Focus issue: ASI174MM PST pairing: redux

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Jordan Konisky
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Focus issue: ASI174MM PST pairing: redux

Post by Jordan Konisky » Thu Dec 03, 2020 1:48 am

I have previously sought advice from members of Solar Chat on how best to bring my ASI174MM into focus with my PST40, a well-known and much discussed issue with this pairing. The consensus solution was to adapt the out-of-the-box camera with a low-profile nose piece and to add the bottom component of a 2X Barlow. To make everything fit, one must first remove the camera’s IR protective filter. Apparently, this solution works for many folks. No problem and that is what I did.

I thought all was going well, and my images (SharpCap, Autostakkert, Registax wavelets or ImPPG) seemed ok, although not great. Seemed not quite in focus, but I had never imaged the sun before. I attributed the lack of quality to a difficulty in focusing on a naked sun. The test came this past week when I tried to image sunspots 2785 and 2786. As I cranked the focusing knob clockwise the focus improved, yet never reached a crisp focus as the knob reached tightness at the end of its run. The sunspots were smudgy looking, out of focus. I tried the bottom component of both a GSO and Cemax 2X, but neither brought me to focus.

At that point, I brought out my ASI224MC, which out-of-the box with no modifications performed beautifully in easily reaching a crisp focus and a nice image. Interestingly enough, not at the clockwise end of the focusing knob, but rather closer to the counterclockwise end. So clearly the issue is with the ASI174MM.

Could it be that some Barlow will work, but not others? If a particular Barlow works for any of you using a PST AI174MM pair, I would appreciate knowing the exact Barlow make and model that you are using. Any other suggestions are most welcome.









I had the problem solved months ago by using the ZWO low profile nose in combination with the bottom component of a 2X Barlow, a solution that apparently works for many folks here



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Re: Focus issue: ASI174MM PST pairing: redux

Post by marktownley » Thu Dec 03, 2020 6:39 am

You sure it's not the prism out of alignment in the PST? Sounds like it to me... The glue that holds it in place flows over time and the prism sags.


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Re: Focus issue: ASI174MM PST pairing: redux

Post by AndiesHandyHandies » Thu Dec 03, 2020 9:42 am

Hi

All the pictures of the inside of PSTs I have seen have the penta prism at a slight angle so seem to be made that way.

To reduce reflections at the cost of some aberrations I assume.

Have Meade ever made a statement about the penta prism angle?

The grease over time does contaminate the surfaces.

Cheers. Andrew.



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Re: Focus issue: ASI174MM PST pairing: redux

Post by marktownley » Thu Dec 03, 2020 11:44 am

AndiesHandyHandies wrote:
Thu Dec 03, 2020 9:42 am
All the pictures of the inside of PSTs I have seen have the penta prism at a slight angle so seem to be made that way.
Indeed, but the glue is gloopy and this sag generally increases over time. After a certain point the sag introduces astigmatism which is what the issue could be here.


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Re: Focus issue: ASI174MM PST pairing: redux

Post by Jordan Konisky » Thu Dec 03, 2020 12:58 pm

Thank you for your responses. My PST was purchased new less than two yrs ago. Also, as mentioned it comes to focus with my ASI224MC. And importantly, but left out of my post, is that by eye-ball observation, it focuses just fine. So smells like a pairing issue to me. How to fix?



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Re: Focus issue: ASI174MM PST pairing: redux

Post by marktownley » Thu Dec 03, 2020 4:36 pm

You got any pictures you can show with the 174? Dont forget with this camera you are hugely undersampling and so will have a loss of resolution which may be what you are seeing?


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Re: Focus issue: ASI174MM PST pairing: redux

Post by Montana » Thu Dec 03, 2020 6:21 pm

I was thinking that too. My DMK41 is only just passable on the PST at 4.65um, I get much better resolution with the Chameleon with a smaller pixel size. The ASI174 is for very hires work as it has such large pixels, I can't imagine using mine on my PST. If the ASI224MC works well, why not keep using it?
This has a pixel size of 3.75µm just like the Chameleon, far better!

Alexandra



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Re: Focus issue: ASI174MM PST pairing: redux

Post by Jordan Konisky » Fri Dec 04, 2020 12:49 am

Well that certainly explains the basis for my fool’s errand. Thanks everyone for your constructive comments. I learned much from them.



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