OAG on visual setup for plate solve

[Jonk]

I’ve had a thought and wondered if anyone else has tried this.

If I have an eq mount, pc controlled with an OAG setup, ready to take a ccd or diagonal / eyepiece, I assume that there are no issues removing the ccd and replacing with the diagonal and still being able to use the OAG camera for platesolving? This would would make observing faint objects a lot quicker and easier right?

[vlaiv]

In principle - it could work, but there are a few issues that need to be addressed.

1. FOV and prism placement. Sensors are usually rectangular and often smaller than field stop of eyepieces. If you just take off camera and put diagonal and eyepiece - you'll likely have vignetting / OAG prism shadow on one side of the field of view. This really depends on field stop of eyepiece - but it needs to be taken into account

2. Back focus distances / issues. Simply unscrewing camera and putting diagonal is not going to work as you expect. There is significant difference in optical path between the two. Diagonals can have up to 100mm of optical path over cameras that have maybe dozen of so mm from thread to sensor. This also translates to OAG - you need to push guide camera away the same distance. This can be done with extension tubes (T2 extensions or whatever) - but prism further in optical path will certainly start to vignette and you'll effectively have aperture stop on OAG.  It will just require longer exposure to get enough stars to plate solve

3. It is Off Axis guider after all - so you need to account for that - if you plate solve OAG to particular part of the sky - eyepiece will actually be centered on different coordinates than those resolved by plate solving.

You need to figure out this displacement direction and magnitude and account for it when directing scope and plate solving.

With all of the above in mind, I have couple of observations:

1. Too much hassle?

2. Maybe setup with ONAG will serve this purpose better

3. Maybe smart phone + prism will act better as sort of DSC - to tell you where you are - Celestron seems to have this in their beginner telescopes like these:

https://www.celestron.com/collections/starsense-explorer-smartphone-app-enabled-telescopes

Not sure if there is open source or at least stand alone application that does the same (no reason why there would not be one)

4. Guide scope can probably serve the same purpose and solve most of the problems listed above

[Jonk]

All good points - I was thinking of ways to reduce weight and 'hassle', but needed a nudge in the right direction, so thanks for that.

Looks like to achieve the easiest setup to use, a small guidescope for platesolving is the least hassle.

Another reason for thinking about this was platesolving a manual dobsonian, perhaps with a tablet running ASTAP or something. An initial platesolve would show how far out I was pointing, and I guess a quick lookup table of distance vs pixels of error could be produced, giving a good 'rough pointing' in the general direction.

I dunno, maybe I think too much!

[vlaiv]

I think that platesolving manual scope is going to be a challenge.

How long exposure do you think you will use?

Let's say you use some "regular" gear - like 3.75µm camera and 50mm finder scope as "guider". These finders have about 180mm of FL, thus working resolution will be 4.3"/px

Sidereal rate is about 15"/s if I'm not mistaken, so near equator, one second exposure will lead to ~4 px trailing. These both deform stars (harder to plate solve) and also smear light over more pixels - requiring longer exposure to get good SNR - and increased exposure further smears stars in little streaks.

Don't know if you'll be able to plate solve such image successfully.

You don't really need great precision between plate solved location and scope pointing, so you could perhaps use fast short lens instead? This will reduce working resolution (reducing blur) while still giving you good precision?

Something around 40-50mm of focal length perhaps? Old M42 50mm lens with appropriate adapter?

[Jonk]

Exposure time using a small guidescope I reckon could be as low as 3 seconds.

Or I can use my 10-100mm cctv lens / asi 120mm-s combo that I use for PA. Sharpcap can platesolve a 0.5s high gain image in less than a second...

I do have an equatorial platform so that should help if setup correctly.

Yes a challenge, but all things I could get to grips with... all in good time!