Quark Combo - conditions for capturing the whole solar disk at once

[Florin Andrei]

I'm thinking to get a DayStar Quark Combo filter for hydrogen alpha (chromosphere version). The instrument I will use is the Orion ED80, at D=80mm, F=600mm, f/7.5 focal ratio.

A 4x telecentric barlow will put it at F=2400mm, f/30 focal ratio, which is how the Quark Combo likes it. After the Quark I will use a focal reducer to keep the f-ratio in line with the pixel size on the camera (I have not decided yet which camera I will use, but it will likely be something along the lines of ASI290MM, 2.9 micron pixels, implying a reducer around 0.5x). That's all good.

Occasionally, I would like to observe, and perhaps also image, the whole disk of the Sun at once. To observe it, I will just have to use an eyepiece with the appropriate focal length and FoV. To image it in prime focus, I have an APS-C sensor, which is 15.6mm on the small axis. I realize imaging quality in this case will not be best, but that's okay, and I hate doing mosaics. The question is, how to do it.

If you look at the Quark Combo product page...

http://www.daystarfilters.com/COMBOQuark.shtml

...they say: "Full disk viewing possible on refractors up to ~1800mm focal length". If that's the case, for full disk I could use a 3x telecentric, which would give me F=1800mm at the etalon. To keep the focal ratio at f/30 (so the etalon maintains good contrast), I could stop the aperture down to 60mm. The Sun would be 16.8 mm in the focal plane. To make the Sun fit on the APS-C, I would only need a mild focal reducer of around 0.8x, which I already have. Or I could just plug an eyepiece in after the Quark for visual observations.

How does that sound? Anybody done this before? Am I missing something? Thanks!

Edited October 7, 2020 by Florin Andrei

[vlaiv]

Avoid focal reducers.

Full disk viewing is only possible up to 1800mm because of the size of blocking filter. It has only 21mm of clear aperture.

Radius of solar disc is about 1/4 of a degree (1/2 for diameter). Tan(0.25) * 1800mm = 7.854mm

This makes disk of 15.7mm in diameter at focal plane. Blocking filter is some distance from focal plane, so to avoid vignetting - Solar disk must be smaller than 21mm at focal plane.

Want to use ASI290 to capture full disk?

First calculate what is critical F/ratio for 2.9um pixel size. It is F/8.85 - this is value above which you should not sample - there simply is no point.

Now let's calculate focal length at which sun fits on ASI290 sensor. It has height of about 3.2mm - so let's take 3.1mm to be solar diameter at focal plane. Radius will then be 1.55mm.

You need something like 330mm to get full disk onto ASI290 - that is smaller than focal length of 80ED.

Maybe consider using larger sensor instead? How about ASI178? It has 5mm of height so needed FL will be about 530mm. Use x0.85 FF/FR or get ASI174 or ultimately use focal reducer - but most available in 1.25" format are low quality.

Btw, optimum sampling rate for ASI178 is F/7.32.

In order to get that sampling rate with small pixels and telecentric lens - you should consider using aperture mask and pixel binning.

[Florin Andrei]

No, for the full disk I plan to use an APS-C sensor, which is 23.6 x 15.6mm, with 4.78 micron pixels. I am aware this is far from ideal sampling, and I don't care. I have other plans for ideal sampling imaging.

My concern is how to get the full, unobstructed image of the solar disk in the focal plane with the Quark Combo. This is for both visual observations and imaging, and the quality of imaging will not be ideal, I know.

Knowing the diameter of the blocking filter, to determine the size of the largest unobstructed image in the focal plane, seems like I would have to assume a certain distance between the focal plane and the blocking filter, is that right?

Edited October 7, 2020 by Florin Andrei

[vlaiv]

1 minute ago, Florin Andrei said:

No, for the full disk I plan to use an APS-C sensor, which is 23.6 x 15.6mm, with 4.78 micron pixels. I am aware this is far from ideal sampling, and I don't care. I have other plans for ideal sampling imaging.

My concern is how to get the full, unobstructed image of the solar disk in the focal plane.

Knowing the diameter of the blocking filter, to determine the size of the largest unobstructed image in the focal plane, seems like I would have to assume a certain distance between the focal plane and the blocking filter, is that right?

Yes indeed. You need to both account for expected distance of of blocking filter from imaging plane and F/ratio of system.

Slower the system - larger sun disk you can have as vignetting will be less.

At for example F/15 and 50mm distance from blocking filter to sensor, you will have 21 - 50/15 = 17.666mm of field without vignetting. We have seen that 1800mm of focal length is going to give you 15.7mm sun disk, so you should be fine - with some room to spare for tracking and centering issues.

[Florin Andrei]

I see.

If I plug the eyepiece directly in the Quark Combo, that would definitely work, because it's very close to the filter. If it's further away (there's a diagonal, etc), then it depends.

With the camera, I'll have to keep the sensor as close as possible to the filter. Shorten that side of the stack.

Do you know where the 21mm aperture is? Is it at the exit from the Quark? Where do I start measuring?

[vlaiv]

8 hours ago, Florin Andrei said:

Do you know where the 21mm aperture is? Is it at the exit from the Quark? Where do I start measuring?

As far as I know it should be the last thing on the exit at eyepiece side - you should be able to see it.

[Florin Andrei]

Thanks for all the information, this is very helpful.

I've found a CN thread where the stop is said to be 21mm, located at the exit:

https://www.cloudynights.com/topic/619797-help-calculating-fov-for-visual-eyepieces-with-quark-chromosphere/

More interestingly, there's another CN thread where Jen (the owner of DayStar) says "a 90mm F/6 = 540mm using a 4X powermate, then a Combo Quark will pass a full disk".

https://www.cloudynights.com/topic/534692-daystar-quark-combo-the-new-sctrefractor-model-impressions/

That's D=90mm, F=2160mm, f/24. I only need it to work at D=60mm, F=1800mm, f/30, which should be easier.

Edited October 8, 2020 by Florin Andrei

[Rattler]

I use the Combo but can't get a full disc due to my rig. I'd like to though but I suppose that's down to the camera.