ZWO ASI290mm-s vs QHY5L-iim

[SteveBz]

Hi Guys,

I've been guiding with a QHY5L-iim/OAG setup for the last year or so.  I can nearly always expect to find a guide star in any frame I GOTO, however there have been a few cases when that has not been true and my sequence has failed.

Recently an unexpected storm drenched all my electronics including the QHY5L-iim, which doesn't appear to have recovered.  So I now need to replace it.

The question is, do I chose another QHY5L-iim, and maybe increase the exposure time for some frames from 1s to 3s, for instance, or should I choose a ZWO ASI290mm-s?  It seems a bit more sensitive, although the chip seems a littler smaller.  What are your thoughts?

Thanks

Steve.

[vlaiv]

At what distance do you keep your OAG?

Larger sensor is hard to cover with small pick off prism without vignetting on a fast scope. You need to have prism very close to sensor.

I guide with OAG and with ASI185 which is 1/1.9" sensor (tiny bit larger than your QHY with 1/2" size) and at F/8 I get visible vignetting:

At F/5 you need to keep prism to sensor distance less than about 30mm in order to have similar level of vignetting as above. If you have it more than 40mm - you'll effectively stop down your guide aperture (in case you have 8mm prism).

In any case - get ASI290, bin x2 guide pixels (there is option in PHD2 to bin 2x2 in software - you should turn that on) and yes, use 3s guide exposures - that both helps with seeing and gets you enough guide stars. Another thing - use ASCOM driver instead of native as that enables 16bit data to be downloaded instead of 8bit data.

Put your OAG as close to sensor as you can to avoid guider being stopped down.

[SteveBz]

52 minutes ago, vlaiv said:

At what distance do you keep your OAG?

Larger sensor is hard to cover with small pick off prism without vignetting on a fast scope. You need to have prism very close to sensor.

I guide with OAG and with ASI185 which is 1/1.9" sensor (tiny bit larger than your QHY with 1/2" size) and at F/8 I get visible vignetting:

At F/5 you need to keep prism to sensor distance less than about 30mm in order to have similar level of vignetting as above. If you have it more than 40mm - you'll effectively stop down your guide aperture (in case you have 8mm prism).

In any case - get ASI290, bin x2 guide pixels (there is option in PHD2 to bin 2x2 in software - you should turn that on) and yes, use 3s guide exposures - that both helps with seeing and gets you enough guide stars. Another thing - use ASCOM driver instead of native as that enables 16bit data to be downloaded instead of 8bit data.

Put your OAG as close to sensor as you can to avoid guider being stopped down.

Hi Vlaiv,

Your stars are very round. I guess that's the RC in action! Mine are distorted even with a coma corrector. My guide camera is screwed straight into the OAG. As close as it can. I don't see any vignetting, but I don't get so many stars either. I was binning x3 with the qhy.  I think I'll go with the 290. It seems more sensitive.

Kind regards

Steve.

Edited March 25, 2021 by SteveBz

[vlaiv]

2 minutes ago, SteveBz said:

Your stars are very round. I guess that's the RC in action! Mine are distorted even with a coma corrector.

Indeed it is. Some of mine also start showing astigmatism. I have slight advantage as I place OAG closer to optical axis since I'm imaging with ASI1600 - which is 4/3" sensor.

With APS-C sensor - you need to place OAG further out from optical axis and it is reasonable that stars will be more distorted further out.

[vlaiv]

Just realized - if you are using DSLR - than you'll have at least 44mm of distance between prism and main sensor or even more. This is because Canon DSLR has flange focal distance of 44mm - and anything you put on lens mount will be at least that far.

You are most likely operating with stopped down OAG because of this. What is the size of the prism on your OAG?

[SteveBz]

19 minutes ago, vlaiv said:

Just realized - if you are using DSLR - than you'll have at least 44mm of distance between prism and main sensor or even more. This is because Canon DSLR has flange focal distance of 44mm - and anything you put on lens mount will be at least that far.

You are most likely operating with stopped down OAG because of this. What is the size of the prism on your OAG?

So you are exactly right, I'd forgotten that.  I have a Thin OAG (like the Orion one) with a 55mm distance, and I think it's a 9mm prism (or maybe 8mm).

Are you still thinking the 290?

Steve.

[vlaiv]

4 minutes ago, SteveBz said:

So you are exactly right, I'd forgotten that.  I have a Thin OAG (like the Orion one) with a 55mm distance, and I think it's a 9mm prism (or maybe 8mm).

Are you still thinking the 290?

Steve.

I think that 290 is going to be a bit better than QHY but not by much.

It will have lower read noise - which is good with short exposure and just a bit better QE. You'll still need to bin it to get enough sensitivity and use longer exposures.

Fact that you have quite a bit of coma does not help. Would refocusing a bit help? Thing with coma is that it moves light from the star into coma tail and this lowers brightness of the star - it is harder to detect then - almost like being defocused.

You could also try to carefully position pick off prism. Place it on the longer side of sensor ("below" or "above" the DSLR - not sideways) and move it to the point where you start getting shadow on the sensor. This will minimize coma.

 

[SteveBz]

2 minutes ago, vlaiv said:

Would refocusing a bit help?

I think it probably would, but as it means playing with hex-keys, I'm a bit reluctant.

3 minutes ago, vlaiv said:

move it to the point where you start getting shadow on the sensor.

That's a good idea.  So I think I'll probably get the 290 and do as you suggest.

Kind regards,

Steve.

[vlaiv]

17 minutes ago, SteveBz said:

That's a good idea.  So I think I'll probably get the 290 and do as you suggest.

Just be careful - if you over do it - you'll introduce diffraction of prism edge It will be very small effect - but I've seen it in some of my images.

above is one of my images - look at bright stars near to edge where OAG sits - they have single spike due to diffraction of the edge. As soon as you move away from that area - stars are normal again.

Here is shadow of prism starting to show on flat.

[SteveBz]

That would be hidden by my normal diffraction spikes!