Using OAG with DSLR camera

[Xav1999]

Hello everyone! 🙂

I'm into astrophotography for some time and I want to take another step ahead - guiding.

I'm new to this topic, but I read many threads on many forums and I chose to use an OAG.

I have SW150/750 on NEQ6 and my main camera is Canon 450D. 

I have two questions:

1) I'm planning to use ASI120MM mini as guiding camera. Would it be enough for OAG?

2) I heard that I may came across some problems with focusing both cameras at the same time. What adapters would I need to get the right backfocus?

PS. Or maybe it's better to use separate guiding scope in my case?

 

 

[alacant]

14 minutes ago, Xav1999 said:

Would it be enough for OAG?

Hi

Yes, easily enough and will guarantee you pinpoint stars every time.

We use a 120mm on a 150/1200. The 120 has bin 2x2 in firmware in case you need it. I think at f5 however, you'll be fine native.

No adapters needed if you use a standard m42x0.75 helical focuser on the oag stalk. See our setup.

Otherwise, a separate guide telescope will work but to get the same level of quality as with the OAG, we'd advise a longer full width dovetail, a top rail to tie the top of the rings and 6 replacement 1.4mm diameter wire springs for the main mirror. 3 to replace the existing springs and 3 more passive over the locking screws. Beware though  that this adds significantly to the mass of the telescope.

HTH 

[vlaiv]

I have two concerns.

First is back focus and use of coma corrector.

Most coma correctors have working distance of 55mm, while Canon flange distance is 44mm.

Usually people use EF/EF-S  to T2 adapter that is 11mm of optical path and that gives them 55mm required by coma corrector.

Ideally, you want OAG to be placed between CC and camera - and there is no room for that in standard setup. One solution is to use dedicated OAG for DSLR cameras that is T2 adapter and OAG in one unit:

https://www.teleskop-express.de/shop/product_info.php/info/p2722_TS-Optics-Off-Axis-Guider-for-Canon-EOS-cameras---replaces-the-T-ring.html

Second concern is how much light is going to be gathered by OAG prism. When used with fast systems, OAG needs to be placed very close to main imaging sensor to reduce distance. This is because of limited size of pick off prism. With DSLR, you have no choice but to place it at about 50mm away from sensor.

In order to fully exploit F/5 system - you need pick off prism that is larger than 10mm. Most models are built with standard prism that is 8mm on it side. That means that OAG will operate at more than F/6 - probably at F/7 or so. Still pretty good performance and you should not have any issues guiding (I guide with OAG at F/8 since I use it at F/8 scope and it works fine - but my scope is 8" and gathers quite a bit of light).

One way of improving OAG performance is to bin your guide camera. That will make it more sensitive. Since you'll be using 750mm of FL to guide on - binning x2 will still give you more than enough guide precision.

[Xav1999]

Thanks for quick response 😀

2 hours ago, vlaiv said:

First is back focus and use of coma corrector.

Most coma correctors have working distance of 55mm, while Canon flange distance is 44mm.

Usually people use EF/EF-S  to T2 adapter that is 11mm of optical path and that gives them 55mm required by coma corrector.

Ideally, you want OAG to be placed between CC and camera - and there is no room for that in standard setup. One solution is to use dedicated OAG for DSLR cameras that is T2 adapter and OAG in one unit:

https://www.teleskop-express.de/shop/product_info.php/info/p2722_TS-Optics-Off-Axis-Guider-for-Canon-EOS-cameras---replaces-the-T-ring.html

 

I don't have a coma corrector for now. I'm planning to buy it together with a dedicated camera so this isn't a problem now.

One more quick question. @alacant you'r OAG is from ZWO? If it is I make my mind and buy it. 😀

[vlaiv]

41 minutes ago, Xav1999 said:

I don't have a coma corrector for now. I'm planning to buy it together with a dedicated camera so this isn't a problem now.

You'll have quite a bit of issues using OAG then.

Coma in newtonian telescope grows with distance from center of the frame. By the time it reaches edge of APS-C sized sensor (usually found in DSLR) it will be too large and star will be very deformed. Small level of deformation usually does not impact guiding - but large does as it lowers level of available light in bulk of the star - used for guiding.

Look at this comparison:

These are stars affected by coma, and here is version with good coma corrector:

You won't be able to guide on most stars in upper image - but you should be able to guide on almost any star from lower one (they are well formed and software can tell the difference between star and background).

[alacant]

3 hours ago, Xav1999 said:

OAG is from ZWO?

Hi

No, it's the TS version made especially for eos cameras.

There is absolutely no problem with or without a coma corrector. For guiding, it matters not the shape of the star. Just our hands on experience. The theory may differ significantly.

We would however strongly recommend the use of a cc. Without, much of an aps-c frame will be unusable; any benefit from guiding will be negated. Unless you're ok with processing hacks.

HTH

Edited July 3, 2021 by alacant