ZWO Camera on Skywatcher 190MN

[CraigD1986]

I’ve just acquired a Skywatcher 190MN (Maksutov-Newtonian) and I can’t figure out how I attach my ZWO ASI533 camera. I’ve got the spacers for the 55mm backfocus on the camera but coming from a refractor (Skywatcher 72ED), I’m not sure how to attach the camera. Does it just slot all the way in?

[Enceladus Dan]

Hiya Craig, 

I use a 150PDS for me it's a case of MPCC fully inserted, then 55mm backfocus from the flattener. As the 190MN has a built in flattener I guess you keep the optical path as short as possible as there is no need for the 55mm backfocus, unless you decide to use a focal reducer at some point. 

Clear skies 

Danny

[CraigD1986]

So here is how I attach the camera to my refractor. Does this just slot all the way in as in the second image?

[alacant]

Hi

If it reaches focus like that, then yes. You're done.

If not, pull it out bit by bit until it does. Add more extension tubes if necessary, but forget about the 55mm.

Cheers 

Edited March 13 by alacant

[CraigD1986]

Brilliant, thanks. Just need the clouds to clear now

[Enceladus Dan]

My PDS came with a 2 inch nose piece with an M42 thread on it. If you don't have one they can be picked up cheap. This would screw to your camera via any filter system if used. 

https://www.firstlightoptics.com/adapters/astro-essentials-2-inch-t-mount-camera-nosepiece-adapter.html (skywatcher one looks shorter)

Or another option is to change the compression thumbscrew fit for a direct screw fit ( if the compression fit can be removed, that is. Your focuser looks a lot different to my PDS)

 

[michael.h.f.wilkinson]

I have a Meade Schmidt-Newtonian, and just attach the camera (ASI183MM) and filter wheel into the focuser, and achieve focus quite easily. I would assume the same works on the Mak-Newt

[CraigD1986]

Looks like I’ll need a clear night to find out. I didn’t get any adapters as I’ve borrowed the scope from my local astronomy club.

[Enceladus Dan]

If like me you don't have any clear skies forecasted, you could try and focus on a distant object in the daytime. I did it on some on some trees about 2 miles from my house, when I was figuring out the best fit for my Barlow and dslr when i wanted to image jupiter. Got me in the right area, without all the palaver at night. 

[CraigD1986]

Good idea, ill have a go tomorrow. Thanks!

[Enceladus Dan]

Your welcome, little off topic for your question, how did you find the 72ED DS-Pro? I find my 150PDS has too little FOV for larger targets even with my 600D, so was looking at the 72ED as an option for these targets, rather than getting a full frame DSLR and modify it. 

2 scopes with my 600D and asi533 would give me more FOV options. 

[CraigD1986]

Thanks for the advice. I’ve ordered an adapter from Amazon so it will arrive tomorrow in time for a clear night.

 

I love my 72ED. The field of view is great, although I do use it with an ASI533MC Pro, which has a relatively small sensor. You can check out the FOV of different setups here is that helps:

 

https://astronomy.tools/calculators/field_of_view/

[michael8554]

Hi Craig

"I’ve got the spacers for the 55mm backfocus on the camera "

You don't need 55mm Backfocus, unless you have a FF or CC with 55mm Backfocus.

And not all FFs and CCs have 55mm Backfocus.

Is this idea from the misleading diagrams on the ZWO site ?

Michael

[CraigD1986]

Sorry, I’m not sure what you mean by “FF” and “CC” Field Flattener and Coma Corrector?

[wimvb]

1 hour ago, CraigD1986 said:

Sorry, I’m not sure what you mean by “FF” and “CC” Field Flattener and Coma Corrector?

Correct. The 55 mm rule is from the old dslr days. Most dslrs have their sensor 45 mm behind the front of the camera body (the lens flange). Add to that 10 mm for a camera adapter, and you have 55 mm. This made connecting a camera to optics easy, you screw the adapter onto the corrector or flattener, and then to the camera. You then slide the assembly into the focuser tube.

The 190MN doesn't need a coma corrector, because the front meniscus corrects any coma. So there are no critical distances on the focuser side. You can dispens with the rings and just use one to attach the camera to the focuser. Easy enough.

Btw, if you have the stock focuser, don't try to use the built in extension tube to fix the focus position; it is wobbly when extended. Push that tube all the way in and secure it in place so that it absolutely cannot move.

Edited March 14 by wimvb

[AstroMuni]

On 12/03/2023 at 21:11, CraigD1986 said:

Brilliant, thanks. Just need the clouds to clear now

Just watch out to check if the focuser tube peeks into the OTA. As far as possible you dont want that to add to the diffraction spikes.

[wimvb]

3 hours ago, AstroMuni said:

Just watch out to check if the focuser tube peeks into the OTA. As far as possible you dont want that to add to the diffraction spikes.

The 190MN has no diffraction spikes and the focuser is low profile.

[CraigD1986]

20 hours ago, wimvb said:

Correct. The 55 mm rule is from the old dslr days. Most dslrs have their sensor 45 mm behind the front of the camera body (the lens flange). Add to that 10 mm for a camera adapter, and you have 55 mm. This made connecting a camera to optics easy, you screw the adapter onto the corrector or flattener, and then to the camera. You then slide the assembly into the focuser tube.

The 190MN doesn't need a coma corrector, because the front meniscus corrects any coma. So there are no critical distances on the focuser side. You can dispens with the rings and just use one to attach the camera to the focuser. Easy enough.

Btw, if you have the stock focuser, don't try to use the built in extension tube to fix the focus position; it is wobbly when extended. Push that tube all the way in and secure it in place so that it absolutely cannot move.

Thanks for explaining. I get you now. I was using the spacers to achieve the 55mm backfocus as I've been using a refractor until now (SW 72ED) so I just assumed the same was needed here. I'm using a ZWO EAF, which making focus so easy and it did achieve focus with the spacers on but the focus tube on the scope didn't extend much so I guess this is why. I'll remove the spacers for the next clear night though.

If I secure the focuser tube all the way in, how would I achieve focus though? This part extends and retracts as the EAF runs through it's autofocus procedure.

[AstroMuni]

9 hours ago, wimvb said:

The 190MN has no diffraction spikes and the focuser is low profile.

I get that there is no spider but if the focus tube sticks into the light path it would create some sort of diffraction is my guess?

[AstroMuni]

3 hours ago, CraigD1986 said:

If I secure the focuser tube all the way in, how would I achieve focus though? This part extends and retracts as the EAF runs through it's autofocus procedure.

Try and achieve focus manually (or take a look at the position after EAF has achieved focus) so you know approx how far the focus tube extends into OTA (if at all).

Another point...your spacers are just helping you position the camera as far in or out of the focuser tube. i.e. serving the function of a nosepiece. In your image you have effectively pushed all the spacers into the focuser tube so camera is almost touching the focuser. (If you were using a CC then the CC would have been attached at the end of the spacers thus giving the 55mm space between sensor & CC).

Edited March 15 by AstroMuni

[wimvb]

5 hours ago, CraigD1986 said:

Thanks for explaining. I get you now. I was using the spacers to achieve the 55mm backfocus as I've been using a refractor until now (SW 72ED) so I just assumed the same was needed here. I'm using a ZWO EAF, which making focus so easy and it did achieve focus with the spacers on but the focus tube on the scope didn't extend much so I guess this is why. I'll remove the spacers for the next clear night though.

If I secure the focuser tube all the way in, how would I achieve focus though? This part extends and retracts as the EAF runs through it's autofocus procedure.

I have a filterwheel to achieve focus at about 15 mm or so for the focus tube to extend out of the focuser. In stead of using the wobbly extension tube, it's better to leave the spacers in place. Just do a rough manual focus first to get into the ball park, as they say. Then use just enough spacers to get to focus.

1 hour ago, AstroMuni said:

I get that there is no spider but if the focus tube sticks into the light path it would create some sort of diffraction is my guess?

Normally, it would. But as I wrote, the focuser is a low profile type, and the focus tube won't extend past the internal baffles. This is by design, so you will get APO-like stars.

[CraigD1986]

It looks like the adapter I bought was causing some wobble between the camera and focus tube, which resulted in slightly elongated stars. I'll try next time the clouds break to insert the camera just using the ZWO spacers and see If that improves things.

[wimvb]

3 hours ago, CraigD1986 said:

It looks like the adapter I bought was causing some wobble

Just make sure you (hand-)tighten the thumbscrew of the built in extension tube as firmly as you can,mor it may cause a wobble.