How to get multiple values from a texture in parallel in HLSL (ShaderLab)?

Question

I am new to HLSL (ShaderLab) and this site. I need some help.

Shader "TEST/UnlitMultiStarTest"
{
    Properties
    {
        _BaseColor("BaseColor", Color) = (0, 0, 0, 1)
        _StarColor("StarColor", Color) = (1, 1, 1, 1)
        _StarStrength("StarStrength", Float) = 100
        _Pos("Star Potisions Texture", 2D) = "White" {}
    }
    SubShader
    {
        Tags { "RenderType"="BackGround" "Queue"="BackGround+0"}
        Cull OFF
        LOD 100

        Pass
        {
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #include "UnityCG.cginc"

            struct appdata
            {
                float4 vertex : POSITION;
                float2 coord0 : TEXCOORD0;
            };

            struct v2f
            {
                float2 uv : TEXCOORD0;
                float4 vertex : SV_POSITION;
                float3 eyeDir : TEXCOORD2;
            };

            sampler2D _MainTex;
            float4 _MainTex_ST;
            uniform float4 _StarColor;
            uniform float _StarStrength;
            uniform float4 _BaseColor;
            sampler2D _Pos;
            float3 _Pos_TexelSize;

            float2 decodetex(float2 uv, uint a, uint b){
                float NtexSize = _Pos_TexelSize.x;
                float e = NtexSize/2;
                float4 pos4 = tex2Dlod(_Pos, float4(float2(NtexSize*a +e, NtexSize*b +e), 0,0));
                float2 pos2 = float2((pos4.g*90 -45)*2, (pos4.r*360));
                return pos2;
            }

            float3 PolarToCartesian(float2 pd)
            {
                return float3 (sin(pd.y)*cos(pd.x), sin(pd.x), cos(pd.x)*cos(pd.y));
            }

            v2f vert (appdata v)
            {
                v2f o;
                o.vertex = UnityObjectToClipPos(v.vertex);
                o.uv = v.coord0;
                o.eyeDir = UnityWorldSpaceViewDir(mul(unity_ObjectToWorld, v.vertex).xyz);
                return o;
            }

            fixed4 frag (v2f i) : SV_Target
            {
                fixed4 col = _BaseColor;
                float texWidth = _Pos_TexelSize.z;
                for (uint a = 0; a<4; a++){
                    for (uint b = 0; b<4; b++){
                        float2 StarDir = decodetex(i.uv,a,b);
                        float3 StarPos = normalize(PolarToCartesian(float2(radians(StarDir.x), radians(StarDir.y))));
                        float4 StarOut = mul( step(0.999, pow( max( dot(StarPos, mul(normalize(i.eyeDir), -1)),0),_StarStrength)),_StarColor);
                        col += StarOut;
                    }
                }
                return col;
            }
            ENDCG
        }
    }
}

This shader gets the numbers from a Pos texture with spherical coordinates written into the rg values and draws the stars at infinity. Right now I am using 16 of them in a 4*4 texture for testing. However, I would like to eventually draw a large number of stars like in a planetarium, but using a for loop makes it very computationally infeasible. Is there any way to get the position of each star from the texture without using a for loop?

I tried simply passing the uv value as an argument to the decodetex function, but only one point was drawn.

            float2 decodetex(float2 uv){
                float4 pos4 = tex2Dlod(_Pos, float4(uv, 0,0));
                float2 pos2 = float2((pos4.g*90 -45)*2, (pos4.r*360));
                return pos2;
            }