approx. 25mm of drawtube from  my  130pds as it protrudes a long way into the tube when my dslr is attached  and focused. However i might want to occasionally use it visually and it looks like my shorter fl  hyperion eyepieces will not focus without the full length of draw tube. I was hoping that to help this occasional use i might fit a 20mm Revelation 20mm (M48) 2" extension ring onto the coma corrector (after it) before the supplied extender - will this work?

I am not sure at all what the statement "the coma corrector requires 55mm back focus" actually means?

Is it that you will need an EXTRA 55mm of distance to achieve focus ? 

Or - that the distance between the coma corrector and eyepiece/ camera sensor always needs to be 55mm?

I am thinking it is the latter and that probably any extension ring needs to go be before the coma corrector. to preserve the CC to eyepiece distance. The big problem with this is that i can't find any extension tubes shorter than 35mm?

 

Any help much appreciated

[billyharris72]

Hi pushrod:

I did indeed get it fixed. The coma corrector needed a little more back focus (it should be 55mm from the camera sensor), so I put another extension tube between the corrector and the camera to take it to (around) 55mm, at which point I was back to being able to focus.

Mine is still just about usable for visual. Rather than the 1.25 inch fitting that came with the scope I use the 2' eyepiece fitting and 2 to 1.25 converter. That gives me just a tad extra length and allows me to attain focus. Not sure it works with all my eyepieces as I only use a 25mm plossl for star alignment, but if not it should be possible to find a suitable extension tube online.

Overall I did find the mod worthwhile, but you might want to consider the other approach, which is to take the tube out of the focuser and cut two wide 23mm deep "notches" at that depth, leaving a runner at each side (you can see where from the Crayford track marks). That way you would have the full focus range, but without the obstruction. That was my original plan, but thanks to the famous Skywatcher build quality (in this case a screw that would not engage and would need to have been drilled out) I was not able to remove the tube and opted for the "brute force and ignorance" option.

Hope this is some help.

Billy.

[pushrod]

1 hour ago, billyharris72 said:

Hi pushrod:

I did indeed get it fixed. The coma corrector needed a little more back focus (it should be 55mm from the camera sensor), so I put another extension tube between the corrector and the camera to take it to (around) 55mm, at which point I was back to being able to focus.

Mine is still just about usable for visual. Rather than the 1.25 inch fitting that came with the scope I use the 2' eyepiece fitting and 2 to 1.25 converter. That gives me just a tad extra length and allows me to attain focus. Not sure it works with all my eyepieces as I only use a 25mm plossl for star alignment, but if not it should be possible to find a suitable extension tube online.

Overall I did find the mod worthwhile, but you might want to consider the other approach, which is to take the tube out of the focuser and cut two wide 23mm deep "notches" at that depth, leaving a runner at each side (you can see where from the Crayford track marks). That way you would have the full focus range, but without the obstruction. That was my original plan, but thanks to the famous Skywatcher build quality (in this case a screw that would not engage and would need to have been drilled out) I was not able to remove the tube and opted for the "brute force and ignorance" option.

Hope this is some help.

Billy.

Thanks for the quick reply. Very interesting as well. I put the dslr straight onto the CC via the correct M48 canon adapter and use a bhatinov mask to check focus and have been very pleased with the focus and lack of coma in the corners of images ( only the smallest of amounts and only visible at very high mags). I think i may have misread your original post am i right in now thinking that your problems only started when you incorporated a filter wheel?

Yes i have seen the "leaving the 2 runners" idea but am guessing they might still cause problems with diffraction. At the moment i think i will go for cutting a huge (scary) 30+mm off the draw tube - i know the camera will still focus with that much missing and just add it back at the other end when needed with a 35mm extension tube fitting before the coma corrector - if you can follow my meaning. Will just have to do a bit more careful measuring tonight (hopefully) to check that i am not limiting my options and that i can get my EPs still focusing.

With my canon dslr it seems that virtually all of the back focus is inside the camera!

[billyharris72]

With a DSLR you should be fine - as you say most of the back focus is covered by the recess between the lens fitting and the sensor. I was using a Nikon and it was pretty much spot on.

The problem with the  ZWO was partially redressed by the filter wheel rather than caused by it. The recess between the camera body and the sensor is only about 6mm, meaning I needed to rack the focuser way out, relative to my DSLR. The filter wheel bought me 20mm and the rest I made up with extension tubes (29mm). That achieved my 55mm CC to sensor distance and also moved the sensor out enough to reach focus.

I would try cutting less than 30mm off. From an old issue of Astronomy Now I seem to remember that 23mm will do the job for a Canon. You can always shave the other 7mm off, but you can't put it back.