Starter scope DIY focuser upgrade to allow Mirrorless/DSLR camera

[deanchapman2705]

Hi,

So I'm into my astrophotography and have been doing it for a couple of years now. I usually image with a Sony A7iii (full-frame) using a WO Z61ii on a iOptron SkyGuider Pro. In the near future, I would like to upgrade to a goto mount (eq6r pro/gem-45) and get an 8" F4 newtonian. The trouble is, I have never owned a reflector before and with how high maintenance they can be compared to refractors, I bought a cheap second-hand SW Skyhawk 114p (500mm F4.4, light enough to fit on my mount) to experiment with. I have taken it apart fully, cleaned the mirrors etc to get to know them better and feel more confident with them.

However, I have just realised with it's tiny 1.25" focuser, there's no way I can focus with my Sony A7iii. I can physically hand-hold the naked sensor up to the focuser tube (wheeled fully inwards closest to the mirror) without adapters and it can focus nearly touching the tube but with the adapters to attach it, it's nowhere near enough to focus.

As I'm new to this, is there any possible way of getting this to work with a 2" focuser (would seem WAY to big for it) or is there a 1.25" focuser out there that does allow a camera to focus?

I didn't pay much for this scope at all so don't really care what needs to be done to it like drilling new holes etc. hence why I have put this in the "DIY Astronomer" forum. Also if there really is nothing I can do, I don't mind using it as a visual scope as it's actually a lot better than I thought it would be!

On a side note, I am aware of the secondary mirror being too small for the full-frame sensor and there will be heavy vignetting maybe even a bit in its crop-sensor mode and the coma will be horrific but again, this is just an experiment

Any help would be much appreciated,

Thanks and clear skies!

Dean

[DaveL59]

I can't answer regarding focusers but someone else may be able to add something there.

One option might be to raise the main mirror which would bring the focal point further up the focuser tube. Had to do this on my TAL-1 when I switched that from the original 32mm russian focuser to the later 1.25-inch version. I simply replaced the collimation & lock bolts with longer ones and longer springs to match. This raised the mirror cell without needing to modify the tube.

Thinking about the OEM focuser on yours, doesn't that ring on the focuser tube (with the 2 locking screws for eyepieces) unscrew so you can then screw on the T2 adapter directly to the focuser tube?

Edited June 13, 2022 by DaveL59

[vlaiv]

You should be able to use mirrorless camera on such scope with just T2 ring screwed directly to focuser tube (see if eyepiece clamp unscrews to reveal T2).

If not - there are designs for "low profile" 1.25" focuser that you can 3D print in order to be able to attach DSLR to focuser.

See this as example

https://www.thingiverse.com/thing:2552565

(it is for celestron astromaster so it has different base - but you get the idea).

I do have to warn you that you will run into multiple issues with that scope / focuser for imaging.

- That focuser is not really capable of holding anything heavy and remain straight

- At F/4.4 you are going to get so much coma - only very narrow central region of APS-C or 4/3 sensor will be usable (forget full frame).

- There is no coma corrector for 1.25" format.

- You'll get serious vignetting due to small secondary.

[vlaiv]

Btw, you can use barlow lens to move focus point outward or shorten the tube like in this video

 

[deanchapman2705]

Thanks guys,

Raising the primary mirror sounds like a good idea actually, I'm guessing this will reduce the focal length and perhaps make it faster though?

Re the T2 screw in adapter, I have unscrewed the eye piece clamp on the end of the focuser and this is what I can see (pic below) - so I take it this is a T2 thread? The diameter of the thread is 3.7mm if that helps...

If so, this adapter should work then? - https://www.firstlightoptics.com/zwo-accessories/zwo-t2-sony-e-mount-adapter-for-asi1600-asi294-asi183-asi174.html also with this in-between - https://www.firstlightoptics.com/zwo-accessories/zwo-t2-male-to-male-adapter.html

Don't worry, I am aware of how terrible the pictures would be even using super 35 mode (crop) but like I said, this is just an experiment.

I do have a 2x Barlow lens so could see what that does when I’m next free today.

The weight though... I did not think of this. You're right, there's no way it could take the weight of a 650g camera. Even if the tension screws are tightened to the max and it doesn't move, the amount of flex would be ridiculous. Although saying that, if the focused is fully wound in, then maybe flex wouldn’t be much of a problem? And if I have the tube rotated so the focused sticks out horizontally, then slipping of the focus would not be a problem?

If not then I guess that gives me no choice but to use a different focuser. It would need to be a good quality, dual-speed focuser that could take its weight. The only problem is, dual-speeds only come in 2" right? (unless you can modify a good quality 1.25" to be dual speed?) so I can't see any way of it fitting on this scope...

If not, then I think I'll just be using it for visual then

Thanks,

Dean

Edited June 14, 2022 by deanchapman2705

[PeterStudz]

9 hours ago, vlaiv said:

Btw, you can use barlow lens to move focus point outward or shorten the tube like in this video

 

What a nice video. The guy has a lot of enthusiasm and whatever he’s doing you can tell he’s had great fun doing it, which is what it’s all about.

[AstroMuni]

23 minutes ago, deanchapman2705 said:

I'm guessing this will reduce the focal length and perhaps make it faster though?

Thats not true. The focal length and F-ratio will be the same. Its just that the point at which it reaches focus will be pushed outwards (i.e further away from OTA) in the focuser. You may need a larger secondary mirror as well to make better use of the available FOV.

Edited June 14, 2022 by AstroMuni

[DaveL59]

7 minutes ago, AstroMuni said:

Thats not true. The focal length and F-ratio will be the same. Its just that the point at which it reaches focus will be pushed outwards in the focuser. You may need a larger secondary mirror as well to make better use of the available FOV.

Agree, tho if only moving the mirror up 5-10mm the small loss in FoV isn't much to be worth replacing the secondary imho, given OP is experimenting with this rig. 

Edited June 14, 2022 by DaveL59

[vlaiv]

3 hours ago, deanchapman2705 said:

Re the T2 screw in adapter, I have unscrewed the eye piece clamp on the end of the focuser and this is what I can see (pic below) - so I take it this is a T2 thread? The diameter of the thread is 3.7mm if that helps...

I can't really tell from the image, but I'm starting to doubt that it is in fact T2 thread.

T2 thread is actually M42 x 0.75 metric thread and you want it in male variety - thread being cut on the outside of  the tube so you can simply screw on the camera with adapter.

It looks like this:

This is from video showing how to attach DSLR to ST80 - it also has 1.25" focuser, but that one has outer T2 thread. Here is short video itself

https://www.youtube.com/watch?v=UBq0yKbtsuQ

3 hours ago, deanchapman2705 said:

I do have a 2x Barlow lens so could see what that does when I’m next free today.

This is probably your best option.

There are few other options - like moving primary or replacing focuser all together (for example, you can maybe adopt this one:  https://www.teleskop-express.de/shop/product_info.php/info/p7836_TS-Optics-1-25--Crayford-Newtonian-Focuser---metal---with-T2-connection.html but I suspect you would need to 3d print base as this looks like unnecessarily tall and suited for 6" tube and I think there is still risk of it being too high for imaging), but those all require investment and cutting up the scope.

Using barlow is straight forward as it moves focal plane further out, but there are some caveats that you should be aware of.

- You will often hear that barlow slows down the scope, and that is true, but what is implied by that is not true. F/ratio is not the measure of the speed of astrophotographic setup. In effect with x2 barlow, you will be working at F/8.8 rather than at F/4.4 and that is fine

- You will need to bin your data to get the same working resolution as without barlow. You can bin your data after you stack it, while it is still linear, before you start processing it. This will recover any SNR loss due to using "zoom". If you increase focal length of telescope (using a barlow) and you increase pixel size (using binning) - you will end up with same effective setup. Research how to best bin your data with softare that you use.

- You will need longer individual exposures compared to not using barlow - to swamp read noise. Total integration time can stay the same when you bin, but you need longer subs. Instead of using 240 x 1minute - use 60 x 4 minutes (for example).

- Barlow usually narrows field of view, but in this case, it is ok as you'll be using only central portion of FOV anyway because of Coma and also because of vignetting. It effectively helps you utilize more of your sensor (although to the same effect as not using one and cropping more)

- Drawback is that it adds more weight to already unsuitable focuser.

 

If you want to solve all of these issues and still practice with newtonian design - solution is fairly simple provided that your star tracker can handle more weight:

https://www.firstlightoptics.com/reflectors/skywatcher-explorer-130p-ds-ota.html

It has adequate 2" focuser, it has properly sized secondary so it will illuminate APS-C sized sensor, you can use coma correctors with it to get large usable field. Only drawback is that it is heavier at 4kg and together with camera and accessories will push your mount to its limits (I think ~5kg is max payload for SkyGuider Pro?)

Bonus is - above OTA will be still usable on larger mount for wonderful images.

 

[deanchapman2705]

5 hours ago, vlaiv said:

I can't really tell from the image, but I'm starting to doubt that it is in fact T2 thread.

T2 thread is actually M42 x 0.75 metric thread and you want it in male variety - thread being cut on the outside of  the tube so you can simply screw on the camera with adapter.

You're right, it's too small to be a T2 thread - here's an M42 to Canon adapter for comparison: 

5 hours ago, vlaiv said:

- You will need to bin your data to get the same working resolution as without barlow. You can bin your data after you stack it, while it is still linear, before you start processing it. This will recover any SNR loss due to using "zoom". If you increase focal length of telescope (using a barlow) and you increase pixel size (using binning) - you will end up with same effective setup. Research how to best bin your data with softare that you use.

I did not know you could bin your data after taking the pictures. I thought it was only something software related with your camera so will look into that.

 

5 hours ago, vlaiv said:

There are few other options - like moving primary or replacing focuser all together (for example, you can maybe adopt this one:  https://www.teleskop-express.de/shop/product_info.php/info/p7836_TS-Optics-1-25--Crayford-Newtonian-Focuser---metal---with-T2-connection.html but I suspect you would need to 3d print base as this looks like unnecessarily tall and suited for 6" tube and I think there is still risk of it being too high for imaging), but those all require investment and cutting up the scope.

I'm pretty sure that would still have the same problem unless moving the primary mirror closer works then it would be worth it. Trouble is, I don't necessarily want to pay for a focuser 3x the cost if the scope if it might not work so need to be sure (would definitely be worth it if I knew it worked though) Will try finding a way of moving the mirror next then.

Re the 130PDS, I did look into this even as my future scope so thought it would be ideal but with the camera, guiding, dew strap etc. it comes to almost exactly 5kg which sounds possible but with it being a large scope in size (compared to a compact refractor) I believe this has a negative impact on guiding.

Thanks,

Dean

 

 

[deanchapman2705]

Update - I ordered a 1.25" to T2 and a low profile M42 to Sony NEX adapter:

 

The 1.25" to T2 has arrived so just waiting for the NEX adaptor. Although, I have an M42 to Canon EOS and an EOS to NEX adaptor to try it out. Obviously using the EOS to NEX on top of the M42 adapter puts the sensor too far away so it can only focus on things up close but once the low profile M42 to NEX adaptor arrives, it should focus properly. But for now, with a 2x barlow it does work! The quality is awful with it though as expected haha.

   (without barlow)

I tested out the vignetting and was surprised how "not terrible" it was! I forgot to test it with the barlow but here's the results:

Full-frame

   

 

Super 35 mode (APS-C)

   

 

Tonight is clear so will be trialling it out with the barlow to see what the stars look like.

Thanks,

Dean

Edited June 17, 2022 by deanchapman2705

[deanchapman2705]

Update - SO CLOSE!!!

The adaptors arrived and I also ordered a crappy light pollution filter to, quite literally, stick on the end to protect the exposed sensor. Weirdly the thread on the nose piece was wrong so guess it's just threaded to stop reflections rather than a functional thread. With a bit of force, I managed to screw the filter on very slightly but blue-tacked it JIC as I wouldn't want it to fall onto the mirror!

You can see from the third pic of it attached to the focuser that it is as close as it could possibly be.

        

 

Bad news is... I tested it out last night and it still couldn't focus! But what makes it more annoying is that it was SOOO close!

    This is what a star looked like with the focuser wheeled in fully.

   So I unscrewed the collimation screws all the way until they came loose then screwed them back in a couple of turns to push the mirror forward as much as possible and this is what I get.

    I held up the bahtinov mask from my WO Z61 and this just shows how close it is! How annoying!

 

So I guess the solution is to buy longer collimation screws as DaveL59 suggested and that should put the mirror close enough  I have no idea what I'm doing but I'm going to take the primary mirror apart to see how the screws work and measure them but is there anything I should know about collimation screws in general? Is there a universal thread size? I've heard of Bob's Knobs so assume they are the best to buy from.

Thanks,

Dean

 

PS. on the plus side, I brought out my WO Z61 ii later than night and managed to get some time on the comet C/2017 K2 PANSTARRS passing through the Summer Beehive cluster which made it worth it - it was just a tester to see how bright it was so only 17.5 mins of data. Also bare in mind humidity and seeing was bad, some misty clouds about:

35x 30secs @ ISO 1600 (couldn't be bothered to set up auto guiding so short exposures)

Edited June 21, 2022 by deanchapman2705

[DaveL59]

6 minutes ago, deanchapman2705 said:

 

So I guess the solution is to buy longer collimation screws as DaveL59 suggested and that should put the mirror close enough  I have no idea what I'm doing but I'm going to take the primary mirror apart to see how the screws work and measure them but is there anything I should know about collimation screws in general? Is there a universal thread size? I've heard of Bob's Knobs so assume they are the best to buy from.

 

ouch that's so near but yet not quite near enough 😞

When I did this mod on my TAL-1 after replacing the russian 32mm focuser with a 1.25 inch one I checked the screw sizes and also spring lengths and then ordered replacements that were suitably longer to lift the mirror the desired amount. Hopefully this'll take you to the pic of my modified mirror mounting

 

Posts above in that thread may help in terms of what I'd ordered but of course you'd need to alter to suit your scope, being a different make etc. You could also use spacers to offset the spring requirement and that'd probably work just fine too. Sounds like you only need to shift the mirror a small amount so might be worth seeing how much slack there is in the existing screws if that'd allow you to lift it just a few mm.

[deanchapman2705]

On 21/06/2022 at 14:10, DaveL59 said:

Hopefully this'll take you to the pic of my modified mirror mounting

I took the mirror apart last night to measure the collimation screws and springs but looked through the pics of yours before hand and it really helped - Thanks!

   

Turns out the collimations screws were long enough but the locking screws were too short. So I could move the mirror forward enough to focus but couldn't lock it down.

But when putting it back together, I noticed the cork pads that sit behind the mirror and thought of sticking something else there to push the mirror forward so found some hard wool sticky pads that worked perfectly -

It now focuses! I can even focus a little bit too much both ways as well - It was pretty cloudy last night but could just about see Vega and yes the coma is HORIFFIC haha. Literally just the centre of the image is "ok". I took a shot of Vega in the centre and in the corner and this is with the crop mode.

        

For a laugh, I thought I'd image the globular cluster whilst it was there as well - 

But like I said at the start, I wasn't expecting to actually image with this scope as it was just to experiment and play around with. The main thing is, I definitely feel a lot more confident taking a Newtonian apart knowing how it all works now.

Thank you for your help on this guys!

Clear skies!

Dean

Edited June 22, 2022 by deanchapman2705