Off-axis guider? Considerations and suggestions?

[turboscrew]

I've been thinking of getting an off-axis guider for a future bigger astro camera.
I just wish to get ready with the extension and spacers.
What are the things I should consider?

I was planning to use a 2" power mate (yet to be purchased) and I have the ES HR CC, with which I can barely get some eyepieces to focus (it that makes a difference). I also have a planetary camera Omegon Camera veLOX 178 C Color, that I thought I could then use as a guide camera.
My scope is 300 mm F/4, if that makes some difference.

[seigell]

Your biggest concerns will be: 1) Vignetting because of the "steep" light cone of a fast f/4 Scope; and 2) Additional Backfocus required to add the OAG to the Imaging Rig.

The Power Mate will help with #1 (at the expense of doubling the Focal Ratio). But it places a hard-stop at about 1.75" for largest Sensor diagonal (you didn't indicate your Imaging Camera).
Your Scope being a Newt, if even an Imaging Newt, means that Backfocus is a critical resource. The OAG will consume 20-30mm of that.

[SkyHiker]

Vignetting is definitely an issue with a 2" focuser and a CC. I can just barely get by with my 2" in my case. The Paracorr II could be a major contributor because it is a long tube. My focuser is actually 3.25" but it has a 2" eyepiece holder. Check out your vignetting with your CC first. Of course flats will help to quite an extent, you just don't want the corners to be totally black. Testing with an APS/C sized DSLR reflects an ASI2600 that is that size.

Next, I got a medium size QHY OAG. The QHY come in 3 sizes: small, medium and large. Here you have to think about whether the size will limit the filters that you want to work with. In my case I got 31 mm filters for which a medium size OAG works. From experience I recommend a mono camera and mono filters (I have the ZWO 7x 31mm filter wheel).

One more feature that not all OAGs have, is a helical focuser. The QHY has one. I like it for fine tuning. It's a one-time setting.

Keep in mind that an OAG needs to be in a specific spot relative to the sensor. You can't put it in a corner of the sensor because it will lead to ugly gradients in your images. So framing becomes a concern - you may have to rotate the whole assembly (not just the camera) and refocus. In that case, an electronic focuser is handy (I use it a lot, also to refocus between filter changes).

[AstroBee]

Juha,

You mention use of a PowerMate and also mentioned a planetary camera for guiding. I would not use the PowerMate if you are just getting into DSO imaging. Your best option is to start out using as wide a f.o.v. as possible. Besides, as seigell mentioned, using an OAG may be very difficult due to the backfocus issue unless your scope is specifically designed for imaging.

[turboscrew]

My plan is to use the planetary camera without guiding, and, maybe, with powermate (before the CC).
And alternatively some bigger astro camera without powermate ,using the planetary camera as the guide.
Mono camera with filter wheel, if possible.

Without the CC I need to use 50 mm extension tube to get the eyepieces to focus. The CC changes the situation, but there should still be that 55 mm between the T2-end of the CC and the sensing plane of the camera. For imaging, the helical focuser of the CC comes off.

I still don't know what the bigger astro camera might be. I just wish to see what I can fit between the CC and a camera.
Maybe I need to replace the CC...

What is, usually, the order of stuff in the imaging rig, if you have CC, OAG, filter wheel and camera?

Sorry, if my posting appears confusing, but that's just because I AM confused, and that's why I'd rather ask first, to avoid major mistakes.

[KathyNS]

I picked up a second-hand ZWO OAG, that came with a helical focuser. I haven't used it in operation yet (I got it to prepare for "galaxy season"), but I did experiment with getting it set up to focus both cameras.

Normally, one would put the helical on the guide camera. However, to do that with my focal reducer would have required a bunch of spacers that I didn't have. After puttering around with a spreadsheet, I worked out that I could get proper focus and reducer backspacing if I put the helical on the imaging camera. I set it up that way, focused on a treetop, and everything worked perfectly.

It is a bit odd, because I have to focus the guider first, and then adjust the imaging camera's focus, which is backwards from the normal sequence. But once it is dialed in and locked, it is good to go.

[turboscrew]

I wonder if it might work if the main camera was Atik ACIS 7.1?
And would it be better to have separate filter wheel, or a camera with filter wheel included?

[turboscrew]

And this is one OAG I was thinking about.
https://www.astroshop.eu/off-axis-guide ... r_1_select

I'm a bit afraid of ZWO cameras, because they seem to have low limit of -5°C. I think I saw somewhere that Atiks have -10°C limit.

[Star Dad]

I have the ZWO ASI294MC-Pro and I routinely set it for -5C. I find in most images the noise is so tiny that I don't bother using the noise reduction feature of Star Tools. I find occasionally in the hot summer months that the camera does not cool that far down - usually 0C seems to be the best it can do- but considering nighttime temps can be in the 20C+ range that's not bad.

I used to use the Orion Thin Off Axis Guider - but grew tired of trying to get the guiding camera to obtain a star bright enough to guide on and went with a separate guiding scope. The TOAG has a tiny mirror which "steals" some of the light and the amount it stole was too little for many areas of the night sky. It was also a bit of a pain to get the imaging camera and the guiding camera into focus at the same time - especially once you start to add any filters, focal reducers, barlows etc to the imaging camera.

[turboscrew]

The thing with the temperature is, that here, the night starts emerging somewhere at the end of august, and in november there are often temperatures below -10°C, and the temperature seldom raises above -5°C until March/April.
That is, temperature limit of -10°C probably doubles or even triples the usable time compared to temperature limit of -5°C.
The time between "dark enough" and "below -5°C" is something around 4 months. 2 months in spring and 2 months in autumn.

https://www.timeanddate.com/sun/@644451

http://www.tampere.climatemps.com/

[turboscrew]

My plans (I wonder if I'm in the "hood"):
The CC makes the image about 56.5 mm from the CC T2 to image plane for 1200 mm telescope.
For the planetary (velox 178C)
CC | extension (30 mm) | 7.5 mm spacer | velox + nose piece (19 mm)
For the Atik (maybe?):
CC | 7.5 mm spacer | OAG(16.5 mm) | filter wheel (20 mm) | Atik (12.5 mm, flange)

What should be the order?

Is the linked ZWO off-axis guider fine?
https://www.astroshop.eu/off-axis-guide ... r_1_select
What would you say about Atik ACIS 7.1?
https://www.astroshop.eu/astronomical-c ... no/p,65785
Then, maybe a filter wheel like this?
https://www.astroshop.eu/filter-wheels- ... 2-/p,64911
Is separate filter wheel better or maybe integrated, like in Atik Camera One 6.0?
https://www.astroshop.eu/astronomical-c ... r_1_select

[SkyHiker]

My plans (I wonder if I'm in the "hood"):
The CC makes the image about 56.5 mm from the CC T2 to image plane for 1200 mm telescope.
For the planetary (velox 178C)
CC | extension (30 mm) | 7.5 mm spacer | velox + nose piece (19 mm)
For the Atik (maybe?):
CC | 7.5 mm spacer | OAG(16.5 mm) | filter wheel (20 mm) | Atik (12.5 mm, flange)

What should be the order?

Is the linked ZWO off-axis guider fine?
https://www.astroshop.eu/off-axis-guide ... r_1_select
What would you say about Atik ACIS 7.1?
https://www.astroshop.eu/astronomical-c ... no/p,65785
Then, maybe a filter wheel like this?
https://www.astroshop.eu/filter-wheels- ... 2-/p,64911
Is separate filter wheel better or maybe integrated, like in Atik Camera One 6.0?
https://www.astroshop.eu/astronomical-c ... r_1_select

About the integrated filter wheel, it only has 5 positions. I like having everything I need (Lum,RGB,HalphaOIIISII) so I never have to change it.
You have the order right.
The Atik has a smaller size and smaller well depth than the ASI2600MM, those are 2 numbers I look at. I could not find anything about the minimum temperature.
The Atiks have nice on-the-fly stacking software I believe.

[turboscrew]

My plans (I wonder if I'm in the "hood"):
The CC makes the image about 56.5 mm from the CC T2 to image plane for 1200 mm telescope.
For the planetary (velox 178C)
CC | extension (30 mm) | 7.5 mm spacer | velox + nose piece (19 mm)
For the Atik (maybe?):
CC | 7.5 mm spacer | OAG(16.5 mm) | filter wheel (20 mm) | Atik (12.5 mm, flange)

What should be the order?

Is the linked ZWO off-axis guider fine?
https://www.astroshop.eu/off-axis-guide ... r_1_select
What would you say about Atik ACIS 7.1?
https://www.astroshop.eu/astronomical-c ... no/p,65785
Then, maybe a filter wheel like this?
https://www.astroshop.eu/filter-wheels- ... 2-/p,64911
Is separate filter wheel better or maybe integrated, like in Atik Camera One 6.0?
https://www.astroshop.eu/astronomical-c ... r_1_select

About the integrated filter wheel, it only has 5 positions. I like having everything I need (Lum,RGB,HalphaOIIISII) so I never have to change it.
You have the order right.
The Atik has a smaller size and smaller well depth than the ASI2600MM, those are 2 numbers I look at. I could not find anything about the minimum temperature.
The Atiks have nice on-the-fly stacking software I believe.

You mean, 7 position wheel would be better? what about the filter types (sizes, mounted/non-mounted)?

It seems like the temperature range has been removed from the ASI2600-specs, but ASI2400 still has it:

Operating temperature -5 - 45

And I remember seeing that in ASI2600-specs too. That's why I dropped the ASI2600 from my options.
I wonder why - the chip can tolerate much colder conditions.
Omegon Camera veTEC 571 C Color has the same chip (Sony IMX571) and for it:

Operating temperature -10°C - +50°C

Also ASI2600 has flange focal distance 17,5 mm. I think it might be "tight fit" (but should fit).

I also wonder if I'd benefit from full frame size sensor? At least without major modifications to the telescope?

I didn't include calculations for guide camera, because that path length doesn't seem to be included in the OAG specs.
But due to the 30 mm extension in planetary camera use, I think it can be made to work.

[SkyHiker]

If you think about going full frame you probably need at least a 2.5" focuser and certainly 2" filters. I have no experience with this but check Rob's thread viewtopic.php?f=35&t=20857&p=182466#p182466 . He plans to go full frame and based his CC on that, filter size as well, 2" IIRC. If the ATIK is less deep it will be easier. I could not quite get close enough (1 mm off I believe) looking at the specs but it works OK. The OAG guide camera should not be much of a problem, if you get the ASI120MM it is small and easy to manipulate length wise, it fits completely in the 1.25" tube.

[turboscrew]

I kind of thought I could select something, but seems like I need to find a combination that fits - if there is such a combination. The 56.5 mm is quite tight for OAG, filter wheel and camera. Most combinations seem to need adapters, that eat up the precious millimeters.
Like, the ZWO OAG : 16.5 mm, Atik Camera ACIS 7.1 Mono: 12.5 mm (T2), Atik Electronic Filter Wheel EFW3 7x 2": 21.8 mm (M54/M54). That's 50.8 mm without adapters (T2 - M54 and M54 - T2).
The adapters would be probably about 5 mm a piece, if I only could find any.

[turboscrew]

It, actually, starts to look like you select the camera, and buy filter wheel and OAG of the same brand. They don't seem to fit to each other without thick adapters, if they are of different brand.

They actually say it:

As a complement to the EFW2 filter wheel, Atik has developed an Off-Axis Guider...

Or ZWO EFW 7x2" filter wheel:

This filterwheel was developed for the ZWO ASI 6200 full-frame camera. It is being screwed directly onto the camera...

[yobbo89]

I'll check out some dimensions for you, you want the oag pick off and filters to be as close to the chip as possible.

2" filters will be fine for aps-c and full frame sized chips, you should be total vignetting free

you could even possibly use 31mm-36mm unmounted filters if you just intend to use aps sized chips

2" corrector is fine for aps-c, any bigger chips you'll want a 2.5" to 3"

I used one of my setups to measure where the filter sits, it's about 35mm from the chip, other setup on my main rig sits alot closer as it is a all in one filter oag wheel.

This totally depends where your filters sit to the chip
.
Full frame calls for 50mm filter, close to 2"

The aps-c calls for 30mm filter, "36mm unmounted"

I think 1.25" filters actually have 28mm of usable filter.

https://astronomy.tools/calculators/ccd_filter_size

You lose alot of back focus with a oag and it can be a problem with the standard 55mm back focus corrector, you may need to find a corrector that has a longer backfocus, or find a slim filter wheel and slim oag, or find a setup where you bolt the filter wheel to the camera, it saves space without using ring adapters.i Machine my adapters, it's slow but I'm able to match any camera with other brand parts to fit in for the backfocus. Sometimes it can be near impossible to mix different brand astronomy gear..

And it is a must have that all your filters are in spec so they are "parfocal, I believe the thickness of the filter is the variable on getting this right, you may need to test all filters and send any back for a replacement, note it is near impossible to get all filters perfectly parfocal, they just have to be close enough so the oag stays in focus with a suitable fwhm to guide.

i think a starting point is have a solid idea on what camera you want to use and keep, we can build around that .

[turboscrew]

i think a starting point is have a solid idea on what camera you want to use and keep, we can build around that .

It starts to look that way. My problem is the weather. Most ZWOs have operating temperature range -5°C and up.
That about halves the usable time in a year (the time when it's dark enough, and frost limit).
There are cameras with operating temperature range -10°C (which would be enough), but they don't seem to fit any slim filter wheel without (fat) adapters (at least the "native" connections are often not mentioned in the specs).

An example:
Atik Camera ACIS 7.1 Mono:
- Connection (to the telescope) T2 (M42 x 0.75)

Atik Electronic Filter Wheel EFW2.2 5x2":
" For example, the distance between any Atik camera and the wheel itself is 0mm, while still maintaining the possibility of rotating the camera to any desired position."
- Connection (to the telescope) 2"
- Connector (at camera end) 2"
and
What's in the box?
- EFW2.2 body with your choice of filter disk
- 2" adapter
...

BTW, I just checked. Paracorr type2 has 56 mm distance from the flange to image plane. Not much different from 56.5 mm of ES HR 2". Both numbers for 1200 mm telescope.

How the heck people manage to stuff OAG, filter wheel and camera in that about 55 mm?
[edit]
Maybe OAG instead of guide scope...?

[SkyHiker]

How the heck people manage to stuff OAG, filter wheel and camera in that about 55 mm?

Here's how I put my rig together, pretty common I think. In the rear is the "Medusa" rig, it helps to have the computer, power and connectivity right there.

[turboscrew]

How the heck people manage to stuff OAG, filter wheel and camera in that about 55 mm?

Here's how I put my rig together, pretty common I think. In the rear is the "Medusa" rig, it helps to have the computer, power and connectivity right there.

I guess the camera is ASI2600 mono?, Which filter wheel, and which OAG?