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Best guide camera for long focal length OAG


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Hi folks, I currently use an old QHY5-ii mono cam for off-axis guiding with my RC8 (focal length 1600mm), but often fail to locate a single guide star or only manage to find one due to the narrow field of view. I would like to try out the new multi-star guiding facility of PHD, so wondered what would be the best guide camera to use.

I have a ZWO ASI290MM for planetary imaging which I understand is more sensitive than the QHY5, but its sensor area is only half that of the QHY. Looking at the alternatives (see my plan below), The 174MM looks like a good option, and its sensor is more than double the area of the QHY5. However, it's pretty expensive, as is the Lodestar.

Does anyone have any suggestions?

50841387997_282c2c921f_b.jpg

Edited by lukebl
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What is diameter of your pick off prism and what is the distance to guide camera?

This is important to minimize vignetting - but also means that larger sensor might not be used fully.

I have RC8" as well and use ASI185mc for guiding. I did not have any issues with finding stars so far with it. That is 8.6mm diagonal sensor. Look at vignetting caused by my OAG:

image.png.c15e65bada37e0f1f584d3ec5c9aa132.png

In current configuration, I don't think I would need larger sensor than this, and even this one is not fully illuminated.

Distance is important as well, although at F/8 you are using rather tight beam, but if you don't pay attention to spacing - you might be stopping down your pick off prism as well and running guide camera at lower aperture as well.

Another trick is to use ASCOM drivers at 16bit data format and use exposures of 2-3s.

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Thanks, Vlaiv.

Here's the current configuration. It's a nice big prism, and you can see the whole of the sensor reflected up the OAG tube (please excuse the dust!). I think that the prism is about far into the light beam as possible without affecting the imaging camera. Maybe it could go a little further. I'm not sure of the make of the OAG. Exposures are 3.5 seconds.

The distance from the prism to the sensor is about 45mm.

50840742728_21ba6dc55d_b.jpg50841478981_421062f18f_b.jpg

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3 minutes ago, lukebl said:

Here's the current configuration. It's a nice big prism, and you can see the whole of the sensor reflected up the OAG tube (please excuse the dust!). I think that the prism is about far into the light beam as possible without affecting the imaging camera. Maybe it could go a little further. I'm not sure of the make of the OAG. Exposures are 3.5 seconds.

The distance from the prism to the sensor is about 45mm.

It indeed looks like quite large prism.

I'd say something like at least 8-9mm diameter. At F/8, you need to be at least 60mm to stop it down, so you are ok with 45mm.

Mine prism is also 8mm on the side - so there is benchmark for you - you don't need sensor larger than about 8-9mm diameter as it will vignette.

You want mono sensor with high QE and low read noise. Pixel size is not as important as you can bin your pixels.

ASI290 seems like obvious choice - you can bin your pixels even 3x3 with 2.9µm pixel size. That should give you high sensitivity while still being precise enough (1.1"/px guide resolution - more than enough).

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4 minutes ago, vlaiv said:

ASI290 seems like obvious choice - you can bin your pixels even 3x3 with 2.9µm pixel size. That should give you high sensitivity while still being precise enough (1.1"/px guide resolution - more than enough).

Thanks for the info vlaiv. Unfortunately, I don't think you can bin the ASI290 in PHD.

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5 minutes ago, lukebl said:

Thanks for the info vlaiv. Unfortunately, I don't think you can bin the ASI290 in PHD.

Opps. Sorry, it looks like you can bin the ASI290 2x.

Edited by lukebl
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20 minutes ago, lukebl said:

Opps. Sorry, it looks like you can bin the ASI290 2x.

...and apparently you can bin the QHY5 too. That's something I didn't realise. So maybe I don't need a new camera after all.

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14 minutes ago, lukebl said:

...and apparently you can bin the QHY5 too. That's something I didn't realise. So maybe I don't need a new camera after all.

Yes, give that a try.

Btw, ASI290 should have ASCOM driver and that one should let you bin as well (maybe even with selection between x2 and x3) - but PHD2 has noise reduction and you can select 2x2 bin and 3x3 median.

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First clear night for weeks last night and, great, binning with the QHY5 works!

A quick session on the Horsehead still only had 3 stars in the field of view, but the guide star was bright and guiding seemed very good. I think my focus was a bit off. 15 x 600 sec exposures, RC 8, Atik 428ex, Baader Ha filter.

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12 minutes ago, vlaiv said:

Atik 428ex is a bit small sensor for that scope, have you considered replacing it with something bigger?

I agree, they’re not well matched, but they’re all I have at the moment!

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On ‎16‎/‎01‎/‎2021 at 15:54, vlaiv said:

It indeed looks like quite large prism.

I'd say something like at least 8-9mm diameter. At F/8, you need to be at least 60mm to stop it down, so you are ok with 45mm.

Mine prism is also 8mm on the side - so there is benchmark for you - you don't need sensor larger than about 8-9mm diameter as it will vignette.

You want mono sensor with high QE and low read noise. Pixel size is not as important as you can bin your pixels.

ASI290 seems like obvious choice - you can bin your pixels even 3x3 with 2.9µm pixel size. That should give you high sensitivity while still being precise enough (1.1"/px guide resolution - more than enough).

Vlaiv, i see you mentioned 45cm for the RC8, but what about for the F/9 RC6? What prism to sensor distance would you have to stay under to keep the guide cam from effectively being stopped down?

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3 hours ago, Xiga said:

Vlaiv, i see you mentioned 45cm for the RC8, but what about for the F/9 RC6? What prism to sensor distance would you have to stay under to keep the guide cam from effectively being stopped down?

It is really simple math - what is your prism diameter? Let's say it is 8mm - then 8 x 9 = 72mm.

At 72mm distance - only central point will be fully illuminated as per this diagram:

image.png.ebef538a5e11f7d82a75e1b7e54a9cf2.png

If you have more path - prism entrance will act as aperture stop and you'll get F/10 or F/11 beam instead of F/9.

Take few mm off the path for part where light travels thru prism. In principle, for F/9 beam and 8mm prism, I'd say you are good with to about 65mm.

If you are using focal reducer - you are no longer at F/9 beam but faster one - so prism / guide sensor distance needs to be reduced. Also - if you want larger illuminated field, you again need to reduce this distance as above is good for %100 of few central mm on sensor.

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10 hours ago, vlaiv said:

It is really simple math - what is your prism diameter? Let's say it is 8mm - then 8 x 9 = 72mm.

At 72mm distance - only central point will be fully illuminated as per this diagram:

image.png.ebef538a5e11f7d82a75e1b7e54a9cf2.png

If you have more path - prism entrance will act as aperture stop and you'll get F/10 or F/11 beam instead of F/9.

Take few mm off the path for part where light travels thru prism. In principle, for F/9 beam and 8mm prism, I'd say you are good with to about 65mm.

If you are using focal reducer - you are no longer at F/9 beam but faster one - so prism / guide sensor distance needs to be reduced. Also - if you want larger illuminated field, you again need to reduce this distance as above is good for %100 of few central mm on sensor.

Thanks Vlaiv, that makes perfect sense 🙏

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