Off-axis guider on an Newtoninan telescope?

[stoffe]

Hello! I start a new post here, is there anybody that have any experience with off-axis guider on a newton telescope? My setup is following.

TS Optics (GSO) 8" f/4

Skywatcher (GPU) Aplanatic coma corrector with M48 male thread connection.

QHY9 KAF8300 Mono CCD

Xagyl Filterwheel with low profile female M42 thread connection, I got about 16-17 mm space for an offaxis guider.

Which one should I choose? I got a ZWO ASI120MM camera that I want to use as a guide camera.

[Earl]

No direct experiance on a newt, however..

On ebay you will see an auction for a OAG they tend to go around the £50 mark, I just picked one of these up and its well made and im quiet pleased espsically for the price.

Camera side its T-2 scope side its M48 so this could work for you. its arounf 11mm think, ill do a acurate measure in amoment for you.

EDIT: its 16mm thick so it could be perfect for you.

http://www.ebay.co.uk/itm/Brand-NEW-High-Quality-Off-Axis-Guider-with-Extra-Extension-Tubes-RRP-239-50P-/141426209024?pt=UK_Photography_Telescopes&hash=item20eda8b100

Place a £50 bid and you stand a chance of getting a second chance offer or may go for less and get a lucky lower price, up to you.

[stoffe]

Hello! Thanks for that tip, it looks like that TS OAG 9 mm that are on the market. I have to check this out.

Btw, should I divide the optical length in my Baader LRGB filters for the exact distance to the corrector? How do I do that?

[Earl]

at a rough guess your filter will add around .5mm, its a case of plugging it all together and seeing how the image looks, then add shims if needed.

Spaceing is a bit of trial and error im affraid.

[stoffe]

Thanks for the info, it should be possible to calculate. I know the exactly distance from the corrector, but I do not know if it is from the back of the glass or if it is from the thread the optical length should be done.

Btw, do you have that OAG in the ebay link? I couldn't find any reviews about it.

[stoffe]

I mean, where should I start measure the distance from the corrector?

[steppenwolf]

Normally, the spacing is specified from the mounting face of the coma corrector rather than the glass surface.

[stoffe]

Thanks for the info, my English is not perfect but to be absolutely sure, the mounting face. Is that where the threads begins at the corrector?

Here is the corrector that I use.

http://www.teleskop-express.de/shop/product_info.php/language/en/info/p5836_GPU-Aplanatic-Newton-Coma-Corrector-4-element-fuer-Newtons-ab-f4.html

[steppenwolf]

, the mounting face. Is that where the threads begins at the corrector?

Yes:-

[stoffe]

Many thanks Steppenwolf!

The back focus for my Baader LRGB filters is 0.67 mm. That OAG would fit me perfectly if it is enough space for M48 thread to reach 4 mm travel into the OAG.

(The last sentence that I just wrote could have grammatical errors but it is good training for me to write in English, its my second language)

[stoffe]

I just won that ebay auction today, I will hope it will fit. I also need a M42 male thread, to connect my filterwheel with my camera with zero space. I think that 6-7 mm length would do fit perfectly, where can I found that kind of adapter?

[Earl]

if you get a lenght of m42 thread  that will connect them together

this might:

http://www.teleskop-express.de/shop/product_info.php/info/p556_Baader-continuous-male-T2-thread-with-1-25--Filterthread.html

[stoffe]

Thanks for the link, I will not risk any vignetting and I just saw that this was slightly cheaper and also wider.

http://www.teleskop-express.de/shop/product_info.php/language/en/info/p561_Baader-Adapter--Durchgehendes-T2-Au-engewinde---16.html

[Earl]

yeah missed that, better choice.

[stoffe]

Yes, but that one that you linked to can be used to mount a focal reducer just before the guide camera. But I think the 5.6 mm is a little to narrow to do that on the OAGs guide camera mount.

[Singlin]

I have the Quattro f4 10 " and the Skywatcher f4 coma corector.

I tried the 9mm oag and I could not get it to focus . I just use a finfer guider.

I would have liked to get the oag to work though.

[stoffe]

Singlin - I think you only didn't got three right distance to the coma corrector with your guide camera. Maybe you need some sort distance or extension tube to get it into focus. Is it the same aplanatic 4-lens coma corrector that both of us have? Your QHY5 camera could need to have one of these distance between.

For example

http://www.optcorp.com/telescope-accessories/focusing/t2-extension-tube-10mm.html

I also got a QHY5 camera but I will use my ASI120MM camera for guiding, its way more sensitive. I can tell you which adapter I needed to got a successful OAG-guide package when I got all the adapters that I may need.

[Singlin]

That's great Soffe.

I have an Atik 383 too.

[Singlin]

I alos have a ASI 120 but don't use it for guiding at the moment as the QHY5 is fine with the wide angle finder scope.

[stoffe]

Singling, I do not own a Atik 383, I asked if you had the same coma corrector that I got. I have The Skywatcher Aplanatic 4-lens corrector, my distance from the mounting face to the sensor is 53.66 mm on my 800mm f/4 newtonian. This table is only for the newly mentioned corrector..

The optimal back focus depends on the telescope's focal length:
F=600mm, working distance = 51.66mm
F=800mm, working distance = 53.66mm
F=1000mm, working distance = 55.0mm
F=1200mm, working distance = 54.66mm
> 1500mm, working distance = 54.60mm

Do you have the colour or the mono version of the ASI120 camera? I got the mono version and the correct name for it is ASI120MM, it is far more sensitive then my QHY5 camera and much lower noise, I think you will need that for your off axis guider due to the higher sensitivity and chance to find a guide star. You can also use 2x2 or 3x3 to increase the sensitivity for guiding. I am using pulse guiding and I am glad to not be needing that ST4 cable, it is very stiff at lower temperatures. With lower temperatures I mean down -10 degree Celsius and lower. I can have down to -32 Celsius here at the winter.

[whizzbang]

i could never get my QHY5 to work with an OAG, just not sensitive enough.....the SVX loadstar works fine though 

[stoffe]

Test your OAG now when it is full moon, can you reach focus with your QHY5 on the moon? Try to bin your camera to increase sensitivity if you not see anything, you will see the moon and you can adjust focus on both camera so they are correctly focused. There exist a T2 internal thread adapter for 1,25" filter, you can try to mount a focal reducer.