Off-Axis Guiding - Vibration ?

[skybadger]

On 13/12/2017 at 16:52, ollypenrice said:

I knew it was there with the first OAG I used. It rocked on its turret so I had to build a steel strap to hold it still. This design fault was addressed by Starlight Xpress in a revised version. The forces of darkness are everywhere in this game! On balance I agree that the OAG is the solution of choice but I won't make the change myself till I find a need. It's perfectly true that flexure can be a devil to track down.

BTW, as Sara points out, you can't use an OAG on an autofocusing dual rig.

Olly

Now that's something I don't understand. One ota has the oag , one doesn't, both autofocus, what's the issue ?. Once the oag is parfocal with the main sensor on the same ota, you can autofocus as you like.

.

[RayD]

10 minutes ago, skybadger said:

Now that's something I don't understand. One ota has the oag , one doesn't, both autofocus, what's the issue ?. Once the oag is parfocal with the main sensor on the same ota, you can autofocus as you like.

Because if the auto focus runs it will stop guiding, and if the other OTA is in the middle of an image it will ruin that sub.

[skybadger]

Ah, that's a problem that is nice to have. I recommend not to do that. Surely that's within your control and you tell it when you want to autofocus after n exposures, or are they not the Same for each ota?

You can write a script to pause for the end of the exposure of one ota before focusing ?

[ollypenrice]

4 minutes ago, skybadger said:

Ah, that's a problem that is nice to have. I recommend not to do that. Surely that's within your control and you tell it when you want to autofocus after n exposures, or are they not the Same for each ota?

You can write a script to pause for the end of the exposure of one ota before focusing ?

This bit of the conversations starts with my post about a rocking turret, which has nothing to do with Sara's point about OAG and dual rigs.

However, the dual rig OAG issue is that you are very unlikely to want to do exposures of the same length on both sides of a dual rig. I tend to be doing 30 minute Ha on one side and either 10 or 15 min LRGB on the other. This is not a problem if you are using a separate guide scope.

Olly

[RayD]

5 minutes ago, skybadger said:

Ah, that's a problem that is nice to have. I recommend not to do that. Surely that's within your control and you tell it when you want to autofocus after n exposures, or are they not the Same for each ota?

You can write a script to pause for the end of the exposure of one ota before focusing ?

You can, but if you are shooting different length subs in each OTA (HA in one and RGB in another) you could be waiting half hour to focus, and then have to do it again right away for the other focuser, which will then need to pause the guiding again.  I'd trust Sara if she says it really can't be done as she would definitely have tried all angles.

[skybadger]

Fairy nuff.

Ta for elaborating.

 

[malc-c]

Guys, I  took some measurements as best I could today which hopefully someone will be able to interpret and let me know for sure if the OAG on FLOs website will work with my 400D and QHY5

This image below is when the camera is in focus

  

So with the coma corrector in place, the camera body to the focus draw tube housing is 46mm (with 32.8mm of the draw tube protruding).

The image above gives the dimensions of the coma corrector and T ring adaptor.  The front lens is recessed a little, and beyond my means to accurately measure.

The focus draw tube travels from 12mm (just the thumbscrew ring protruding) to 49.7mm fully extended.

I wasn't able to measure the distance from the camera body to the CCD, but I'm guessing that this distance is documented somewhere on Canons website ?

 

I assume that I would need to fit the OAG between the T-Ring and Corrector.  One thing that does bug me is that there are no measurements or technical spec on the OAG on FLO's website, so have no idea  how thick the body is so can't work out if there is enough travel (presumably its less than the 20mm inward travel I have to play with) in the draw tube.  Or would the OAF be required to be fitted before the corrector so that the corrector maintains it's focus distance to the CCD ?? - I'm confused !!

[skybadger]

Mine is telescope service oag which is 9mm thick. So would fit happily in your system. Increasing the distance between the flattened and camera will cause a further reduction of in-focus so I think it will be touch and go. What is the design distance for the reducer ? I'd expect it be 80 mm or so, for which you are currently short. 

[malc-c]

1 hour ago, skybadger said:

Mine is telescope service oag which is 9mm thick. So would fit happily in your system. Increasing the distance between the flattened and camera will cause a further reduction of in-focus so I think it will be touch and go. What is the design distance for the reducer ? I'd expect it be 80 mm or so, for which you are currently short. 

A lot of that went straight over my head !

Its a Baader Coma Corrector SKU 100134 which was purchased second hand from a fellow SGL member probably around 5 years ago, maybe more. - So no idea on which MK it is or reducer distance - sorry

Edit: 

If you are meaning the back focus, this is 55mm from reading up some of the blurb on this and Baaders website.  According to this site https://en.wikipedia.org/wiki/Flange_focal_distance the FFD (flange focal distance ) for Canon is 44mm - hope this all means something to someone 

So I assume that the thickness of the T ring added to the FFD equals the back focus of 55mm (+/- 1mm), which is why it all works.  So I presume adding an OAG between the T-ring and coma corrector will increase this distance and thus cause the corrector not to work ?

[skybadger]

Sorry for the confusion, mixed a few things in there. You will need a 48mm oag to put the corrector on the outside , based on your 2" adapter in the photo. 

Then I don't see why it won't fit, but your guide camera stalk will be quite long to match the 55+corrector thickness.

 

[malc-c]

The 2" "adapter" in the photo is the coma corrector, and whilst it has a threaded section at the front by the lense, it's not (as far as i can tell) a standard thread that will permit something like an OAG fitted in front of it.  If I placed the OAG between the T ring and Coma corrector, the 55mm back focus distance will be increased and thus the image will never focus. - at least that's my understanding

[alacant]

1 hour ago, malc-c said:

between the T ring and Coma corrector, the 55mm back focus distance will be increased and thus the image will never focus. - at least that's my understanding

Hi. You remove the t-ring. This oag is the t-ring:

https://www.teleskop-express.de/shop/product_info.php/language/en/info/p2722_Lacerta-Off-Axis-Guider-for-Canon-EOS-cameras---11mm-length.html

AFAICT, at 55mm it's your only option. The TS 9mm will take you over 55mm, even with a low profile t adaptor.

HTH.

[malc-c]

28 minutes ago, alacant said:

Hi. You remove the t-ring. This oag is the t-ring:

https://www.teleskop-express.de/shop/product_info.php/language/en/info/p2722_Lacerta-Off-Axis-Guider-for-Canon-EOS-cameras---11mm-length.html

AFAICT, at 55mm it's your only option. The TS 9mm will take you over 55mm, even with a low profile t adaptor.

HTH.

That looks interesting.  So basically I just screw the Coma corrector to to the front of that, then mount the OAG to the camera body and job done.  It looks like there is an adjustment for the guide camera to get focus independent of the DSLR too...  

Googling the item seems to come back with mixed results, some reporting that they can fit the OAG perpendicular to the DSLR CCD, and that the prism easily rotates, but nothing else seems available so that will be my only option if I opt to go down the OAG route.  I'm not going to rush into this, and as it's quite expensive (I'm on  a limited income at the moment being between jobs as mentioned) so might end up placing this on my wish list for my Birthday next March.  Thanks guys for everyones input

[alacant]

9 hours ago, malc-c said:

So basically I just screw the Coma corrector to to the front of that

Yes.

9 hours ago, malc-c said:

then mount the OAG to the camera body

Yes.

9 hours ago, malc-c said:

there is an adjustment for the guide camera to get focus independent of the DSLR

Yes.

Or forget the corrector, get a cheap oag from e-bay and fix the coma in software. HTH

[skybadger]

Interesting - if the coma corrector screws on to the t adapter as shown in the photo, it will screw on to an oag.

I haven't seen an OAG where the guide camera stalk wasn't separately focusable, by spacing the camera if anything. 

The TS 9mm also comes with direct adapters for Nikon and Canon cameras. Its the slimmest there is afaik and supports 48mm adapters too to prevent vignetting on large sensors. 

Check out the connection options on the vendor sites. It sounds like you just need the corrector to sit after the oag. Work out what the corrector thread is and you're done. 

M

[malc-c]

1 hour ago, skybadger said:

Interesting - if the coma corrector screws on to the t adapter as shown in the photo, it will screw on to an oag.

I haven't seen an OAG where the guide camera stalk wasn't separately focusable, by spacing the camera if anything. 

The TS 9mm also comes with direct adapters for Nikon and Canon cameras. Its the slimmest there is afaik and supports 48mm adapters too to prevent vignetting on large sensors. 

Check out the connection options on the vendor sites. It sounds like you just need the corrector to sit after the oag. Work out what the corrector thread is and you're done. 

M

 

Looking at the TS website the TSOAG9  is 159 euro, and the direct adaptor for Canon EOS cameras is optional at an additional 25 euro (although they do offer a TSOAG9EOS at the same combined price of 179 Euro.  It also confirms that the Baader coma Corrector can fit directly, but then states a confusing focal distance.  I'm confused as it gives a distance from the connection adaptor to the sensor of 46.6mm and not 55mm.

The TSOAG9 fits directly to the M48 connecting thread of the correctors (Image at left). The combination offers coma correction and guiding in a very small space. From the camera connecting adapter of the off-axis guider to the sensor, you have 46.6 mm left - enough space even for a DSLR camera.

The TSOAG9 does look more sturdy compared to the TSOAG11 EOS that was linked to above at a cost of 99 euro, but is that enough to warrant the additional 80 euros ??

[alacant]

19 minutes ago, malc-c said:

The TSOAG9 does look more sturdy compared to the TSOAG11

But the TSOAG9 still needs a t-ring or -extra-expense- the eos adaptor. This may take you over the 55mm required by your cc.

Another -maybe cheaper- alternative is to swap your current cc for the Rowe or GSO ccs. Then with around 80mm to play with, just about any -cheap-solid-fat-robust- OAG will fit. Cheers and HTH.

[malc-c]

18 minutes ago, alacant said:

But the TSOAG9 still needs a t-ring or -extra-expense- the eos adaptor. This may take you over the 55mm required by your cc.

Another -maybe cheaper- alternative is to swap your current cc for the Rowe or GSO ccs. Then with around 80mm to play with, just about any -cheap-solid-fat-robust- OAG will fit. Cheers and HTH.

Really?, the TS website seems to suggest that no additional parts are required https://www.teleskop-express.de/shop/product_info.php/info/p1188_TS-Optics-Off-Axis-Guider-for-EOS-Cameras----length-only-9-mm.html and that it works with the CC that I have.

[alacant]

3 minutes ago, malc-c said:

the TS website seems to suggest that no additional parts are required https://www.teleskop-express.de/shop/product_info.php/info/p1188_TS-Optics-Off-Axis-Guider-for-EOS-Cameras----length-only-9-mm.html and that it works with the CC that I have.

Hi. I was thinking of the economy; it's €80 more to do the same thing!

[malc-c]

1 hour ago, alacant said:

But the TSOAG9 still needs a t-ring or -extra-expense- the eos adaptor. This may take you over the 55mm required by your cc.

Another -maybe cheaper- alternative is to swap your current cc for the Rowe or GSO ccs. Then with around 80mm to play with, just about any -cheap-solid-fat-robust- OAG will fit. Cheers and HTH.

 

48 minutes ago, alacant said:

Hi. I was thinking of the economy; it's €80 more to do the same thing!

Thanks for the suggestion, however the Baader Rowe CC retails for  £145 and the cheapest OAG I found on E-bay is around the £70 mark, which when combined is more than the 179 Euro for the TSOAG9 EOS device.

My Birthday is at the beginning of March so I'll have words with the family and hope that they can club together, at least this would make your suggestion viable.

[alacant]

? Keep what you have. The OAG11 is €99, the OAG9, €179.