Help with using a Powermate

[Crispy5573]

Hi all, I bought a 4x2" powermate and I am attempting to use it for planetary photography with a Meade LX10 8" ZWO filter wheel and ASI290MM USB3 camera.

My original set up was simply slot the powermate into my baader click lock attach the teleview T adaptor and use T2 extension tubes and the filter wheel to get 56mm spacing between the T adaptor and the camera sensor. I could only get a very soft focus with this set up. 

I was then led to believe I needed to move the Powermate back from the scope so the front element was at the standard back focus ( 109mm is the figure someone here said they were quoted by Meade ) so I bought a Celestron SCT to T2 adaptor, Baader T2 2" click lock and a Baader 29 - 46 T2 Vari Lock and got to 109mm but can't achieve any sort of focus.

Now I am hoping someone can please tell me what I am doing wrong apart from spending £500 on equipment that has almost had flying lesson

[skybadger]

You only need to get the Barlow in place and a detector behind it. You don't need the filter wheel or spacers unless you want to increase the magnification of the Barlow by increasing the distance between lens and detector. 

You shouldn't care about the detector being at a particular position, just focus. The position will be different than at prime focus due to using the Barlow. 

The problem of soft focus is nothing to do with the Barlow other than it magnifying what your scope is capable of. You could check collimation at this point or reduce the magnification slightly, get some video for stacking and see what comes out. 

Was it soft in the eyepiece or the camera ? If viewing on a screen, the image is quite magnified so will be soft for lots of reasons including scope cooling and seeing. 

 

[Cornelius Varley]

12 hours ago, Crispy5573 said:

Hi all, I bought a 4x2" powermate and I am attempting to use it for planetary photography with a Meade LX10 8" ZWO filter wheel and ASI290MM USB3 camera.

My original set up was simply slot the powermate into my baader click lock attach the teleview T adaptor and use T2 extension tubes and the filter wheel to get 56mm spacing between the T adaptor and the camera sensor. I could only get a very soft focus with this set up. 

I was then led to believe I needed to move the Powermate back from the scope so the front element was at the standard back focus ( 109mm is the figure someone here said they were quoted by Meade ) so I bought a Celestron SCT to T2 adaptor, Baader T2 2" click lock and a Baader 29 - 46 T2 Vari Lock and got to 109mm but can't achieve any sort of focus.

Now I am hoping someone can please tell me what I am doing wrong apart from spending £500 on equipment that has almost had flying lesson

A quick check on the astronomy tools calculator suggests that the telescope/camera combination works best without the need of the Powermate. The small pixel size of the camera (2.9microns) doesn't require any extra increase in focal length to reach the ideal sample rate. 

1 hour ago, skybadger said:

You don't need the filter wheel or spacers unless you want to increase the magnification of the Barlow by increasing the distance between lens and detector. 

Powermates don't increase in magnification with increased distance, unlike barlow lenses, except for the 1.25" x5 powermate.

[skybadger]

1 hour ago, Cornelius Varley said:

Powermates don't increase in magnification with increased distance, unlike barlow lenses, except for the 1.25" x5 powermate.

Thanks for reminding me. In that case you don't need the spacers at all. 

 It is fair to say that your 2000mm focal length and x4 powermate are overkill for this and can probably get away with nothing at all; hence the mushy focus. 

Sample images ?

[Crispy5573]

Thanks for the replies guys.

As I said it is for planetary imaging and I checked the astronomy tools CCD calculator and yes it does lead to over sampling but to quote astronomy tools "This combination leads to over-sampling. Will require a good mount and careful guiding. OK for high magnification solar, lunar or planetary imaging ".

I also asked 365 for advice before I bougt it and was told that with their 8"SCT they used a 3x PM for Saturn and Jupiter and a 5x PM for mars so the 4x would be a good compromise.

I will use this combination and work it out I was just hoping someone may have use something similar and could point me in the right direction. 

Sorry Badger no pics yet I'm just doing day time testing. The good thing is I appear to have plenty of time to test according to the weather forecast lol . The collimation is very good. 

 

 

[Cornelius Varley]

24 minutes ago, Crispy5573 said:

This combination leads to over-sampling. Will require a good mount and careful guiding. OK for high magnification solar, lunar or planetary imaging ".

My point exactly. Using a high magnification barlow/powermate with this size pixel gives no better resolution than using the telescope at its native focal length. The planetary disc may be bigger using the powermate but you don't get any increase in detail.

[markse68]

Are you going to be lucky imaging Crispy? My limited understanding is that much higher than”normal” resolutions can be gained by that technique (edit-though not higher than Dawes limit of scope. edit again- Damian Peach says Dawes limit is irrelevant to planetary imaging) Not sure how you’d get focus sharp though as without the processing step the image would look mushy i guess- edit- couldn’t you just use a bahtinov mask on a bright star and lock it there?

Mark

Edited August 18, 2021 by markse68

[skybadger]

I wouldn't say much higher, I'd aim for slightly higher than expected if all other factors are in your favour. Id want to sample at at least 2x the Dawes resolution per pixel.  if Dawes is 0.6, aiming for better than 0.3 per pixel to make sure you don't miss anything. 

You can focus on a moon or a star to get focus right for each wavelength. 

[Crispy5573]

9 hours ago, markse68 said:

Are you going to be lucky imaging Crispy? My limited understanding is that much higher than”normal” resolutions can be gained by that technique (edit-though not higher than Dawes limit of scope. edit again- Damian Peach says Dawes limit is irrelevant to planetary imaging) Not sure how you’d get focus sharp though as without the processing step the image would look mushy i guess- edit- couldn’t you just use a bahtinov mask on a bright star and lock it there?

Mark

Sorry Mark the emails appear to have ceased again??? Yes I will be lucky imaging and as you (and Mr Peach ) say the normal limits are irrelevant and long focal length / small sensor is (according to chaps like Mr.Peach) the way to go and let the software sort it out. 

I spoke to someone at Telview today and he told me just to focus with an eyepiece (no powermate ) then replace the eyepiece with the powermate and camera and you shouldn't be far away as the 4x is parfocal . Pin sharp focus isn't going to happen and the main thing to keep an eye on is exposure and video length to avoid saturation and blurring.

Edited August 18, 2021 by Crispy5573
Forgot something

[The Lazy Astronomer]

General rule of thumb is to go for an f ratio approx 5x - 7x your pixel size.

Focus as best you can, then use high gain, exposures of sub 10ms - the image will look terrible on the capture software, but stacking and sharpening will fix it (if conditions are good; if your sky condition is bad, then you've got no hope).

[CraigT82]

As above, a focal ratio of around 5x the pixel size is a good place to be for lunar and planetary.   Using the 4x PM  with an f/10 scope will put you at f/40 so with the ASI290m that's a whopping 14x the pixel size. 

The resulting image would be massive and very blurry and wouldn't contain anymore detail than an image captured at a much shorter focal length. It would also require much longer exposures and that is exactly what you don't want for lucky imaging as it'll really slow down your frame rate.

As an example this Jupiter image was capture at f/13.5 which is about 4.5x pixel size (with gain at 40% and exposures of 2.2ms which gave 247fps).