question about plate solving

[Fafa]

Hi Robin,
I hope you're doing well!

Premise: I have a C6 that I normally use with HyperStar (f/2) and an ASI585MC — and I've had zero issues with plate solving (the native SharpCap solver is absolutely outstanding, thank you for that!).

Now to the question:
When I do planetary imaging at f/10, or even f/15 with a Barlow, I assume (due to weather I haven’t tested yet) that plate solving wouldn't work because of the much smaller FOV.

So, to avoid the tedious 3-star alignment (I’ve been using Celestron StarSense but would like to move away from it), I'm thinking of using a cheap GSO 0.5x reducer, screwed directly onto the nose of the ASI.
This should increase the FOV enough to allow a plate solve and sync. Once that’s done, I would simply unscrew the reducer and screw on the Barlow (which also threads onto the ASI nose), then start my planetary session.

Here’s the key info:
With the reducer, the focal length should be around f/5.32 (≈798 mm) — measured in the past via the Astronomy.net solver.
This gives me approximately:

Width: ~47.8 arcmin
Height: ~26.7 arcmin
→ Roughly 0.8° x 0.45°

Do you think that with these parameters, SharpCap's plate solving would work reliably?
(Of course, I’ll test it as soon as the weather allows!)

Thanks in advance for your time and for SharpCap
Fabio

[Menno555]

Hi Fabio

It might work but only with the extra 0.25 degrees index. Robin added that especially for the smaller FOVs.
You can find and download it in the Plate Solving settings, Smallest Field Of View. Click the Download button and then Tick the 0.25 degrees box.
Also, make sure that you set the Focal Length part there on Do Not Use.

I have an Meade LX200 8" f/10 and when I use my Zwo ASI482MC, it has a smaller FOV than your combi. When enough stars are in the FOV, the plate solve works fine. I do use longer exposures too for that plate solve.
So I think only removing your barlow and then plate solve would already do the trick.

Menno

[Fafa]

Hi Menno
thank you so much for your detailed and kind reply – I really appreciate it!

I didn’t know about the extra 0.25 degrees index – that’s a great tip. I’ll definitely download it and adjust the plate solving settings as you suggested (including setting the focal length to “Do Not Use”).

It’s also very encouraging to hear that your setup works even with a smaller FOV than mine. That gives me a lot more confidence to try plate solving even at f/10 or f/15.

Thanks again, your help really made my day!
Fabio

[admin]

Hi,

I would be tempted to try it with the 0.25 degree index and *without* the reducer first... At f/10 that would give you 0.4x0.22 degrees. I have had reasonable success at 0.37x0.21 using a 26C with at a total focal length of ~3600mm (2x barlow, 6" f/12 classical cassegrain). Not every field solves, but it works often enough to be usable. As Menno says, at longer focal lengths you will need more exposure (and possibly binning) to help with star detection and a bit of luck to get enough stars in the field of view.

cheers,

Robin

[Julian Sanz]

Hi, how can I download the Smallest Field of View file? Thanks.

[Menno555]

Hi, how can I download the Smallest Field of View file? Thanks.

You can find and download it in the Plate Solving settings, Smallest Field Of View. Tick the 0.25 degrees box and then the Download button.
Also, make sure that you set the Focal Length part there on Do Not Use.

Menno

[Julian Sanz]

Hi, the smaller field of view of 0.25 degrees is not activated, how can I activate it? Thanks

[Menno555]

You first have to download it with the download button next to it. I got the order wrong I see now
So first download and then you can activate 0.25 degrees.

Menno

Clipboard_05-08-2025_01.jpg (373.58 KiB) Viewed 816 times

[Julian Sanz]

Gracias