Focussing with OAG on a DSLR

[Leeps]

SO I tried out my new OAG setup tonight, and the DSLR I'm using as a primary can't focus  I can't get it to go in towards the body of the scope enough... I'm sure I should be using some jargon around "Back focus" but I don't want to get it wrong! 

What are my options? It's a 200p scope. I was looking forward to trying this one! It's a 16mm OAG (TS Optics TSOAG16). Can I modify the draw tube to let me focus closer?

[Merlin66]

Leeps,

Using an OAG on a Newtonian is not easy...

What about moving the primary mirror up the tube (towards the secondary) to get more room for your OAG/ DSLR?

The other alternative (the one I had to use years ago), is to replace the focuser with a "low profile" version.

 

[tooth_dr]

Depends how far out you are.  As Merlin suggests you could move the mirror forward.  I did that on my own 250 scope and that gave me just enough inward travel.  It's free so nothing to lose trying it (apart from your current collimation!)

 

[wimvb]

Do you need an oag? Using a guidescope firmly attached to the main scope may be the easiest option.

@tooth_dr and @Merlin66: won't moving the primary further in cause (more) vignetting? Especially with a dslr, which I suspect, has a large sensor. If so, that would create the need for a larger secondary.

[tooth_dr]

1 minute ago, wimvb said:

Do you need an oag? Using a guidescope firmly attached to the main scope may be the easiest option.

@tooth_dr and @Merlin66: won't moving the primary further in cause (more) vignetting? Especially with a dslr, which I suspect, has a large sensor. If so, that would create the need for a larger secondary.

I'm sorry cant answer that, but I see no difference in my camera, although it's only 8MP and has a slightly smaller sensor than the DLSR sensor.   Any vignetting I get is corrected easily with flats.    I've attached a comparative FOV below.

As a no cost option, nothing to lose trying it.

[Merlin66]

Wim,

I think the limited change in the primary position required will have minimal impact.

 

[Leeps]

Thanks for the info guys. Little frustrated, I wanted this to be the cheaper and simpler option! My collimation is unchecked and all out of whack anyway, so I'll try that when my cheshire arrives. Will that make a difference to the focal length?

I went for an OAG because I was originally going to use an old M42 mount lens that I got, but I couldn't figure out mounting it. Once I looked into the price of a guidescope and the mounting hardware I'd need I figured an OAG would be cheaper and simpler in that respect. That plus it'd be 1000mm focal length guiding. so seemed better? I also find them cool

 

I'll try to adjust the mirror first, it's pretty close to coming into focus.

[wimvb]

1 hour ago, Leeps said:

Will that make a difference to the focal length?

Not a bit. But depending on how far you push the primary in during collimation, you just might reach focus.

One for the experts: is it an option to replace the six collimation screws on a sw newtonian with longer ones, and push the mirror in that way? Or maybe this is even the standard way of moving the mirror cell up? One would probably also need to lengthen the three springs. Just wondering, since I've never had this problem myself. 

[tooth_dr]

5 hours ago, Leeps said:

Once I looked into the price of a guidescope and the mounting hardware

I was guiding with a finderscope and slightly modified bracket.  A lot of folks do it this way. The adapter is about £25 and that’s really all you need to spend extra if you already have the finderscope.  Plus pretty much any camera will get you a star to guide on. With an OAG you end up starting to need a sensitive camera to find stars which can cost more money again.  

Edited May 31, 2020 by tooth_dr

[Leeps]

18 hours ago, wimvb said:

Not a bit. But depending on how far you push the primary in during collimation, you just might reach focus.

One for the experts: is it an option to replace the six collimation screws on a sw newtonian with longer ones, and push the mirror in that way? Or maybe this is even the standard way of moving the mirror cell up? One would probably also need to lengthen the three springs. Just wondering, since I've never had this problem myself. 

I did it yesterday, and managed to reach focus! It made a remarkable difference, I actually have a little bit of wiggle room either side of my focal point now. Surprised it makes so much difference!

14 hours ago, tooth_dr said:

I was guiding with a finderscope and slightly modified bracket.  A lot of folks do it this way. The adapter is about £25 and that’s really all you need to spend extra if you already have the finderscope.  Plus pretty much any camera will get you a star to guide on. With an OAG you end up starting to need a sensitive camera to find stars which can cost more money again.  

I bought the scope used, and unfortunately it only came with a red dot finder. So I'd have to get the mounting hardware and the finder. I am seeing what you mean about finding a star with the OAG though, that might be the tricky part. 

I got it working last night and tried PHD2 for the first time. It's a bit of a learning curve, and I've stumbled onto my next issue now - the mount isn't smooth enough to be guided, so I might need to do some tweaking  When I'm guiding east/west I can hear a loud audible clang as the mount sticks and releases. Onto the next issue!...

[tooth_dr]

3 hours ago, Leeps said:

I'd have to get the mounting hardware and the finder.

Finderscopes with the bracket are on sale (used) frequently at about £20-30, and you might even find one with the right camera adapter included. You don’t need any fancy mounts or guide rings etc.
Sounds like you are making progress anyway 👍🏼

[Robindonne]

Lol.  I recently asked about oag’s and almost all advices were directing to get a seperate guidescope.  I didn’t want to believe its that hard to use an oag but i found out😬.  I bought a 16 mm oag and the first thing i noticed was that i had only 1 mm inward focus left.  A bit of luck otherwise it brought me in the same position as you.   Not even using a coma corrector so probably the same problems will face as soon as i get a cc.  

i use an asi120 as guidecam, on an oag, on a quattro 8” without cc.  So my only available guidestars are egg-shaped😬.  
 

why is it so hard to take advice from experienced people?  But i learned in one night more than i would have learned when going directly to the best solution, a seperate guidescope.   
 

The other mistake i made was with the guide-settings.   Epx time on 3 sec helped me a lot. Also the aggressiveness of correction was a bit to much.   I still have to finetune a lot, but its also very very very much fun when spending a night alone, and finding out the practical side, instead of the (you think you know) theoretical side of getting your setup to work.  
 

oh and i forgot to mention that i probably didn’t get in focus, using a dslr also, with a standard eos t-mount.  I bought a es mpcc for eos, a flattener for use with an dslr.  The camera-adapter that came with that flattener is like 1,5 mm thick instead of the bulky 10 mm orso that a standard eos t-mount use.  

Edited June 1, 2020 by Robindonne

[Leeps]

Yeah I think I'll keep an eye out for a guidescope. I chose this because I figured more magnification would result in better tracking, but I am super inexperienced, so who knows. lol

[tooth_dr]

28 minutes ago, Leeps said:

Yeah I think I'll keep an eye out for a guidescope. I chose this because I figured more magnification would result in better tracking, but I am super inexperienced, so who knows. lol

I have got the same figures guiding with a SW guidescope at 185mm as I do with my current OAG setup at 1200mm. The difference is there can be movement between the main camera and the guidescope so that good tracking in the guidescope doesn’t always correlate to the same in the main imaging scope. I think you are on the boundary of guidescope/OAG at 1000mm

[wimvb]

3 hours ago, Robindonne said:

i use an asi120 as guidecam, on an oag,

So did I, but now I've ordered an ASI290, which is supposed to be more sensitive. Many people use a Lodestar with their oag.

[Datalord]

1 hour ago, wimvb said:

So did I, but now I've ordered an ASI290, which is supposed to be more sensitive. Many people use a Lodestar with their oag.

I went to an OAG and tried the ASI120 with many tears as result. Once I got the LodestarX2, everything just worked.

[wimvb]

5 minutes ago, Datalord said:

Once I got the LodestarX2, everything just worked.

I hope the ASI290 mini will do that for me. Otherwise I might go back to using a guide scope. 

[Robindonne]

Yes 3 weeks after buying a 120mc-s, i already know i have to find a place in my cabinet where it can stay very long without being touched.   Its basically very useless 

[RayD]

4 minutes ago, Robindonne said:

Yes 3 weeks after buying a 120mc-s, i already know i have to find a place in my cabinet where it can stay very long without being touched.   Its basically very useless 

They make pretty good all sky cameras, so not totally useless 👍

[wimvb]

2 hours ago, Robindonne said:

Yes 3 weeks after buying a 120mc-s, i already know i have to find a place in my cabinet where it can stay very long without being touched.   Its basically very useless 

I'm sure you will need a paper weight sooner or later. 😉

[Robindonne]

11 hours ago, wimvb said:

I'm sure you will need a paper weight sooner or later. 😉

Ha yes but already owned some astro paper weights🙄