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Camera + [Rotator] + Coma Corrector = Spacer Hell


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Hi!

A while ago, I acquired a ZWO ASI294MC Pro and a SkyWatcher F/5 Coma Corrector for my SkyWatcher 8" Newtonian telescope. The coma corrector requires a backfocus distance of 55mm which was straight forward to achieve:

  1. Camera (back focus = 6.5mm)
  2. 11mm M42 (female to female) extender (11mm)
  3. 21mm M42 (male) to M42 (female) extender (21mm)
  4. 16.5mm M42 (male) to M48 (female) extender (16.5mm)
  5. Coma corrector with a M48 (male) thread

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This all adds up to exactly 55mm which was all perfect. However, I've been thinking of getting a M42 CAA 360 ° Rotator Camera Angle Adjuster or a similar M48 one. Both of these have an extension thickness of 12.5mm. Furthermore, the length of the male thread on the rotator is 3mm while the female thread is 5mm long.

If I opt for the M42 rotator, I have only 1 option:

  1. it will have to replace the existing 21mm spacer with an additional 8.5mm (21 - 12.5) spacer which is a very odd spacing and I can find no satisfactory option on AliExpress (which is I'm forced to buy items from as only it ships to Pakistan). My new assembly should become:
    1. Coma corrector
    2. 16.5mm M42-M48 extender (16.5mm)
    3. M42 SPACER (8.5mm) (16.5 + 8.5 + 12.5 + 11 + 6.5 = 55) (I can break the 8.5 down into (4 + 4 + 0.5) or (5 + 3 + 0.5) but both seem odd
    4. M42 ROTATOR (12.5mm)
    5. 11mm extender (11mm)
    6. Camera

If I opt for the M48 rotator, I have only 1 option and that is even worse:

  1. it will come between the coma corrector and the 16.5mm extender and the 21mm extender will be replaced by some other extender. This option is bad as the M42 female thread on the rotator is only 5mm long whereas the male thread on the coma corrector is 6mm long. This means there will be a gap of (not exactly) 1mm in the assembly. So my spacings become:
    1. Coma corrector (male M48 6mm)
    2. Empty space (1mm approx)
    3. M48 ROTATOR (female M48 5mm) (12.5mm)
    4. 16.5mm M48-M42 extender (16.5mm)
    5. M42 SPACER (7.5mm) (1 + 12.5 + 16.5 + 7.5 + 11 + 6.5 = 55)
    6. 11mm extender (11mm)
    7. Camera

Does my "spacer math" check out? How do I go about solving this problem? I don't see anywhere outside this assembly where the rotator can go. On one side is the camera, while on the other side beyond the coma corrector, there is the focuser entrance. Also, the coma corrector simply slides into the focuser and I don't see how I can attach the rotator outside this assembly area.

I would love to hear the thoughts of you awesome people who might have the same combo as this (Coma Corrector + Rotator + ZWO ASI294MC Pro) and would love to see how you guys have approached the problem.

Finally, if having these weird spacers is the only way, which option should I go for? And given 8.5mm or 7.5mm of spacing, is there a preferred way of building this spacing (4 + 4 + 0.5) or (4 + 0.5 + 4) or (3 + 5 + 0.5) or (8 + 0.5)?

Thanks a lot!

Asim Sohail
Pakistan

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26 minutes ago, Waldemar said:

Hi Asim,

Camera and rotator take 19mm. The coma corrector needs 55. That leaves 39mm to bridge.
This: https://www.baader-planetarium.com/en/baader-varilock-46-lockable-t-2-extensiontube-29-46mm-with-spanner-tool-(t-2-part-25v).html
will solve your problems. 

Hi @Waldemar,

Thanks for the suggestion. I can see some problems with this that I hope you will address:

  1. The variable adapter you proposed is almost EUR 63 which is a bit expensive for my budget at the moment.
  2. My coma corrector has an M48 male thread whereas the camera has a M42 male thread. Since most spacers have a male thread at one end and a female thread at the other, I will have to use the stock 11mm M42 female-to-female extender supplied so that the camera + 11mm spacer exposes a female thread and the coma corrector exposes a male thread. Total spacing required in this case is 55 - 6.5 - 12.5 - 11 = 25mm. Since I have a stock 16.5mm M42-to-M48 spacer supplied with the camera itself, if I use it, the spacer required will be, once again, 25 - 16.5 = 8.5mm. I don't think there are many variable spacers within this range. Besides, as noted in #1, they are way more expensive.

Thanks a lot!

Asim

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I'm by no means expert on this, but here are my thoughts on the subject and hopefully it will help in one way or another.

Why do you need camera rotator in the first place?

Camera rotators are very useful if you have threaded connection to telescope - which means everything is screwed together and screwed in focuser. Once everything is screwed together - you cannot adjust camera angle any more by rotation in focuser - as any rotation will disassemble the whole thing.

As is, with your setup, you already have suitable mechanism for rotation - loosen focuser screws, rotate whole camera / CC assembly in focuser, tighten focuser screws. Simple as that. Although camera and CC are screwed together - that assembly is inserted into focuser rather than screwed onto it.

If you think that having numbers for orientation is helpful - just start plate solving - it will tell you exact orientation of camera in degrees so you can match it between sessions. You can also DIY some sort of dial do you know how much you are turning your camera (but you really don't need one - you'll quickly learn by feel how much to turn).

Even if you don't plate solve there are ways to orient your camera the same between imaging sessions - align X/Y to RA/DEC - either by slewing in exposure or drifting in exposure. You start exposure on a bright star and either slew telescope at low speed, or stop telescope tracking. Star will make a trail. Rotate camera until that line is horizontal (or vertical - depends how you want to frame your shot).

If you really want to add this part, then here are other tips that I can think of:

- longer male thread is a problem, longer female thread is not a problem. You calculate from "shoulder" to "shoulder". In your case you had 5mm female and 6mm male - so yes, there is a "gap" of 1mm that you should add to calculation. Don't worry about light leak - no way it will leak in the light

- I don't know about coma correctors (never used one), but field flatteners and reducers need to be "dialed" in for particular telescope in question. This usually means that prescribed distance is only a guideline - you actually need to try it out and adjust it for best results. For this, variable length extension tubes are very good solution, and so are shims. You can get either plastic or aluminum shims of 0.1 to 0.5mm and 1mm that you put on M42 or M48 threads (between parts to add some distance - opposite of above "gap")

- You can actually make different order of elements. Problem that you have is that you need to keep both female-to-female 11mm for camera since both camera and CC have male thread so you need to switch order at some point, and you need to keep T2/M48 adapter since you need to switch thread size as well. But you have some room to do things differently.

You can arrange it like this:

Camera - rotator (in reverse, it should not matter) - 11mm - whatever you need here - T2/M48 adapter

This gives you another option - you can combine 11mm and whatever you need to reach distance into single variable length extension female/female T2. This is for T2 version of rotator.

 

 

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15 minutes ago, vlaiv said:

I'm by no means expert on this, but here are my thoughts on the subject and hopefully it will help in one way or another.

Why do you need camera rotator in the first place?

Camera rotators are very useful if you have threaded connection to telescope - which means everything is screwed together and screwed in focuser. Once everything is screwed together - you cannot adjust camera angle any more by rotation in focuser - as any rotation will disassemble the whole thing.

As is, with your setup, you already have suitable mechanism for rotation - loosen focuser screws, rotate whole camera / CC assembly in focuser, tighten focuser screws. Simple as that. Although camera and CC are screwed together - that assembly is inserted into focuser rather than screwed onto it.

If you think that having numbers for orientation is helpful - just start plate solving - it will tell you exact orientation of camera in degrees so you can match it between sessions. You can also DIY some sort of dial do you know how much you are turning your camera (but you really don't need one - you'll quickly learn by feel how much to turn).

Even if you don't plate solve there are ways to orient your camera the same between imaging sessions - align X/Y to RA/DEC - either by slewing in exposure or drifting in exposure. You start exposure on a bright star and either slew telescope at low speed, or stop telescope tracking. Star will make a trail. Rotate camera until that line is horizontal (or vertical - depends how you want to frame your shot).

If you really want to add this part, then here are other tips that I can think of:

- longer male thread is a problem, longer female thread is not a problem. You calculate from "shoulder" to "shoulder". In your case you had 5mm female and 6mm male - so yes, there is a "gap" of 1mm that you should add to calculation. Don't worry about light leak - no way it will leak in the light

- I don't know about coma correctors (never used one), but field flatteners and reducers need to be "dialed" in for particular telescope in question. This usually means that prescribed distance is only a guideline - you actually need to try it out and adjust it for best results. For this, variable length extension tubes are very good solution, and so are shims. You can get either plastic or aluminum shims of 0.1 to 0.5mm and 1mm that you put on M42 or M48 threads (between parts to add some distance - opposite of above "gap")

- You can actually make different order of elements. Problem that you have is that you need to keep both female-to-female 11mm for camera since both camera and CC have male thread so you need to switch order at some point, and you need to keep T2/M48 adapter since you need to switch thread size as well. But you have some room to do things differently.

You can arrange it like this:

Camera - rotator (in reverse, it should not matter) - 11mm - whatever you need here - T2/M48 adapter

This gives you another option - you can combine 11mm and whatever you need to reach distance into single variable length extension female/female T2. This is for T2 version of rotator.

 

 

Hi @vlaiv, thanks a lot for your valuable suggestions and some great insights. I didn't know that the backfocus of 55mm was an approximate value. How should it be dialed in and how can it be tested to see if its improving things or not? As in, is there a non-subjective way other than take an exposure, add shim, take another and judge whether things look better or worse?

Thank you for understanding the constraints of my assembly as well. Indeed I need a female-to-female adapter and a M42-to-M48 adapter at some point in my assembly. Your suggestions about the solution make sense, and if I go ahead and purchase a rotator, I will act upon your advice definitely.

As for why am I opting for a rotator instead of simply rotating the assembly myself is this: I have tried doing it exactly like youve said many times: taking an exposure, plate-solving to find rough orientation, repeat. But it is time consuming to get it right and everytime I loosen the screws and fiddle with the assembly, my focuser inadvertedly slips a bit and I have to go through focusing again and again. So I was thinking maybe a rotator that allows rotation without any resistance (on bearings) will be more helpful than my current technique. I even marked angles on my assembly using a pencil.

Keeping your advice in view, perhaps a better option for me might be to ditch the rotator and fix my focuser first so it doesn't slip. Your thoughts?

Thanks a lot!

Asim Sohail

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3 minutes ago, AweSIM said:

I didn't know that the backfocus of 55mm was an approximate value. How should it be dialed in and how can it be tested to see if its improving things or not? As in, is there a non-subjective way other than take an exposure, add shim, take another and judge whether things look better or worse?

I'm not sure if 55mm is exact thing with coma correctors and in particular with Skywatcher CC and Skywatcher scope - they could be a good match and 55mm exact distance.

I just mentioned that Field Flatteners have this issue where you have to dial in exact spacing - and yes - it is done like that - trial and error. You repeat procedure until you get best stars in your corners. This is why variable extensions come in handy - no need for bunch of different spacers. If you want to save some imaging time - then you do it during the day or cloudy night by using artificial star (ball bearing that is smooth and shiny really far away with torch pointed at it can be budget friendly artificial star).

In any case, it would be worth checking if spacing is exact - either someone could contribute who used coma corrector or a quick search online.

9 minutes ago, AweSIM said:

Keeping your advice in view, perhaps a better option for me might be to ditch the rotator and fix my focuser first so it doesn't slip. Your thoughts?

I think that you should make sure your focuser is in good shape - regardless of how you proceed with rotator. It should have lock screw and you should make sure that it stays in place once you lock it. There should be no tilt or play in focuser as well.

10 minutes ago, AweSIM said:

So I was thinking maybe a rotator that allows rotation without any resistance (on bearings) will be more helpful than my current technique.

Rotators are not quite resistance free, so I'm not sure it will solve the problem of focuser slipping. I have rotators for my systems (threaded connection) - one large in focuser and one separate M68. They both require some force and are not quite smooth although I don't have much weight hanging at the back  - ASI1600 cool and filter drawer (which is light weight, much lighter than filter wheel).

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