9 replies to this topic

[arshadwm]

Hi all. Need some help

I have Redcat51 and I attached ZWO OAG L, ZWO EFW and ZWO ASI 2600 MC. but I failed to achieve the correct focus. I followed the diagram provided by ZWO to achieve 56 mm back focus but it’s not working. The helical Focuser reached its limit.

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[rkinnett]

How much backfocus did you measure?

 

Do you have the camera angle adjuster and tilt plate and M54-M48 adapter on the RedCat?  If so, then you need 59.7mm between the rear shoulder of the M48 thread on the M54-M48 adapter to your sensor plane.  Find a way to add 4mm additional space between the scope and OAG (assuming you measured 56mm like shown in the ZWO diagram).

[BlakPhoenix]

The Redcat doesn't have a required backfocus as it's a Petzval design so ignore the 56mm spacing requirement. I use the OAGL, EWF & 2600MM with my redcat just fine, just remove the tilt plate off the camera. Also not exactly sure why you're using a EFW with a colour camera? Could save some space without it if need be.

Edited by BlakPhoenix, 20 March 2023 - 06:30 PM.

[james7ca]

Technically, there is no backfocus requirement for the Redcat 51 so I'm not sure what you mean by achieving the "56 mm back focus." 

 

Thus, you don't need to have a fixed spacing between the Redcat 51 and the camera's sensor, whatever spacing that brings the scope to focus with an adjustment to the helical focuser is just fine.

 

That said, if after adding the OAG and filter wheel you can't reach infinity focus (not enough inward travel on the helical focuser) then that means you have too much spacing between the Redcat and the camera (i.e. there is a limited range of spacing that can exist between the end of the Redcat and the point of infinity focus which varies upon the position of the helical focuser). So, it's likely that you need to find something that can be removed to shorten that spacing.

 

However, I'd be certain that you are testing the focus correctly before you begin any changes. Are you doing this testing during the day on a VERY distant object? Sometimes it can be hard to find focus when you have nothing but stars to go by.

 

What you probably want is a spacing that allows you to reach focus when the helical is near to its mid-point, or (better?) at a point where it is nearly fully retracted. Of coarse, if you have very little spacing between the Redcat and the camera then there is nothing particularly wrong with having to extend the helical to its full outward travel.

 

In any case, the Redcat offers anywhere from 59.7 to 77mm of available spacing between itself and the point of focus depending upon whether you are using its M48 or M56 threads. So, worse case (and if you can't remove anything else) then perhaps you can find a shorter M56 to M42 adapter to sit between the Redcat and your OAG. However, I'm not certain where the helical was positioned when WO determined those 59.7 to 77mm of available spacing (or it you want to call it so, the "backfocus").

Edited by james7ca, 20 March 2023 - 06:41 PM.

[arshadwm]

@rekinnett. Yes I do have adjuster.

[arshadwm]

@blakpheonix great suggestion. I will remove it. I have attached different light pollution filter just to experiment. I’ll receive the mm version soon. Are you using unmourned filter on EFW, if so how is your experience what size should of unmounted filters should I buy against 2 inches mounted filter

[arshadwm]

@james I’m trying to fix it during the night on stars.

[BlakPhoenix]

@blakpheonix great suggestion. I will remove it. I have attached different light pollution filter just to experiment. I’ll receive the mm version soon. Are you using unmourned filter on EFW, if so how is your experience what size should of unmounted filters should I buy against 2 inches mounted filter

FYI the Redcat has a 2" filter thread inside it if you want to use a single filter for light pollution or dual narrowband.

 

I use 2" mounted filters in my EFW for the 2600MM but you can get away with 36mm just fine.

[james7ca]

@james I’m trying to fix it during the night on stars.

Do the setup during the day, much easier. But, you'll need to focus on a very distant object on your far horizon or a good half mile away (i.e. don't use a tree in your front yard or even something in your neighbor's yard).

Edited by james7ca, 20 March 2023 - 06:46 PM.

[andysea]

The diagram for the shows a back focus of 59.7mm from the m48 connection. That is the available back focus. Ideally your sensor placement should  be less than that because you want to have to extra travel past infinity when focusing. I use the M54 connection and that gives me an extra 4.5mm of wiggle room.